Introduction I

Contents

I
Introduction
ATPL Book 9 Meteorology

1. The Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
2. Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3. Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
4. Pressure Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
5. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
6. Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
7. Adiabatics and Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
8. Turbulence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
9. Altimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
10. Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
11. Upper Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
12. Clouds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
13. Cloud Formation and Precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
14. Thunderstorms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
15. Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265
16. Icing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283
17. Air Masses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299
18. Occlusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
19. Other Depressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 341
20. Global Climatology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 365
21. Local Winds and Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 403
22. Area Climatology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413
23. Route Climatology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437
24. Satellite Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 463
25. Meteorological Aerodrome Reports (METARs) . . . . . . . . . . . . . . . . . . . . . . . 469
26. Terminal Aerodrome Forecasts (TAFs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485
27. Significant Weather and Wind Charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 495
28. Warning Messages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509
29. Meteorological Information for Aircraft in Flight . . . . . . . . . . . . . . . . . . . . . . 531
30. Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 543
31. Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 639

v
 I Introduction
I
Introduction

vi
 Chapter

1
The Atmosphere

A Definition of Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Reasons for Studying Meteorology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
A Definition of the Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
The Constituents of the Atmosphere (By Volume) . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Properties of the Earth’s Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
The Structure of the Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
The Significance of Tropopause Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Atmospheric Hazards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
The International Standard Atmosphere (ISA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
ISA Deviation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
The ICAO International Standard Atmosphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1
 1 The Atmosphere
1
The Atmosphere

2
 The Atmosphere
1
A Definition of Meteorology

1
The Atmosphere
“The branch of science dealing with the earth’s atmosphere and the physical processes
occurring in it.”

Reasons for Studying Meteorology

• To understand the physical processes in the atmosphere
• To understand the meteorological hazards, their effect on aircraft and how to minimize the
risks posed by those hazards
• To identify the weather information that is required for each flight
• To interpret actual and forecast weather conditions from the documentation provided
• To analyse and evaluate weather information before flight and in-flight
• To devise solutions to problems presented by weather conditions

Weather is the one factor in modern aviation over which man has no control; a knowledge
of meteorology will at least enable the aviator to anticipate some of the difficulties which
weather may cause.

Weather-influenced Accidents to UK Transport Aircraft

Table 1 Transport aircraft accidents, 1975 - 94

All accidents

Aeroplanes Rotorcraft All aircraft

Year Total WI Per cent Total WI Per cent Total WI Per cent

1975-79 52 17 32.69 9 4 44.44 61 21 34.43

1980-84 67 20 29.85 20 7 35.00 87 27 31.03
1985-89 95 22 23.16 20 3 15.00 115 25 21.74
1990-94 216* 25 11.58* 20 6 30.00 236* 31 13.13*
1975-94 430 84 19.53 69 20 28.98 499 104 20.84

* Includes ramp and other minor ground accidents, hence low percentage figures.
WI: Weather-influenced

Accidents excluding selected ramp and other occurrences

Aeroplanes Rotorcraft All aircraft

Year Total WI Per cent Total WI Per cent Total WI Per cent
1975-79 52 17 32.69 9 4 44.44 61 21 34.43
1980-84 67 20 29.85 20 7 35.00 87 27 31.03
1985-89 78 22 28.20 20 3 15.00 98 25 25.51
1990-94 101 25 24.75 20 6 30.00 121 31 25.62
1975-94 298 84 28.18 69 20 28.98 367 104 28.34

WI: Weather-influenced

3
 1 The Atmosphere

Table 2 Weather-influenced accidents to transport aircraft by element of weather, 1975 - 94

1

All Accidents Fatal Accidents

The Atmosphere

Element No. Percentage of No. Percentage of

total total
Visibility 22 21.1 10 66.7
Icing/snow 22 21.1 3 20.0
Wind and turbulence 45 43.3 2 13.3
Rain/wet runway 12 11.5 0 0
Lightning 3 2.9 0 0
All cases 104 100 15 100

For this course a knowledge of advanced physics is not required, but a knowledge of the
elementary laws of motion, heating, cooling, condensation and evaporation will be useful.

A Definition of the Atmosphere

“The spheroidal gaseous envelope surrounding a heavenly body.”

The Constituents of the Atmosphere (By Volume)

Nitrogen 78.09% Argon 0.93%
Oxygen 20.95% Carbon Dioxide 0.03%

Plus traces of:

Neon Nitrous Oxide Helium Nitrogen Dioxide

Krypton Carbon Monoxide Xenon Sulphur Dioxide
Hydrogen Ammonia Methane Iodine and Ozone

Also present are solid particles and, in particular, water vapour which, from a meteorological
point of view, is the most important gas in the atmosphere.

The proportions of the constituents remain constant up to a height of at least 60 km (except for
ozone), but above this the mixing processes associated with the lower levels of the atmosphere
no longer exist and gravitational separation of the gases occurs. Although the trace of ozone
in the atmosphere is important as a shield against ultraviolet radiation, if the whole of the layer
of ozone were brought down to sea level it would only be 3 mm thick.

Properties of the Earth’s Atmosphere

The earth’s atmosphere varies vertically and horizontally in:

• Pressure.
• Temperature.
• Density.
• Humidity.

The earth’s atmosphere is fluid, supports life only at lower levels and is a poor conductor.

4
 The Atmosphere
1
The Structure of the Atmosphere

1
The Atmosphere
• The Troposphere:

• is the lowest layer of the earth’s atmosphere where temperature decreases with an
increase in height.
• consists of ¾ of the total atmosphere in weight.
• contains almost all the weather.

• The Stratosphere is the layer above the troposphere where temperature initially remains
constant to an average height of 20 km then increases to reach a temperature of -2.5°C at
a height of 47 km, then above 51 km temperature starts to decrease again. The reason for
the increase is the action of ultraviolet radiation in the formation of ozone. The boundary
between the stratosphere and the next layer, the mesosphere is known as the stratopause.
The average height of the stratopause is 50 km in temperate latitudes.

• The Tropopause:
• This marks the boundary between the troposphere and the stratosphere and is where
temperature ceases to fall with an increase in height. (Practically taken as the height
where the temperature fall is less than 0.65°C per 100 m (2°C per 1000 ft.)

• T
 he height of the tropopause is controlled by the temperature of the air near the
surface. The warmer the air, the higher the tropopause. The colder the air, the lower
the tropopause. Therefore, temperature variations due to latitude, season, land and
sea, will all cause varying heights of the tropopause. There are two locations where the
tropopause abruptly changes height or “folds”. These are at approximately 40° and 60°
latitude. The average height of the tropopause at the Equator is 16-18 km with an average
temperature of -75°C to -80°C, and at the poles 8 km with an average temperature of
-40°C to -50°C. The average value at 50°N is 11 km (36 090 ft) with a temperature of
-56.5°C.

• T
 he temperature of the tropopause is controlled by its height. The higher it is, the colder
the temperature at the tropopause. The lower it is, the warmer the temperature at the
tropopause. The temperature at the tropopause can be as high as -40°C over the poles
and as low as -80°C over the Equator.

Figure 1.1 The mean height of the tropopause at the Greenwich Meridian

5
 1 The Atmosphere

The Significance of Tropopause Height

1
The Atmosphere

The significance of the tropopause height is that it usually marks:

• the maximum height of significant cloud.

• the presence of jet streams.
• the presence of Clear Air Turbulence (CAT). It is now referred to as TURB.
• the maximum wind speed.
• the upper limit of most of the weather

Temperatures
Temperature in the troposphere increases from the poles to the Equator.

Temperature in the lower stratosphere increases from the Equator to the poles in summer but
reaches max temperature in mid latitudes in winter.

Atmospheric Hazards
As aircraft operating altitudes increase, so concentrations of OZONE and COSMIC RADIATION
become of greater importance to the aviator.

Above 50 000 ft, normal concentrations of ozone exceed tolerable limits and air needs to be
filtered before entering the cabin. The heat of the compressor system will assist in the breaking
down of the ozone to an acceptable level.

Cosmic radiation is not normally hazardous, but at times of solar flare activity a lower flight
level may be necessary.

Advances in meteorological forecasting and communications should result in pilots receiving

prompt and accurate information regarding high altitude hazards, but it is important that they
should be aware of these hazards and prepared to take the necessary re-planning action.

The International Standard Atmosphere (ISA)

Because temperature and pressure vary with time and position, both horizontally and vertically,
it is necessary, in aviation, to have a standard set of conditions to give a common datum for:

• the calibration of aircraft pressure instruments

• the design and testing of aircraft.

The standard atmosphere now used in aviation is the ICAO International Standard Atmosphere
(ISA). ISA defines an ‘average’ atmosphere from -5 km (-16 400 ft) to 80 km (262 464 ft). For
practical purposes we just need to look at the ISA between mean sea level and 20 km.

The ICAO International Standard Atmosphere (ISA) is:

• a MSL temperature of +15° Celsius,

• a MSL pressure of 1013.25 hectopascals (hPa),
• a MSL density of 1225 grams / cubic metre,
• a lapse rate of 0.65°C/100 m (1.98°C/1000 ft) up to 11 km (36 090 ft),
• a constant temperature of -56.5°C up to 20 km (65 617 ft),
• an increase of temperature 0.1°C/100 m (0.3° C/1000 ft), up to 32 km (104 987 ft).

Note: Practically we use a lapse rate of 2°/1000 ft for calculations up to the Tropopause.

6
 The Atmosphere
1

1
The Atmosphere

Figure 1.2 The International Standard Atmosphere (ISA).

7
 1 The Atmosphere

ISA Deviation
1
The Atmosphere

To determine true altitude and for the assessment of performance data it is necessary to
determine the temperature deviation from the ISA at any specified altitude. To do this we
firstly need to determine what the ISA temperature is at a specified altitude, then calculate the
deviation from the ISA.

The ISA temperature at a particular pressure altitude is found by reducing the MSL temperature
by 2°C for each 1000 ft above 1013 hPa datum:

ISA Temperature = 15 - 2× altitude (in 1000 ft)

e.g. find the ISA temperature at 18 000 ft:

ISA temperature = 15 - 2 × 18 = -21°C

Note: Remember the temperature is isothermal above 36 000 ft (11 km) in the ISA at -57°C.

Now to find the deviation from ISA we subtract the ISA temperature from the actual
temperature:

ISA Deviation = actual temperature - ISA temperature

So if the actual temperature at 18 000 ft is -27°C, then the deviation is:

ISA Deviation = -27 - (-21) = -6°

For the temperatures below, calculate the ISA deviations:

Height (ft) Temperature ISA Temperature ISA Deviation

(°C)
1500 +28
17 500 -18
24 000 -35
37 000 -45
9500 -5
5000 +15
31 000 -50
57 000 -67

If the limiting deviation for your aircraft at an airfield 5000 ft AMSL is ISA +10, what is the
maximum temp at which you can operate?

If the deviation at 3500 ft is +12, what is the ambient temperature?

(Answers on page 14)

8
 The Atmosphere
1
The ICAO International Standard Atmosphere

1
The Atmosphere
Height (km) Height (ft) Temp (°C) Pressure Height Change Density (%)
(hPa) (per hPa)
32.00 104 987 -44.7 8.9 1.1
30.48 100 000 -46.2 11.1 1.4
27.43 90 000 -49.2 17.3 2.2
24.38 80 000 -52.2 28.0 3.6
21.34 70 000 -55.2 44.9 5.8
20.00 65 620 -56.5 56.7 7.2
15.24 50 000 -56.5 116.6 15.3
13.71 45 000 -56.5 148.2 19.5
11.78 38 662 -56.5 200 103 ft 26.3
11.00 36 090 -56.5 228.2 91 ft 29.7
9.16 30 065 -44.4 300 73 ft 36.8
5.51 18 289 -21.2 500 48 ft 56.4
3.05 10 000 -4.8 696.8 37 ft 73.8
3.01 9882 -4.6 700 36 ft 74.1
1.46 4781 +5.5 850 31 ft 87.3
0 0 +15 1013.25 27 ft 100

Note: The above height change figures show how the pressure against height change equation is
modified as altitude changes but the figures offered only relate to ISA conditions of Temperature
and Pressure. We can assess changes outside these conditions by using the following formula:

96 ×T
H=
P

where H = height change per hPa in feet

T = Actual Absolute Temperature at that level in kelvin (K)
P = Actual Pressure in hPa

Note: this formula is only valid for calculating the height change per hPa change in pressure at
a specified altitude; it cannot be used to calculate a change in height between two pressure
levels, nor the change in pressure between two altitudes.

9
 1 Questions

Questions
1
Questions

1. How does the height of the tropopause normally vary with latitude in the Northern
Hemisphere?

a. It decreases from south to north

b. It increases from south to north
c. It remains constant from north to south
d. It remains constant throughout the year

2. What, approximately, is the average height of the tropopause over the Equator?

a. 8 km
b. 16 km
c. 11 km
d. 50 km

3. In the International Standard Atmosphere the decrease in temperature with height
below 11 km is:

a. 0.5°C/100 m
b. 0.6°C/100 m
c. 0.65°/100 m
d. 1°C/100 m

4. The 200 hPa pressure altitude level can vary in height. In temperate regions which
of the following average heights is applicable?

a. FL390
b. FL300
c. FL100
d. FL50

5. The temperature at FL110 is -12°C. What will the temperature be at FL140 if the
ICAO standard lapse rate is applied?

a. -6°C
b. -18°C
c. -9°C
d. -15°C

6. At a certain position the temperature on the 300 hPa chart is -54°C, and according
to the significant weather chart the tropopause is at FL330. What is the most likely
temperature at FL350?

a. -48°C
b. -60°C
c. -56.5°C
d. -64°C

7. What is the boundary between the troposphere and the stratosphere called?

a. Ionosphere
b. Stratosphere
c. Atmosphere
d. Tropopause

10
 Questions
1
8. Which constant pressure altitude chart is standard for 4781 ft pressure level (FL50)?

1
a. 500 hPa

Questions
b. 300 hPa
c. 850 hPa
d. 700 hPa

9. An outside air temperature of -30°C is measured whilst cruising at FL200. What is
the temperature deviation from the ISA at this level?

a. 5°C colder than ISA

b. 5°C warmer than ISA
c. 10°C colder than ISA
d. 10°C warmer than ISA

10. What is the most likely temperature at the tropical tropopause?

a. -56.5°C
b. -75°C
c. -40°C
d. -25°C

11. Which one of the following statements applies to the tropopause?

a. It is, by definition, an isothermal layer

b. It indicates a strong temperature lapse rate
c. It is, by definition a temperature inversion
d. It separates the troposphere from the stratosphere

12. In the lower part of the stratosphere the temperature:

a. is almost constant
b. decreases with altitude
c. increases with altitude
d. increases at first and decreases afterwards

13. What is the approximate composition of the dry air by volume in the troposphere?

a. 10% oxygen, 89% nitrogen and the rest other gases

b. 88% oxygen, 9% nitrogen and the rest other gases
c. 50% oxygen, 40% nitrogen and the rest other gases
d. 21% oxygen, 78% nitrogen and the rest other gases

14. How does temperature vary with increasing altitude in the ICAO standard
atmosphere below the tropopause?

a. Remains constant
b. Decreases
c. Increases
d. At first it increases and higher up it decreases

11
 1 Questions

15. How would you characterize an air temperature of -15°C at the 700 hPa level over
1

western Europe?
a. Within +/-5°C of ISA
Questions

b. 20°C below standard

c. Low
d. High

16. If you are flying at FL300 in an air mass that is 15°C warmer than a standard
atmosphere what is the outside temperature likely to be?

a. -15°C
b. -30°C
c. -45°C
d. -60°C

17. If you are flying at FL140 and the outside temperature is -8°C at what altitude will
the freezing level be?

a. FL75
b. FL100
c. FL130
d. FL180

18. What is the most important constituent in the atmosphere from a weather
standpoint?

a. Carbon dioxide
b. Oxygen
c. Water vapour
d. Methane

19. The average height of the tropopause at a latitude of 50° is about:

a. 8 km
b. 11 km
c. 14 km
d. 16 km

20. Between mean sea level and a height of 20 km the lowest temperature in the
international standard atmosphere (ISA) is:

a. -273°C
b. -44.7°C
c. -56.5°C
d. -100°C

21. The international standard atmosphere (ISA) assumes that the temperature will
reduce at a rate of:

a. 1.98°C per 1000 feet up to 36 090 feet after which it remains constant to 65
617 feet
b. 1.98°C per 1000 feet up to 36 090 feet and then will rise at 0.3°C per 1000 feet
up to 65 617 feet when it will remain constant
c. 2°C per 1000 feet up to 65 617 feet after which it will remain constant to 104
987 feet
d. 2°C per 1000 feet up to 36 090 feet and will then increase at 0.3°C per 1000
feet up to 65 617 feet

12
 Questions
1
22. In the mid-latitudes the stratosphere extends on average from:

1
a. 0 to 11 km

Questions
b. 11 to 50 km
c. 50 to 85 km
d. 11 to 20 km

23. In relation to the total weight of the atmosphere, the weight of the atmosphere
between mean sea level and a height of 5500 m is approximately:

a. 1%
b. 25%
c. 50%
d. 99%

24. A temperature of +15°C is recorded at an altitude of 500 metres above mean sea
level. If the vertical temperature gradient is that of a standard atmosphere, what
will be the temperature at the summit of a mountain 2500 metres above mean sea
level?

a. 0°C
b. +2°C
c. +4°C
d. -2°C

13
 1 Answers

Answers
1
Answers

1 2 3 4 5 6 7 8 9 10 11 12
a b c a b b d c a b d a
13 14 15 16 17 18 19 20 21 22 23 24
d b c b b c b c a b c b

Answers to Questions on page 8

Height (ft) Temperature ISA Temperature ISA Deviation

(°C)
1500 +28 +12 +16
17 500 -18 -20 +2
24 000 -35 -33 -2
37 000 -45 -57 +12
9500 -5 -4 -1
5000 +15 +5 +10
31 000 -50 -47 -3
57 000 -67 -57 -10

Max temperature = +15°C

Ambient temperature = +20°C

14
 Chapter

2
Pressure

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Atmospheric Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
The Barograph . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Variations of Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Types of Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
QFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
QNH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
QFF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Pressure Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Analysis Charts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

15
 2 Pressure
2
Pressure

16
 Pressure
2
Introduction
Variations in pressure have long been associated with changes in the weather - the ‘falling

2
glass’ usually indicating the approach of bad weather. The Handbook of Aviation Meteorology

Pressure
makes the statement:

“The study of atmospheric pressure may be said to form the foundations of the science of
meteorology.”

Atmospheric Pressure
Atmospheric pressure is the force per unit area exerted by the atmosphere on any surface in
contact with it. If pressure is considered as the weight of a column of air of unit cross-sectional
area above a surface, then it can be seen from the diagram that the pressure (weight of the
column above) at the upper surface will be less than that at the lower surface.

Thus atmospheric pressure will decrease with an increase in height.

Figure 2.1 The Weight of the Atmosphere on the Surface of the Earth

Units of Measurement
The standard unit of force is the NEWTON (N) and an average for atmospheric pressure at sea
level is 101 325 newtons per square metre (pascals). For simplicity this is expressed as 1013.25
hectopascals (hPa) because the earlier system of measurement was millibars (mb) and 1 hPa =
1 mb. In some countries millibars are still used. Other units which are still in use are related to
the height of a column of mercury in a barometer in inches or millimetres (see overleaf).

Note: mean sea level pressure in the ISA is 29.92 inches or 760 mm of mercury.

17
 2 Pressure

Mercury Barometer
The basic instrument used for the measurement of atmospheric pressure is the mercury
barometer. The atmospheric pressure is measured by the height of a column of mercury, and
2

this height can be read in terms of any of the units shown above. The USA still uses inches of
Pressure

mercury as their measurement of atmospheric pressure.

Figure 2.2 A Mercury Barometer

Aneroid Barometer.
A more compact means of measuring atmospheric pressure is the Aneroid Barometer. It
consists of partially evacuated capsules, which respond to changes in pressure by expanding
and contracting, and a system of levers, these changes of pressure being indicated by a pointer
moving over a scale.

Figure 2.3 An Aneroid Barometer

18
 Pressure
2

2
Pressure
Figure 2.4 Met Office Aneroid Barometer

The Barograph
To enable a continuous record of pressure changes to be made, a paper covered rotating drum
is substituted for the scale and the instrument then becomes a barograph. This instrument is
used by the meteorologist to measure what is known as pressure tendency, the rise and fall of
pressure over a period of time. Pressure tendency is an important forecasting tool.

Figure 2.5 A Barograph

19
 2 Pressure

Variations of Pressure
Height
2

With an increase in height, the weight of air overlying the surface will reduce. Therefore
Pressure

pressure will fall with height. The rate of change of pressure with height (the barometric lapse
rate) reduces as altitude increase (see table on page 9), or the height change per hPa
increases as altitude increases

However, temperature has a dramatic effect on the pressure change with height, i.e. the
pressure lapse rate. Warm air will cause pressure to fall slowly with height, i.e. decreasing
the pressure lapse rate, whereas cold air will cause pressure to fall rapidly with height, i.e.
increasing the pressure lapse rate. Therefore we would expect the pressure at any given height
to be higher over warm air and lower over cold air. The effect of temperature on the rate of
change of pressure with height is an important fact which we will return to in altimetry and
upper winds.

Shown below is how temperature affects the height difference with a 1 hPa change in pressure.
These values have been derived from the formula described in the chapter on the atmosphere.
96T
H=
P

ISA
27 feet at MSL
50 feet at 20 000 ft
100 feet at 40 000 ft

Diurnal Variation
There is a change in pressure during the day which although small (about 1 hPa in temperate
latitudes, can be as much as 3 hPa in the tropics) would need to be taken into account when
considering pressure tendency as an indication of changing weather. The variation is shown in
Figure 2.6.

Figure 2.6 Diurnal Variation

The variation is difficult to explain, but is probably due to a natural oscillation of the atmosphere
having a period of about 12 hours, this oscillation being maintained by the 24 hour variation
of temperature.

20
 Pressure
2
Types of Pressure
QFE

2
Pressure
The atmospheric pressure measured at the aerodrome reference point. With QFE set on the
altimeter the altimeter will read zero feet when the aircraft is on the aerodrome.

Figure 2.7 QFE

QNH
This is the barometric pressure at the airfield (QFE), converted to mean sea level (MSL) using
the ISA temperature at the airfield and the ISA pressure lapse rate. This will provide a pressure
which does not account for any temperature deviation away from ISA. The correction to be
made to the surface pressure will depend solely upon the height of the airfield AMSL. QNH is
always a whole number without any decimal places and is always rounded down. When on the
aerodrome with QNH set the altimeter will read aerodrome elevation.

QFF
Because temperature affects the change of pressure over height QNH is not a true mean sea
level pressure (unless ISA conditions exist). The forecaster needs to know the true mean sea
level pressure in order to construct accurate analysis charts and to help with the forecasting of
future changes.

The meteorological offices, therefore, convert QFE to MSL using the actual temperature and
assuming isothermal conditions between the aerodrome and MSL. This pressure is known
as QFF and, because of the differential rate of change of pressure over height at different
temperatures, may differ from QNH.

We can determine, from the formula above, that at temperatures below ISA we have a relatively
small height change per 1 hPa change in pressure and a relatively large change at temperatures
above ISA.

21
 2 Pressure

Example 1: What is the relationship between QFF and QNH at Oxford (270 ft AMSL) if the
QNH is 1020 hPa and the temperature ISA +10°?
2
Pressure

Figure 2.8

The QNH is calculated using the ISA temperature and the QFF using the actual temperature.
Since the actual temperature is warmer than ISA the change in pressure over 270 ft will be
greater in the ISA than in the actual conditions. As we are above MSL this means that the QNH
will be greater than the QFF.

Example 2: What is the relationship between QFF and QNH at an aerodrome 69 m below MSL
if the QNH is 1005 hPa and the temperature is ISA-10°?

Figure 2.9

This time the change in pressure is greater for the calculation of QFF than for the QNH. As we
are reducing pressure this time it means the QNH will once again be greater than the QFF.

We can use similar arguments to show that at an aerodrome AMSL with a temperature colder
than ISA or at an aerodrome below MSL with a temperature greater than ISA the QFF will be
greater than the QNH. This is summarized overleaf:

22
 Pressure
2
Summary
ISA

2
+- ++

Pressure
QNH < QFF QNH > QFF
MSL
-- -+
QNH > QFF QNH < QFF

Same sign, above mean sea level and warmer than ISA (+,+) or below mean sea level and colder
than ISA (-,-) then QNH is greater than QFF. Otherwise the QFF is greater than the QNH.

Stations AT MSL Regardless of temperature QNH = QFF (=QFE)

The normal range of mean sea level pressure (QFF) extends from 950 to 1050 hPa. The lowest
recorded mean sea level pressure is 870 hPa in the eye of Typhoon Tip in the Western Pacific
in 1979. The lowest recorded in the North Atlantic is 882 hPa in the eye of Hurricane Wilma in
2005. The highest mean sea level pressure was 1085.7 hPa recorded in winter in Siberia in 2001.

Pressure Definitions
QFE The pressure measured at the aerodrome reference point.
QFF QFE converted to mean sea level using the actual temperature.
QNH  QFE converted to mean sea level using the ISA.
ISOBAR 
A line joining places of the same atmospheric pressure (usually MSL pressure
QFF).

Standard Pressure Setting (SPS) 1013 hPa

Analysis Charts
Isobars on analysis charts
are corrected mean sea level
pressures (QFF) and are
drawn at a spacing which is
dependent on the scale of the
chart.

On larger area charts the

spacing may be expanded
to 4 or more hectopascals
but this will be stated on the
chart.

Figure 2.10 Isobars on an Analysis Chart

23
 2 Questions

Questions
1. The barometric pressure at the airfield datum point is known as:
2
Questions

a. QFF
b. QNH
c. QFE
d. Standard Pressure

2. The instrument that gives a continuous printed reading and record of the
atmospheric pressure is:

a. barometer
b. hygrometer
c. anemograph
d. barograph

3. The pressure of the atmosphere:

a. decreases at an increasing rate as height increases

b. decreases at a constant rate as height increases
c. decreases at a decreasing rate as height increases
d. decreases at a constant rate up to the tropopause and then remains constant

4. When considering the actual tropopause which statement is correct?

a. It is low over the poles and high over the Equator

b. It is high over the poles and low over the Equator
c. It is the same height of 36 090 ft all over the world
d. It is at a constant altitude of 26 000’

5. Atmospheric pressure may be defined as:

a. the weight of the atmosphere exerted on any surface with which it is in

contact
b. the weight of the atmosphere at standard sea level
c. the force per unit area exerted by the atmosphere on any surface with which
it is in contact
d. a pressure exerted by the atmosphere of 1013.2 hPa

6. The QFF is the atmospheric pressure:

a. at the place where the reading is taken

b. corrected for temperature difference from standard and adjusted to MSL
assuming standard atmospheric conditions exist
c. at a place where the reading is taken corrected to MSL taking into account the
prevailing temperature conditions
d. as measured by a barometer at the aerodrome reference point

7. The pressure of 1013 hPa is known as:

a. standard pressure setting

b. QNH
c. QFE
d. QFF

24
 Questions
2
8. The aircraft altimeter will read zero at aerodrome level with which pressure setting
set on the altimeter subscale:

2
a. QFF
b. QNH

Questions
c. SPS
d. QFE

9. The aerodrome QFE is:

a. the reading on the altimeter on an aerodrome when the aerodrome

barometric pressure is set on the subscale
b. the reading on the altimeter on touchdown at an aerodrome when 1013 is set
on the subscale
c. the reading on the altimeter on an aerodrome when the sea level barometric
pressure is set on the subscale
d. the aerodrome barometric pressure

10. When an altimeter subscale is set to the aerodrome QFE, the altimeter reads:

a. the elevation of the aerodrome at the aerodrome reference point

b. zero at the aerodrome reference point
c. the pressure altitude at the aerodrome reference point
d. the appropriate altitude of the aircraft

11. The aerodrome QNH is the aerodrome barometric pressure:

a. corrected to mean sea level assuming standard atmospheric conditions exist

b. corrected to mean sea level, assuming isothermal conditions exist
c. corrected for temperature and adjusted to MSL assuming standard
atmosphere conditions exist
d. corrected to MSL using ambient temperature

12. A line drawn on a chart joining places having the same barometric pressure at the
same level and at the same time is:

a. an isotherm
b. an isallobar
c. a contour
d. an isobar

13. An isobar on a meteorological chart joins all places having the same:

a. QFE
b. QFF
c. QNH
d. standard pressure

14. Pressure will _________ with increase of height and will be about __________ at
10 000 ft and ___________ at 30 000 ft.

a. Increase 800 hPa 400 hPa

b. Decrease 700 hPa 300 hPa
c. Increase 200 hPa 800 hPa
d. Decrease 500 hPa 200 hPa

25
 2 Questions

15. An airfield in England is 100 m above sea level, QFF is 1030 hPa, temperature at the
surface is -15°C. What is the value of QNH?
2

a. Impossible to determine
b. Less than 1030 hPa
Questions

c. Same as QFF
d. More than 1030 hPa

26
Questions
2

2
Questions

27
 2 Answers

Answers
2

1 2 3 4 5 6 7 8 9 10 11 12
c d c a c c a d d b a d
Answers

13 14 15
b b b

28
 Chapter

3
Density

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Effect of Changes of Pressure on Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Effect of Change of Temperature on Density . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Effect of Changes in Humidity on Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Effect of Change of Altitude on Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Effect of Change of Latitude on Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Effect of Changes in Density on Aircraft Operations . . . . . . . . . . . . . . . . . . . . . . . . 33
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

29
 3 Density
3
Density

30
 Density
3
Introduction
Density may be defined as mass per unit volume and may be expressed as:

• Grams per cubic metre.

3
Density
• A percentage of the standard surface density - relative density.

• T
 he altitude in the standard atmosphere to which the observed density corresponds -
density altitude.

Effect of Changes of Pressure on Density

As pressure is increased, the air will be compressed which reduces the volume and increases the
density. Likewise, if pressure is decreased, the air will expand which will increase the volume
and decrease the density.

(rho) = density

We can therefore say that:

DENSITY IS DIRECTLY PROPORTIONAL TO PRESSURE.

In the atmosphere density can be decreased by raising the volume of air to a greater height
since we know that pressure decreases with an increase in altitude. Similarly, density can be
increased by lowering the volume of air to a lower height.

Effect of Change of Temperature on Density

If a volume of air is heated it will expand and the mass of air contained in unit volume will be
less. Thus density will decrease with an increase in temperature and we can say:

DENSITY IS INVERSELY PROPORTIONAL TO TEMPERATURE.

Effect of Changes in Humidity on Density

The molecular mass of water is less than that of nitrogen and oxygen. If we increase the
amount of water vapour in a fixed volume of air, then we are replacing the heavier nitrogen
and oxygen molecules with the lighter water molecules so the total mass of that volume will
decrease and hence the density will decrease.

DENSITY IS INVERSELY PROPORTIONAL TO WATER VAPOUR CONTENT

31
 3 Density

Effect of Change of Altitude on Density

In the troposphere as altitude increases both temperature and pressure decrease but, although
they have opposite effects on density, the effect of pressure is much greater than the effect of
3

temperature so density decreases as altitude increases.

Density

(In the ISA ρ = 100% at sea level, 50% at 20 000 ft, 25% at 40 000 ft and 10% at 60 000 ft)

Density will change by 1% for a 3 degree change in temperature or a 10 hPa change in pressure.

Effect of Change of Latitude on Density

At the surface as latitude increases temperature decreases so density will increase as we move
from the Equator towards the poles. At the Equator the surface temperatures are high so
the rate of change of pressure with height is relatively low compared to the poles where
temperatures are low and the change of pressure with height is relatively high. This means
that at, say, 50 000 ft the pressure over the Equator will be relatively high compared to the
pressure at 50 000 ft over the poles. The temperatures are lower at 50 000 ft at the Equator
than at the poles which means that the density at 50 000 ft at the poles will be less than at
50 000 ft at the Equator. So we can summarize the change of density as follows:

• at the surface density increases as latitude increases

• at about 26 000 ft density remains constant with an increase in latitude.

• a
 bove 26 000 ft density decreases with an increase in latitude. (Maximum deviation from
standard occurs at about 50 000 ft.)

Figure 3.1 The Effect of Latitude on Density

32
 Density
3
Effect of Changes in Density on Aircraft Operations
a) Accuracy of aircraft instruments - Mach meters, ASIs.

b) Aircraft and engine performance - low density will reduce lift, increase take-off run,

3
reduce maximum take-off weight.

Density
(L = CL ½ρV2S)
Where L = Lift

CL = Coefficient of Lift

ρ = Density

V = TAS

S = Wing area

Airfields affected would be:

• High Denver Nairobi Sana’a

• Hot Bahrain Khartoum Singapore

c) H
umidity generally has a small effect on density (humidity reduces density), but must
be taken into account at moist tropical airfields, e.g. Bahrain, Singapore.

Figure 3.2 An Illustration of Pressure Decrease with Height in air masses with Different Temperatures and therefore
Different Densities

33
 3 Density
3
Density

34
 Questions
3
Questions
1. Consider the following statements relative to air density and select the one which
is correct:

3
a. Because air density increases with decrease of temperature, air density must

Questions
increase with increase of height in the International Standard Atmosphere
(ISA)
b. At any given surface temperature the air density will be greater in anticyclonic
conditions than it will be when the MSL pressure is lower
c. Air density increases with increase of relative humidity
d. The effect of change of temperature on the air density is much greater than
the effect of change of atmospheric pressure

2. The tropopause in mid latitudes is:

a. lower in summer with a lower temperature

b. lower in winter with a higher temperature
c. lower in summer with a higher temperature
d. lower in winter with a lower temperature

3. Generally as altitude increases:

a. temperature decreases and density increases

b. temperature, pressure and density decreases
c. temperature and pressure increase and density decreases
d. temperature decreases and pressure density increases

4. In the troposphere:

a. over cold air, the pressure is higher at upper levels than at similar levels over
warm air
b. over cold air, the pressure is lower at upper levels than at similar levels over
warm air
c. over warm air, the pressure is lower at upper levels than at similar levels over
warm air
d. the upper level pressure depends solely on the relative humidity below

5. Density at the surface will be low when:

a. pressure is high and temperature is high

b. pressure is high and temperature is low
c. pressure is low and temperature is low
d. pressure is low and temperature is high

6. Which of the following combinations will give the lowest air density?

a. Low pressure, low humidity, low temperature

b. High pressure, high temperature, high humidity
c. High pressure, low temperature, low humidity
d. Low pressure, high humidity, high temperature

35
 3 Answers

Answers
1 2 3 4 5 6
b b b b d d
3
Answers

36
 Chapter

4
Pressure Systems

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Buys Ballot’s Law . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Advection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Depressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Troughs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Depression Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Anticyclones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Ridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Anticyclonic Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Cols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Col Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Pressure Systems Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Annex A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Annex B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Annex C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54

37
 4 Pressure Systems
4
Pressure Systems

38
 Pressure Systems
4
Introduction
Isobars can form patterns, which when they are recognized, can help us forecast the weather.
These patterns are called pressure distribution systems. They include:

• Depressions, or lows.

4
• Anticyclones, or highs.
• Troughs.

Pressure Systems
• Ridges.
• Cols.

Buys Ballot’s Law

In the 19th century the Dutch scientist and meteorologist, Buys Ballot, produced a law based
on the observation of wind direction and pressure systems.

Buys Ballot’s Law states that:

If an observer stands with his back to the wind in the Northern Hemisphere then the low
pressure is on his left. (In the Southern Hemisphere low pressure is to the right.)

This law will prove to be a useful tool in both the study of wind and altimetry.

Advection
Advection is a meteorological term for horizontal movement of air.

Depressions
A depression is a region of comparatively low pressure shown by more or less circular and
concentric isobars surrounding the centre, where pressure is lowest. A depression is sometimes
called a low or a cyclone. In Europe the term cyclone is usually reserved for tropical revolving
storms. However, the term cyclonic circulation implies a low pressure system.

Buys Ballots’ law tells us that the wind will move around a low pressure system in an anti-
clockwise direction in the Northern Hemisphere.

39
 4 Pressure Systems
4
Pressure Systems

Figure 4.1 A Depression in the Northern Hemisphere

There are two types of depression, frontal (large scale) found in our temperate latitudes and
non-frontal (small scale) depressions which can occur virtually anywhere.

In a depression air is converging at the surface, rising from the surface to medium to high
altitude (convection) then diverging at medium to high altitude.

Figure 4.2 Vertical Cross-section

40
 Pressure Systems
4
Frontal depressions are known as Polar Front Depressions and form, in temperate latitudes,
in both the Northern and Southern Hemispheres when warm, moist sub-tropical air masses
meet cold polar air masses. These depressions move from west to east and eventually, in the
Northern Hemisphere, lose their identity over the North American or Eurasian land masses. In
the N. Atlantic these depressions originate in the central to western Atlantic and move rapidly
eastward, eventually losing their identity over the steppes of central Asia. An example of a

4
polar front depression is centred over Greenland/Iceland on the analysis chart on the next
page.

Pressure Systems
Non-frontal depressions are usually formed by surface heating when they are known as thermal
depressions. They occur over land in summer as a result of strong surface heating. They also
occur over the warm sub-tropical oceans where they are known as tropical cyclones. In winter
they occur over sea areas in cold polar or arctic air masses. The different types of depressions
and their formation will be discussed in later chapters. On the analysis chart thermal depressions
(labelled TD) are seen over the Black Sea and the Mediterranean Sea, formed in the cold air
coming out of central Asia

Figure 4.3

Troughs
A trough is an extension of a low pressure system. On the analysis chart there are two troughs
associated with the polar front depression centred over Greenland/Iceland. One extending
north across Greenland is a non-frontal trough. The other extending southward is a frontal
trough formed along the cold front. Troughs are very often associated with the fronts of polar
front depressions. The weather associated with a trough will be similar to that of a depression.

41
 4 Pressure Systems

Depression Weather
Cloud extensive and may extend from low altitude to the tropopause

Precipitation may be continuous/intermittent precipitation or showers and intensity can

range from light to heavy dependent on the type of depression
4

Visibility Poor in precipitation, otherwise good due to ascending air.

Pressure Systems

Temperature dependent on type of depression and time of year. For example, a frontal
depression coming into Europe from the Atlantic in winter will bring warmer air,
but in summer will bring cooler air.

Winds Winds are usually strong - the deeper the depression and the closer the isobars,
the stronger the wind.

Anticyclones
An anticyclone or high is a region of relatively high pressure shown by more or less circular
isobars similar to a depression but with higher pressure at the centre.

Figure 4.4 An Anticyclone in the Northern Hemisphere

Isobars are more widely spaced than with depressions. There are five types of anticyclone,
warm, cold, temporary cold, ridges (or wedges) and blocking. Within an anticyclone, at high
altitude we have air converging, then descent of air within the anticyclone (subsidence) and
divergence at the surface.

42
 Pressure Systems
4

4
Pressure Systems
Figure 4.5 Vertical Cross-section

Cold anticyclones occur as permanent features at the poles and as seasonal features over
continental land masses in the winter. In simple terms the air at the surface is cooled thereby
increasing its density and drawing more air down from above hence increasing the surface
pressure.

Warm Anticyclones
To understand the formation
of warm anticyclones we
need to look at the global
circulation of air. In the 19th
century a British scientist,
George Hadley, proposed a
global circulation based on
hot air rising at the Equator
then flowing up to the
poles at the tropopause,
descending at the poles and
flowing back to the Equator
at the surface. This model
was not quite correct because
in our temperate latitudes
pressure is predominantly
low because of the large Figure 4.6 Hadley Cell, Polar Front, and Associated Wind-Flows.
scale frontal depressions. An
American scientist, William Ferrel, proposed a modification to Hadley’s model introducing the

43
 4 Pressure Systems

modification arising because of the low pressure systems in temperate latitudes. This gives
the three circulation cells, the Hadley cell between the Equator and the subtropics, the Ferrel
cell between the subtropics and temperate latitudes and the Polar cell between temperate
latitudes and the poles.

This circulation means that we have, at the tropopause, air flowing outwards from the Equator
towards the poles and from temperate latitudes towards the Equator. This creates an excess
4

of air at the tropopause in subtropical regions which is forced to descend, hence creating the
Pressure Systems

subtropical high pressure systems which are permanent features over the subtropical oceans,
for example the Azores high in the N. Atlantic.

Ridges
Ridges of high pressure are indicated by isobars
extending outwards from an anticyclone and
always rounded, never V-shaped as seen in a
trough.

Figure 4.7 A Ridge of High Pressure.

Temporary Cold Anticyclones

A temporary cold anticyclone is a ridge of high
pressure found in the cold air between two
frontal depressions. Because the depressions
are moving rapidly the influence of these
anticyclones will be experienced for up to a
maximum of about 24 hours.

Figure 4.8 A Temporary Cold Anticyclone.

44
 Pressure Systems
4
Blocking Anticyclones
A blocking anticyclone is one which prevents the usual eastward movement of frontal
depressions, forcing these depressions to take up northerly tracks in the Northern Hemisphere.
These anticyclones are usually extensions of the warm subtropical anticyclones. They can
persist for weeks giving (usually) warm dry weather in summer and gloomy overcast conditions
in winter with a possibility of drizzle. Over Europe in winter they may be extensions of the

4
Siberian high giving (usually) cold clear conditions.

Pressure Systems
Figure 4.9 High from Azores to Scandinavia.

Anticyclonic Weather
SUMMER (and cold anticyclones in winter):

Cloud None except on the edge of the anticyclone.

Precipitation None.
Visibility Generally moderate with haze
Temperature Dependent on type.
Winds Light.

WINTER (warm anticyclones):

Cloud Extensive stratus with a low base and limited vertical extent.
Precipitation Possibly drizzle.
Visibility Generally moderate to poor with mist and fog likely
Temperature Relatively warm.
Winds Light.

45
 4 Pressure Systems

Cols
Cols are regions of almost level pressure
between two highs and two lows. It is an area
of stagnation as illustrated in Figure 4.10 and
Figure 4.11.
4

Col Weather
Pressure Systems

Col weather is normally settled, but is

dependent on changing pressure.

In autumn and winter cols produce

poor visibility and fog, whilst in summer
thunderstorms are common. Figure 4.11 is an
example of a weather forecast for a day when
a col influenced the weather over the UK.

Figure 4.10 A Col.

Figure 4.11 Col Weather.

46
 Pressure Systems
4
Pressure Systems Movement
Frontal depressions tend to move rapidly. The movement of non-frontal depressions depends on
type and location; they may remain relatively static or move at moderate speeds. Anticyclones
tend to be slow moving and will persist in more or less the same location for long periods. Cols
tend to be static.

4
Movement of the systems is the key to accurate forecasting.

Pressure Systems
The figures below show the movement of weather over a period of four successive days.

Figure 4.12 Maintenance of Shape.

Figure 5.11. Maintenance of Shape.

47
 4 Pressure Systems

Terminology
Depressions will fill up or decay as pressure rises.

Depressions will deepen as pressure falls.

4

Frontal depressions move rapidly, their average lifetime is 10 to 14 days.

Pressure Systems

Anticyclones will build up as pressure rises.

Anticyclones will weaken or collapse as pressure falls.

Anticyclones are generally slow moving and may persist for long periods.

Cols may last up to a few days before being replaced by other pressure systems.

48
 Pressure Systems
4
Questions
1. A steep pressure gradient is characterized by:

a. isobars close together, strengthened wind

b. isobars far apart, decreased wind
c. isobars close together, temperature increasing

4
d. isobars far apart, temperature decreasing

Pressure Systems
2. QNH at Timbuktu (200 m AMSL) is 1015 hPa. What is the QFE? (Assume 1 hPa = 8 m)

a. 1000 hPa
b. 990 hPa
c. 1020 hPa
d. 995 hPa

3. In temperate latitudes in summer what conditions would you expect in the centre
of a high pressure system?

a. Thunderstorms
b. Calm winds, haze
c. Showers
d. Dense cloud

4. If the pressure level surface bulges upwards, the pressure system is a:

a. cold low
b. warm low
c. cold high
d. warm high

5. The QNH at an airfield 200 m AMSL is 1009 hPa; air temperature is 10°C lower than
standard. What is the QFF?

a. Not possible to give a definite answer

b. Less than 1009 hPa
c. 1009 hPa
d. More than 1009 hPa

6. QNH is defined as:

a. the pressure at MSL obtained using the standard atmosphere

b. the pressure at MSL obtained using the actual conditions
c. QFE reduced to MSL using the actual conditions
d. QFE reduced to MSL using the standard atmosphere

7. Landing at an airfield with QNH set the pressure altimeter reads:

a. zero feet on landing only if ISA conditions prevail

b. zero
c. the elevation of the airfield if ISA conditions prevail
d. the elevation of the airfield

49
 4 Pressure Systems

8. Airfield is 69 metres below sea level, QFF is 1030 hPa, temperature is ISA -10°C.
What is the QNH?

a. Impossible to tell
b. Less than 1030 hPa
c. 1030 hPa
d. More than 1030 hPa
4

9. What is the vertical movement of air relating to a trough?

Pressure Systems

a. Descending and diverging

b. Ascending and diverging
c. Descending and converging
d. Converging and ascending

10. What is the vertical movement of air relating to a ridge?

a. Descending and diverging

b. Ascending and diverging
c. Descending and converging
d. Ascending and converging

11. What is subsidence?

a. Horizontal motion of air

b. Vertical down draught of air
c. Vertical up draught of air
d. Adiabatic cooling

12. Aerodrome at MSL, QNH is 1022 hPa. QFF is:

a. greater than 1022 hPa

b. less than 1022 hPa
c. same as QNH
d. cannot tell without temperature information

13. Air at the upper levels of the atmosphere is diverging. What would you expect at
the surface?

a. Rise in pressure with clouds dissipating

b. Rise in pressure with clouds forming
c. Fall in pressure with cloud dissipating
d. Fall in pressure with cloud forming

14. Subsidence would be described as:

a. vertical ascension of air

b. horizontal movement of air
c. the same as convection
d. vertical down flow of air

50
 Questions
4
15. You are flying at FL170. The pressure level which is closest to you is the:

a. 300 hPa
b. 700 hPa
c. 500 hPa
d. 850 hPa

4
16. On a surface weather chart, isobars are lines of:

Questions
a. QNH
b. QFE
c. QFF
d. QNE

17. At FL60 what pressure chart would you use?

a. 700 hPa
b. 500 hPa
c. 800 hPa
d. 1000 hPa

18. (For this question use Annex B) A ridge is indicated by letter:

a. D
b. A
c. B
d. C

19. (For this question use Annex A) Which of the following best describes Zone D?

a. Ridge of high pressure

b. Anticyclone
c. Trough of low pressure
d. Col

20. (For this question use Annex A) Which of the following best describes Zone C?

a. Trough of low pressure

b. Depression
c. Ridge of high pressure
d. Anticyclone

21. (For this question use Annex B) Which of the following best describes Zone A?

a. Col
b. Ridge of High Pressure
c. Depression
d. Trough of low pressure

51
 4 Questions

22. (For this question use Annex B) Which of the following best describes Zone B?

a. Ridge of high pressure

b. Depression
c. Anticyclone
d. Col
4

23. (For this question Annex C) The pressure system at position A is a:

Questions

a. trough of low pressure

b. anticyclone
c. col
d. secondary low

24. (For this question use Annex C) The pressure system located in area “B” is a

a. Ridge of high pressure

b. col
c. trough of low pressure
d. depression

25. At which average height can the 500 hPa pressure level be expected in moderate
latitudes?

a. 12.2 km
b. 3 km
c. 5.5 km
d. 9.0 km

26. The average pressure found at a height of 1620 m in mid latitudes would be:

a. 350 hPa
b. 400 hPa
c. 850 hPa
d. 950 hPa

52
 Questions
4
Annex A

4
Questions
Annex B

Annex C

53
 4 Answers

Answers
1 2 3 4 5 6 7 8 9 10 11 12
a b b d d d d d d a b c
13 14 15 16 17 18 19 20 21 22 23 24
4

d d c c c b d d b b a b
Answers

25 26
c c

54
 Chapter

5
Temperature

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Heating of the Troposphere . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
Temperature Variation with Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Lapse Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Isotherm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Inversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Surface Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

55
 5 Temperature
5
Temperature

56
 Temperature
5
Introduction
One of the important variables in the atmosphere is temperature. The study of temperature
variation, both horizontally and vertically has considerable significance in the study of
meteorology.

Measurement

5
There are three scales which may be used to measure temperature though only Celsius and

Temperature
Kelvin are used in meteorology. The figures show the melting point of ice and the boiling point
of water (at standard pressure) in each scale.

• The FAHRENHEIT scale: +32 and +212 degrees.

• The CELSIUS (or Centigrade) scale: 0 and +100 degrees.

• The KELVIN (or Absolute) scale: +273 and +373 Kelvin.

Conversion factors:
5
°C = × (°F - 32)
9

9
°F = × °C + 32
5

K = °C + 273

Instruments
The standard means of measurement on the
ground is a mercury thermometer placed
in a Stevenson Screen. Electrical resistance
thermometers may be used where the screen is
not readily accessible to the observer.

Figure 5.1 The Stevenson Screen

57
 5 Temperature

A Thermograph (similar in its output to a barograph) will also be found inside the screen. The
Stevenson screen is a louvred box 4 feet (1.22 m) above the ground. This screen, shown in
Figure 5.1, is used worldwide.
5
Temperature

Figure 5.2 Thermograph

Upper air temperature (and pressure and humidity) are measured using a Radiosonde, shown
in Figure 5.3, - a device transmitting continuous readings whilst being carried aloft beneath a
balloon. Rate of climb is 1200 fpm and maximum ceiling between 65 000 and 115 000 ft. Earlier
devices were tracked using radar to determine position and to determine wind speed. Modern
systems use GPS to provide a 3D position to send with the data.

Figure 5.3 A Radiosonde

Aircraft readings, though often the only way in which atmospheric temperature may be
measured over the oceans and other areas far away from meteorological stations, are not as
accurate as they are affected by compressibility and lag. The electrical thermometer will give
a digital readout of temperature and this can be automatically calibrated and transmitted on
some modern aircraft.

58
 Temperature
5

5
Temperature
Figure 5.4 Electrical Thermometer

Heating of the Troposphere

The main source of heat for the troposphere is the sun.

• Solar Radiation. Radiation from the sun is of Short wave-length (λ) and passes through the
troposphere almost without heating it at all.

λ = 0.15 - 4 microns (micron = µ = 10 -6 m)

S ome solar radiation is reflected back to the upper air from cloud tops and from water
surfaces on the earth. The rest of this radiation heats the earth’s surface. The process
whereby the surface is heated by solar radiation is called insolation.

Figure 5.5 Solar Radiation

59
 5 Temperature

There are four processes which heat the troposphere:

• Terrestrial Radiation.

The earth radiates heat at all times. It is relatively long wave radiation λ = 4 to 80 microns,
peaking at 10 m.

This radiation is absorbed by the so-called greenhouse gases giving rise to the lapse rate in
5

the troposphere, principally water vapour, carbon dioxide and methane. The increase in the
amount of carbon dioxide in the troposphere is one of the factors contributing to global
Temperature

warming. (Note: the global warming phenomenon is much more complex than this.)

Figure 5.6 Terrestrial Radiation

Figure 7.6. Terrestrial Radiation.
• Conduction. Air lying in contact with the earth’s surface by day will be heated by conduction.
At night air in contact with the earth’s surface will be cooled by conduction. Because of the
air’s poor conductivity, the air at a higher level will remain at the same temperature as
during the day and an inversion will result.

Figure7.7.
Figure 5.7 Conduction
Conduction.

60
 Temperature
5
• Convection. Air heated by conduction will be less dense and will therefore rise. This will
produce up currents called thermals or convection currents. These will take the warm air
to higher levels in the troposphere. This and terrestrial radiation are the two main processes
heating the troposphere.

5
Temperature
Figure7.8.
Figure 5.8 Convection
Convection Currents
Currents.

• Condensation. As the air is lifted it will cool by the adiabatic process and the water vapour
in the air will condense out as visible droplets forming cloud. As this occurs latent heat will
be released by the water vapour and this will add to the heating of the troposphere.

Figure 5.9 Latent Heat being released through Condensation

Figure 7.9. Latent Heat being released through
Condensation.

61
 5 Temperature
5
Temperature

Figure 5.10 Heat Processes in the Atmosphere

Temperature Variation with Height

We have seen that although our source of heat
is the sun, because of the troposphere’s virtual
transparency to insolation, it is in fact heated
(by long wave IR) from the surface upwards.

Thus as we move further and further from the

surface we would expect the heating effects to
diminish.

Lapse Rate
The rate at which temperature falls with an
increase in height is called the Lapse Rate. Figure 5.11 Temperature Variation with Height
An ideal uniform atmosphere would show a
constant lapse rate rather like the ISA, which is
0.65°C/100 m (1.98°C (2°) per 1000 ft.)

Isotherm
If temperature remains constant with height it is called an isothermal layer.

62
 Temperature
5
Inversions
Where the temperature increases with an increase in height, then we have what is called an
inversion. We have already seen that at night we can expect an inversion above the surface,
but this can occur in many different ways.

Radiation, on a night of clear skies, will also result in a temperature inversion above the surface.
This is called a Radiation Inversion.

5
When we look at cloud formation, we shall see that because of turbulence in the layer closest

Temperature
to the surface we can have an inversion at a height of 2 or 3 thousand feet.

Quite often, at the tropopause instead of the temperature remaining constant, it may show a
slight rise for a few thousand feet.

At the higher levels of the stratosphere, temperature will show an increase with height (in ISA
from 20 km to 32 km the temperature increases at 1°C per km).

In a high pressure system, air descends at the centre. As the air descends it will be heated
adiabatically (more of this later) and will be warmer than the air at a lower level. This is called
a Subsidence Inversion.

Figure 5.12 Inversions

63
 5 Temperature

Surface Temperature
The surface air temperature measured in a Stevenson screen is subject to considerable variations:
Latitude Effect, Seasonal Effect, Diurnal Variation and multiple effects due to cloud and wind.

The Angular Elevation of the Sun

• Latitude Effect. At the Equator only a small area is heated by the sun’s radiation and
therefore will be subject to the greatest heat/unit area. At the poles the sun’s rays will cover
5

a larger area and there will be the least heat/unit area.

Temperature

• T
 he actual distance of polar regions from
the sun is only fractionally more than that
from the Equator, and the effect may be
ignored.

• Seasonal Effect. The Vernal (Spring) and

autumnal equinoxes occur about 21 March
and 21 September respectively. Then
the sun is directly over the Equator and
maximum heating will occur there. About
21 June the sun reaches its most northerly
latitude (Summer Solstice for the Northern
Figure 5.13 The Effect of Latitude
Hemisphere) and maximum heating will
occur in the Northern Hemisphere. But the
land (and sea) continues to heat up and
maximum temperatures are found around
late July or early August in temperate
latitudes. Around 21 December the sun
reaches its most southerly latitude (Winter
Solstice for the Northern Hemisphere) and
minimum heating occurs. But the land
(and sea) continues to cool down and
minimum temperatures are experienced
around late January or early February in
temperate latitudes.

Figure 5.14 The Seasonal Effect

Diurnal Variation - (Note: This Assumes Clear Skies and Light Winds and No Change
in Air Mass)
• T
 he sun is at its highest elevation at noon, but for two to three hours after this time, the
earth is receiving more solar radiation than it is giving up as terrestrial radiation. A balance
between incoming and outgoing radiation is reached on average at 1500 local time when
maximum temperatures can be expected.

 ote: the actual time of maximum temperature varies with latitude and time of year, earlier
N
in winter later in summer, but 1500 local time is a good average for temperate latitudes.

64
 Temperature
5
• From 15:00 onwards, the temperature falls continuously until a little after sunrise. The
lowest temperature occurs at about sunrise plus 30 minutes when once again we get a
balance between incoming and outgoing radiation.

• D
 iurnal Variation (DV) is greatest with clear skies and little wind. DV varies with a number
of factors, but in temperate latitudes is about +/- 6 degrees about the mean.

5
Temperature
Figure 5.15 Diurnal Variation

• Cloud cover by day. By day some of the solar radiation is reflected back by the cloud tops
and maximum temperature (T Max) is reduced.

Figure 5.16 Cloud Cover by Day

65
 5 Temperature

• Cloud cover by night. By night terrestrial radiation is absorbed and radiated back to the
earth’s surface from the clouds. T min is increased.
5
Temperature

Figure
Figure 5.17Cloud
7.17. Cloud Cover
Coverby Night
by Night.
• E
 ffect of wind by day. By day wind will cause turbulent mixing of the warm air at the
surface with cold air above, reducing T max. Wind will also reduce the time the air is in
contact with the warm ground.

Figure
Figure 5.18The
7.18. The Effect
Effectofof
Wind by Day
Wind by Day.

66
 Temperature
5
• E
 ffect of wind by night. By night there will normally be an inversion above the surface and
wind will cause cold air to be turbulently mixed with warm air above thus increasing T min.

5
Temperature
FigureFigure
7.19.5.19 The Effect of Wind by Night
The Effect of Wind by Night.
In summary, wind or cloud cover will cause T max to be reduced and T min to be increased.
Therefore DV will be reduced.

• D
 V over sea. As the Specific Heat (SH) of water is unity, compared to other substances
whose SH is much less, and as the temperature rise is inversely proportional to the Specific
Heat, the diurnal temperature variation over the sea is small, generally less than 1°C.

Nature of the Surface

• Sea. The sea takes a long time to heat (and cool) and as we have seen has a very small DV.

Figure 5.20 Diurnal Variation Over the Sea

67
 5 Temperature

The difference in DV values between land and sea is the cause of sea breezes. The minimal
DV of sea temperature is the reason why the most common form of fog, radiation fog,
never forms over the sea.

When the angular elevation of the sun is low, much solar radiation is reflected back to the
atmosphere.

• Land. Bare rock, sand, dry soil, tarred roads and concrete runways attain a higher temperature
5

by insolation than woods, lakes, grasslands and wet soil.

Temperature

The temperature difference between air above concrete runways and adjacent grass can
be as much as 4 degrees. Higher temperature surfaces provide strong up currents called
thermals or convection currents.

Figure 5.21 July Average Temperatures

68
 Temperature
5
In Figure 5.21 we may note that the sea temperature remains “cool” in July in the Northern
Hemisphere but the desert land areas of Africa and neighbouring Asia get very warm. Air
over snow covered surfaces is very cold. Some 80% of solar radiation is reflected from
snow surfaces.

Snow does not prevent the earth from radiating its heat. Hence surface air temperatures
over snow will become colder day by day. Temperatures in Siberia can reach -72°C after a long
cold winter. This very cold air results in high density and the development of anticyclones.

5
Temperature
Location
• Over Land. Air in a valley will tend to be more static than air in an exposed position. Therefore
by night the air is in contact with the ground for a longer time and the air temperature is
lower than on a hill. Additionally, in a valley, cold air tends to sink from the hills above at
night, again causing lower temperatures. It is for these reasons that mist and fog tend to
form firstly in valleys.

Figure
Figure 5.22 Location
7.23. LocationEffect
Effect.

• Over Oceans. The fact that seas tend to have a very small DV of temperature has been
stated above. On a wide scale this means that in winter the sea is warmer than the land and
thus there is a widespread movement of air from land to sea (monsoon effect). There is an
opposite tendency in summer.

Figure 5.23 Relative Airflow in Winter

Figure 7.24 Monsoon Effect in Winter

69
 5 Temperature

Origin of Air Supply

Air tends to retain its temperature and humidity for a considerable time, therefore air from
high latitudes will bring lower temperatures to UK. A southerly wind, however, will normally
provide an increase in temperature.
5
Temperature

Figure 5.24 Origin of Air Supply

70
 Questions
5
Questions
1. The measurement of surface temperature is made:

a. at ground level
b. at approximately 10 metres from ground level
c. at approximately 4 feet above ground level
d. at approximately 4 metres above ground level

5
Questions
2. The purpose of a “Stevenson screen” is to:

a. maintain a moist atmosphere so that the wet bulb thermometer can function
correctly
b. prevent the mercury freezing in the low winter temperatures
c. protect the thermometer from wind, weather and from direct sunshine
d. keep the wet and dry bulb thermometers away from surface extremes of
temperature

3. If temperature remains constant with an increase in altitude there is:

a. an inversion
b. an inversion aloft
c. uniform lapse rate
d. an isothermal layer

4. The surface of the earth is heated by:

a. convection
b. conduction
c. long wave solar radiation
d. short wave solar radiation

5. Cloud cover will reduce diurnal variation of temperature because:

a. incoming solar radiation is reflected back to space and outgoing terrestrial

radiation is reflected back to earth
b. incoming solar radiation is re-radiated back to space and atmospheric heating
by convection will stop at the level of the cloud layer
c. the cloud stops the sun’s rays getting through to the earth and also reduces
outgoing conduction
d. incoming solar radiation is reflected back to space and outgoing terrestrial
radiation is re-radiated from the cloud layer back to the surface

6. Diurnal variation of the surface temperature will:

a. be unaffected by a change of wind speed

b. decrease as wind speed increases
c. increase as wind speed increases
d. be at a minimum in calm conditions

71
 5 Questions

7. Which of the following surfaces is likely to produce a higher than average diurnal
variation of temperature:

a. rock or concrete
b. water
c. snow
d. vegetation

8. Most accurate temperatures above ground level are obtained by:

5
Questions

a. tephigram
b. aircraft reports
c. temperature probe
d. radiosonde

9. The method by which energy is transferred from one body to another by contact is
called:

a. radiation
b. convection
c. conduction
d. latent heat

10. The diurnal variation of temperature is:

a. greater over the sea than overland

b. less over desert areas then over temperate grassland
c. reduced anywhere by the presence of cloud
d. increased anywhere as wind speed increases

11. The troposphere is heated largely by:

a. absorption of the sun’s short wave radiation

b. radiation of heat from cloud tops and the earth’s surface
c. absorption by ozone of the sun’s short wave radiation
d. conduction from the surface, convection and the release of latent heat

12. An inversion is one in which:

a. there is no horizontal gradient of temperature

b. there is no change of temperature with height
c. there is an increase of temperature as height increases
d. there is a decrease of temperature as height increases

13. The sun gives out________ amount of energy with _________ wavelengths.
The earth gives out relatively___________ amounts of energy with
relatively___________ wavelengths:

a. large, large, small, small

b. small, small, large, large
c. large, large, small, large
d. large, small, small, large

72
 Questions
5
14. With a clear night sky, the temperature change with height by early morning is
most likely to show:

a. a steady lapse rate averaging 2°C per 1000 ft

b. a stable lapse rate of 1°C per 1000 ft
c. an inversion above the surface with an isothermal layer above
d. an inversion from near the surface and a 2°C per 1000 ft lapse rate above

15. Over continents and oceans, the relative temperature conditions are:

5
Questions
a. warmer in winter over land, colder in summer over sea
b. colder in winter over land, warmer in winter over sea
c. cold in winter over land and sea
d. warmer in summer over land and sea

73
 5 Answers

Answers
1 2 3 4 5 6 7 8 9 10 11 12
c c d d d b a d c c d c
13 14 15
d d b
5
Answers

74
 Chapter

6
Humidity

Definition of Latent Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Saturation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Condensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Freezing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Melting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Sublimation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Humidity Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Bergeron Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Measurement of Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Dry-bulb and Wet-bulb Hygrometer or Psychrometer . . . . . . . . . . . . . . . . . . . . . . . 81
Dew Point Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Diurnal Variation of Humidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

75
 6 Humidity
6
Humidity

76
 Humidity
6
Definition of Latent Heat
The latent heat of a substance is the heat absorbed or released without change of temperature
when the substance changes state. Latent heat differs according to the state of the substance.
When ice changes to water or water vapour, or water changes to water vapour, latent heat is
absorbed.

When water vapour changes to water or ice, or water changes to ice, latent heat is released.

Evaporation

6
Humidity
Evaporation is the change of state from liquid to vapour. Latent heat is absorbed.

Evaporation can occur at any temperature. For a particular temperature there is a particular
amount of water per unit volume that the air can hold. When this maximum is reached,
evaporation will cease.

Figure 6.1 The Change of State from Solid to Liquid to Gas and Back Again.

Saturation
Air becomes saturated by adding more water vapour to it. Alternatively, as warm air can hold
more water vapour than cold, saturation can be achieved by cooling the air.

Air is saturated if it contains the maximum amount of water vapour that it can hold at that
temperature. If saturated air is cooled, condensation will occur.

Condensation
Condensation is the change of state from vapour to liquid. Latent heat is released.
Condensation causes cloud and fog to form. Condensation will require minute impurities or
particles called hygroscopic or condensation nuclei; these are usually present in abundance in
the troposphere.

77
 6 Humidity

Freezing
If the water droplet is cooled below zero, then it may change state again to ice. The process
is called freezing. Freezing requires the presence of freezing nuclei; these are less common
in the troposphere than condensation nuclei, so it is possible to have water droplets in the
atmosphere with temperatures below 0°C. These are known as supercooled water droplets
and give us the icing hazard discussed in Chapter 16.

Melting
6

The opposite change of state, from solid to liquid, is called melting. (There is no superfrozen
Humidity

state).

Sublimation
Sublimation is the change of state directly from water vapour to ice without water droplets
being formed. Latent heat is released. This process is also known as deposition.

The change of state from ice directly to water vapour is also called sublimation.

Humidity Measurement
• Absolute Humidity is the weight of water vapour in unit volume of air. Absolute Humidity
is usually expressed in g/m3 .

• H
 umidity Mixing Ratio (HMR) is the weight of water vapour contained in unit mass of dry
air. The Humidity Mixing Ratio is usually expressed in g/kg. In unsaturated air, HMR remains
constant during ascent while temperature and pressure decreases.

• S
 aturation Mixing Ratio (SMR) is the maximum amount of water vapour a unit mass of dry
air can hold at a specified temperature.

• Relative Humidity (RH).

HMR
The ratio × 100%
SMR
or more simply, the amount of water vapour present in a volume of air divided by the
maximum amount of water vapour which that volume could hold at that temperature
expressed as a percentage.

RH 100% = SATURATION

78
 Humidity
6

6
Humidity
Figure 6.2 The Amount of Water Vapour the Air can Hold when Saturated at Different Temperatures

Bergeron Theory
This is more accurately the (Wegener)-Bergeron-Findeissen theory, named after the 3 scientists
who discovered the relationship. Figure 6.3, next page, shows the partial pressure of water
vapour at saturation for temperatures from -30°C to +40°C. As we already know, the maximum
amount of water vapour the air can hold and hence the partial water vapour pressure at
saturation decreases as temperature decreases.

The small sub-diagram shows that at temperatures below 0°C the partial pressure at saturation
for the formation of water is greater than the partial pressure for the formation of ice. This
means that the air becomes saturated for the formation of ice before it becomes saturated
for the formation of water. In other words at temperatures below zero the water vapour will
go directly to the solid state without first going through the liquid state (the converse also
applies). This may be stated as: “the saturation vapour pressure over water is greater than
over ice”.

79
 6 Humidity
6
Humidity

Figure 6.3

The table shows the same effect in terms of relative humidity for water and ice, for example, at
-10°C when the air is saturated for the formation of ice the relative humidity for water is 91%.
The effect of this is that when supercooled water droplets exist (at temperatures below 0°C),
the water droplets will evaporate saturating the air (for the formation of ice) and the water
vapour will now sublime out as ice. This effect is important in the formation of precipitation in
clouds when the temperature is below 0°C and in the formation of fog.

RELATIVE HUMIDITY AT SATURATION FOR ICE

Temperature RH for water RH for ice
0°C 100% 100%
-05°C 95% 100%
-10°C 91% 100%
-15°C 87% 100%
-20°C 83% 100%

80
 Humidity
6
Measurement of Humidity
Atmospheric humidity is measured using a dry bulb and wet bulb hygrometer or psychrometer
or an electrical hygrometer. The dry bulb and wet bulb hygrometer or psychrometer comprises
two thermometers. The dry bulb thermometer gives the ambient temperature. The wet bulb
thermometer has, around its bulb, a muslin cloth the other end of which is in a reservoir of
distilled water. The water rises up the muslin and evaporates drawing heat from the bulb and
hence reducing its temperature. So the wet bulb thermometer gives the lowest temperature to
which the air can be cooled by the evaporation of water. The rate at which the water evaporates
depends on the relative humidity. With high relative humidity the rate of evaporation will

6
be slow so the wet bulb temperature will be relatively high. Conversely if the air is dry the

Humidity
evaporation will be rapid and the wet bulb temperature will be much lower than the dry bulb
temperature.

Dry-bulb and Wet-bulb Hygrometer or Psychrometer

• If air is dry, water will evaporate from the muslin covering the wet bulb and latent heat will
lower the temperature.

• If air is saturated, no evaporation will occur and thermometers will read the same.

• D
 ew point, relative humidity and HMR are read from tables or slide rule by entering with
the two temperatures obtained.

Figure 6.4 Dry-Bulb and Wet-Bulb Hygrometer or Psychrometer

Dew Point Temperature

Dew point (DP) is the temperature to which air must be cooled at constant pressure for
saturation to occur.
 ote that the dew point temperature is not the same as the wet bulb temperature
N
(except at saturation).
The dew point has a lapse rate of 0.5°C/1000 ft
Wet bulb = dry bulb (= dew point) – 100% RH (saturation)

81
 6 Humidity

Diurnal Variation of Humidity

By day, as the temperature increases, RH will decrease because the maximum amount of water
vapour air can hold increases as the temperature rises.

After 1500 hrs, the temperature will start to fall and the maximum amount of water vapour the
air can hold will fall and thus the RH will increase. The higher RH at night is the reason for the
formation of mist and fog after dark in autumn and winter.
6
Humidity

Figure 6.5 Diurnal Variation of Humidity

RH is maximum approximately 30 minutes after sunrise when the temperature is minimum.

Figure 6.6 shows a graph of relative humidity at RAF Waddington over a number of years. The
maximum and minimum times and the sinusoidal curve confirm Figure 6.5.

Figure 6.6

By definition:
Saturated Air: RH=100%
Dry Air: RH<100%
E.g. RH=99.9% - Dry Air

82
 Questions
6
Questions
1. Throughout the 24 hours of a day the Relative Humidity can be expected to:

a. increase during the day and decrease at night

b. stay reasonably constant throughout the 24 hours
c. reduce during the day and increase at night
d. only change with a change of air mass

2. During a night with a clear sky, surface temperature will ____________ RH

6
will______________ and dew point will___________.

Questions
a. fall, rise, rise
b. rise, rise, fall
c. fall, rise, remain the same
d. fall, fall, remain the same

3. A change of state directly from a solid to a vapour or vice versa is:

a. insolation
b. condensation
c. evaporation
d. sublimation

4. The instrument used for measuring the humidity of air is a:

a. hydrometer
b. hygrometer
c. wet bulb thermometer
d. hygroscope

5. The process of change of state from a gas to a liquid is:

a. evaporation in which latent heat is absorbed

b. evaporation in which latent heat is released
c. condensation in which latent heat is absorbed
d. condensation in which latent heat is released

6. The process of change of state from a liquid to a gas is:

a. condensation in which latent heat is released

b. evaporation in which latent heat is released
c. condensation in which latent heat is absorbed
d. evaporation in which latent heat is absorbed

7. Air is classified as dry or saturated according to its relative humidity. If the relative
humidity were 95% the air would be classified as:

a. conditionally saturated
b. partially saturated
c. saturated
d. dry

83
 6 Questions

8. On a wet bulb thermometer in an unsaturated atmosphere there will be a

reduction of temperature below that of the dry bulb thermometer because:

a. heat is absorbed during the process of condensation

b. heat is released during the process of condensation
c. heat is absorbed by the thermometer during the process of evaporation
d. heat is released from the thermometer during the process of evaporation

9. Relative humidity is:

a. air temperature over wet bulb temperature × 100

6

b. air temperature over dew point temperature × 100

Questions

c. the actual amount of water vapour in a sample of air over the maximum
amount of water vapour that the sample can contain × 100
d. the maximum amount of water vapour that a sample of air can contain over
the actual amount of water vapour the sample does contain × 100

10. Absolute humidity is:

a. the number of water droplets in a given quantity of air

b. the amount of water vapour that a given quantity of air holds
c. the maximum amount of water vapour that a given quantity of air can hold
d. the maximum number of water droplets that a given quantity of air can hold

11. Wet bulb temperature would normally be lower than the dry bulb temperature
because:

a. condensation causes a release of latent heat

b. evaporation causes cooling
c. latent heat is absorbed by the bulb thermometer
d. of condensation on the muslin wick of the bulb

12. The wet bulb temperature:

a. is measured using a hydrometer

b. is the minimum temperature to which a thermometer bulb can be cooled by
the evaporation of water
c. measures the dew point of the air
d. is the minimum temperature reached by the surface of the earth as measured
by a thermometer placed 1.2 metres above the ground

13. Which one of the following statements relating to atmospheric humidity is correct?

a. If the air temperature falls then the absolute humidity must increase
b. The absolute humidity is the mass of water vapour contained in unit volume of
air
c. The diurnal variation of dew point temperature is greatest when skies are
clear at night
d. The dew point temperature is the temperature indicated by the wet bulb
thermometer

84
 Questions
6
14. When condensation takes place, the higher the temperature, the __________the
amount of latent heat___________:

a. lesser; released
b. greater; absorbed
c. greater; released
d. lesser; absorbed

15. When water vapour changes to ice:

a. latent heat is absorbed

6
b. specific heat is released

Questions
c. latent heat is released
d. specific heat is absorbed

85
 6 Answers

Answers
1 2 3 4 5 6 7 8 9 10 11 12
c c d b d d d d c b b b

13 14 15
b c c
6
Answers

86
 Chapter

7
Adiabatics and Stability

Adiabatic Temperature Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

The Dry Adiabatic Lapse Rate - DALR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
The Saturated Adiabatic Lapse Rate - SALR . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
Variation of the SALR with Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
The Environmental Lapse Rate (ELR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
Absolute Instability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
Absolute Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
Conditional Instability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93
Neutral Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Stability Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
Answers to Questions on Page 96 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

87
 7 Adiabatics and Stability
7
Adiabatics and Stability

88
 Adiabatics and Stability
7
Adiabatic Temperature Changes
An adiabatic temperature change occurs
when a gas is compressed or expanded with
no external exchange of heat.

We can experience this in everyday life. When

we use a manual pump to inflate a bicycle
tyre we observe that the tyre valve gets hot.
The reason for this is that the compression of
the air in the pump raises its temperature and

7
this heat is transferred to the valve as the air
passes through.

Adiabatics and Stability

The opposite effect is observed when a carbon
dioxide (CO2) fire extinguisher is discharged. Figure 7.1
The CO2 is under very high pressure in the
cylinder, when the release handle is operated the gas expands rapidly as it exits the cylinder
cooling as it does so. (In fact the expansion is so great that the fall in temperature is such that
we risk frost burns if we hold the horn.)

In each case the temperature has changed because of the expansion or compression of the gas;
no heat has been added from or removed to external sources.

In the atmosphere pressure decreases as altitude increases so if a parcel of air is forced to rise
it will expand as it rises and hence will cool by the adiabatic process. Similarly if a parcel of
air is forced to descend it will become compressed and hence heat up, again by the adiabatic
process.

The Dry Adiabatic Lapse Rate - DALR

The Dry Adiabatic Lapse Rate (DALR) is the lapse rate for rising dry (i.e. unsaturated) air. It has
a constant value of 1°C/100 m (about 3°C/1000 ft) as illustrated in Figure 7.2.

The Saturated Adiabatic Lapse Rate - SALR

Saturated air, when forced to rise will also cool, but as it cools condensation will take place,
releasing latent heat which slows the rate at which the air cools. The Saturated Adiabatic Lapse
Rate (SALR) is the lapse rate for rising air which is saturated (RH 100%) and has an average value
in temperate latitudes near the ground of 0.6°C/100 m (1.8°C/1000 ft), as seen in Figure 7.3.

Figure 7.2 Figure 7.3

89
 7 Adiabatics and Stability

Variation of the SALR with Temperature

The amount of water vapour the air can hold is directly proportional to temperature. At high
temperatures the air can hold large amounts of water vapour so that when it cools a much
greater amount condenses releasing a lot of latent heat thus slowing the cooling process even
more. Conversely, at low temperature the air holds a relatively small amount of water vapour,
so little latent heat is released to slow the rate of cooling.

Hence the SALR increases as latitude and/or altitude increase, tending towards DALR at high
altitude and high latitude.

The difference between DALR and SALR is shown in Figure 7.4.

7
Adiabatics and Stability

A comparison between SALRs at different latitudes is shown below.

Zone DALR TEMP SALR

°C / 100 m °C / 100 m
Polar Low Level; High Alt All 1 Cold >0.6
Latitudes

Mid Latitudes Low Level 1 Med 0.6

Equatorial Latitudes Low Level 1 Warm <0.6

Figure 7.4 SALR Differences

The Environmental Lapse Rate (ELR)

The ELR is the actual temperature profile of the troposphere
as measured by radiosonde ascents. It varies with time and
position.

Figure 7.5 Variable ELR

Stability
Stability can be defined as being resistance to change. When dealing with atmospheric stability
we are looking at what happens to air in vertical motion. If a parcel of air is forced to rise, for
example over a mountain, when it gets to the top of the mountain there are 3 things it can do.
It may return to its original height, it may continue rising or it may remain at the height of the
summit. In the first case, in terms of the vertical position, the air is where it started so before
and after are the same so we have a stable situation. In the second case we have continual
change and hence instability. The third situation is a neutral or indifferent case, since the
parcel of air is remaining where it was moved.

Atmospheric stability is determined by comparing the ELR with the DALR and the SALR.

90
 Adiabatics and Stability
7
Absolute Instability
Let us imagine a hill, 300 m high. A radiosonde ascent gives the ELR over the first few hundred
metres as 1.2°C/100 m so the environmental temperature at a height of 300 m is +16.4°C (see
diagram).

7
Adiabatics and Stability
Figure 7.6

The wind blows a parcel of unsaturated air up

the hill and that air cools adiabatically at rate of
1.0°C/100 m and at 300 m has cooled to 17°C.
This air is now warmer than the environment
and hence less dense so will continue to rise.
This is an unstable situation.
Now the wind blows a parcel of saturated air
up the hill which cools at 0.6°C/100 m, cooling
to a temperature of 18.2°C at 300 m. This air
is also warmer than the environment and will
also continue to rise and is hence unstable.

In this scenario when the ELR is greater than

Figure 7.7
the DALR, the air is unstable for both dry and
saturated air. We call this situation absolute
instability.

ELR > DALR: ABSOLUTE INSTABILITY

91
 7 Adiabatics and Stability

Absolute Stability
Let us now take the same situation except that the radiosonde ascent shows a lapse rate of
0.4°C/100 m, giving an environmental temperature at 300 m of 18.8°C.
7
Adiabatics and Stability

Figure 7.8

Once again the parcel of dry air is blown up the

hill cooling adiabatically to 17°C. This parcel
of air is now cooler and therefore denser than
the environment and will now descend on the
opposite side of the hill to its starting position.
Now we have a stable situation.

The saturated air as it is blown up the hill will

cool to 18.2°C and it too will be colder than
the environment and will roll down the other
side of the hill.

This time we have stable conditions for both

dry and saturated air which we term absolute Figure 7.9
stability.

ELR < SALR: ABSOLUTE STABILITY

92
 Adiabatics and Stability
7
Conditional Instability
Now we will look at what happens when the radiosonde ascent shows an average lapse rate of
0.8°C/100 m over the first few hundred metres giving an environmental temperature of 17.6°C
at a height of 300 m.

7
Adiabatics and Stability
Figure 7.10

The parcel of dry air is blown up the hill and

cools as before to 17°C. This air is now colder
than the environment and will descend on
the other side of the hill, the stable condition.

The saturated air will cool to 18.2°C as it is

blown up the hill. Now the saturated air
is warmer than the environment and will
continue to rise, the unstable condition.

The stability of the air is now dependent on

whether the air is saturated or unsaturated.
This state is known as conditional instability,
Figure 7.11
where the atmosphere is stable for
unsaturated (dry) air and unstable for
saturated air.

DALR > ELR >SALR: CONDITIONAL INSTABILITY

Note: The term ‘conditional stability’ is not a meteorological term and, if seen in the answer to
an examination question, can be confidently deleted as an incorrect answer.

93
 7 Adiabatics and Stability

Neutral Stability
If the ELR is the same as the DALR then the temperature at 300 m will be 17°C.
7
Adiabatics and Stability

Figure 7.12

The unsaturated air blown up the hill will cool to 17°C as it rises. The uplifted air now has the
same temperature and hence density as the environment, so it will now remain at 300 m. This
situation is known as neutral (or indifferent) stability for unsaturated (dry) air.

A similar argument holds for saturated air, however this is less likely since the value of the SALR
is a function of both temperature and pressure and is more complex.

ELR = DALR: NEUTRAL STABILITY, for unsaturated (dry) air

(ELR = SALR: NEUTRAL STABILITY, for saturated air)

94
 Adiabatics and Stability
7
Stability Summary
THE RELATIONSHIP BETWEEN THE ELR AND THE DALR AND SALR DETERMINES STABILITY

When ELR < SALR we have absolute stability.

Stable Weather: Clear skies

Moderate to poor visibility

Light turbulence (except at any inversion and in mountain waves – see

chapter on turbulence)

7
Adiabatics and Stability
OR

Stratiform cloud

Possibly fog, especially in winter

Continuous or intermittent light precipitation

• T
 he clouds which form in
stable air tend to be small
in vertical extent and large
in horizontal extent - layer
clouds. Layer clouds may
include stratocumulus
as shown in Figure 7.13.
which is identified by
its well defined shape,
whereas stratus is ill
defined in shape but can
cover equally large areas.

Figure 7.13 Stratocumulus

95
 7 Adiabatics and Stability

When ELR > DALR we have absolute instability.

Unstable Weather: Cumuliform clouds

Moderate to heavy showers

Potential for moderate to heavy precipitation

Good visibility except in showers

• T
 he clouds which form in unstable
7

air tend to be large in vertical

extent and small in horizontal
Adiabatics and Stability

extent - heap clouds.

Figure 7.14 Cumulus of moderate to strong vertical development

Examples
Assuming a constant lapse rate in the layer between 2000 ft and 5000 ft and ignoring the
effects of pressure change, what is the state of stability when:
TEMP AT TEMP AT RH STABILITY
2000’ 5000’ STATE
1 +7° +1° 60%
2 +15° +9° 100%
3 +12° +9° 100%
4 +16° +2° 75%
5 +11° +5° 100%
6 +11° +8° 100%
7 0° -9° 88%
8 +11° +4° 50%
9 +15° +3° 98%
10 +5° 0° 100%
11 +10° +10° 90%
12 +10° +15° 100%

What else is unusual about the environment with regard to questions 11 and 12?

Answers on page 102.

96
 Adiabatics and Stability
7

7
INTENTIONALLY LEFT BLANK

Adiabatics and Stability

97
 7 Questions

Questions
1. If the ELR is 0.65°C / 100 m, the layer is:

a. atmosphere is conditionally stable

b. atmosphere is stable
c. atmosphere is unstable
d. atmosphere is stable when dry

2. ELR is 1°C / 100 m, the layer is:

a. neutral when dry

7

b. absolute stability
c. absolute instability
Questions

d. conditional stability

3. Why does air cool as it rises?

a It expands
b. It contracts
c. The air is colder at higher latitudes
d. The air is colder at higher altitudes

4. From which of the following can the stability of the atmosphere be determined?

a. Surface pressure
b. Surface temperature
c. DALR
d. ELR

5. When the upper part of a layer of warm air is advected:

a. Stability increases within the layer

b. Stability decreases within the layer
c. Wind speed will always decrease with increase in height in the Northern
Hemisphere
d. Wind will back with increase in height in the Northern Hemisphere

6. The temperature at the surface is 15°C, the temperature at 1000 m is 13°C.

The atmosphere is:

a. unstable
b. conditionally unstable
c. stable
d. cannot tell

7. Which of the following gives conditionally unstable conditions?

a. 1°C / 100 m
b. 0.65°C / 100 m
c. 0.49°C / 100 m
d. None of the above

98
 Questions
7
8. A mass of unsaturated air is forced to rise till just under the condensation level. It
then settles back to its original position. What happens to the temperature?

a. Temp. is greater than before

b. Temp. stays the same
c. Temp. is less than before
d. It depends on QFE

9. What happens to the stability of the atmosphere in an inversion? (Temp increasing

with height)

a. Absolutely stable
b. Unstable

7
c. Conditionally stable

Questions
d. Conditionally unstable

10. What happens to stability of the atmosphere in an isothermal layer? (Temp

constant with height)

a. Absolutely stable
b. Unstable
c. Conditionally stable
d. Conditionally unstable

11. What is the effect of a strong low level inversion?

a. Good visibility
b. Calm conditions
c. Turbulence
d. Unstable conditions

12. A layer of air can be:

a. conditional; unstable when unsaturated and stable when saturated

b. conditional; unstable when saturated and stable when unsaturated
c. neutrally stable when saturated and unstable when unsaturated
d. all of the above

13. What happens to the temperature of a saturated air mass when forced to descend?

a. It heats up more than dry because of expansion

b. It heats up less than dry because of evaporation
c. It heats up more than dry because of sublimation
d. It heats up less than dry because of latent heat released during condensation

14. In still air a lapse rate of 1.2°C / 100 m refers to:

a. DALR
b. SALR
c. ELR
d. ALR

99
 7 Questions

15. What happens to the temperature of a saturated air mass when descending?

a. It heats up more than dry because of expansion

b. It heats up less than dry because of evaporation
c. It heats up more than dry because of compression
d. It heats up less than dry because of latent heat released during condensation

16. The DALR is:

a. variable with time

b. fixed
c. variable with latitude
d. variable with temperature
7
Questions

17. An environment cooling at more than 1°C / 100 m is said to be:

a. conditionally stable
b. conditionally unstable
c. unstable
d. stable

100
Questions
7

7
Questions

101
 7 Answers

Answers
1 2 3 4 5 6 7 8 9 10 11 12
d a a d a c b b a a c b
13 14 15 16 17
b c b b c
7

Answers to Questions on Page 96

Answers

Question Answer
1. Stable
2. Unstable
3. Stable
4. Unstable
5. Unstable
6. Stable
7. Neutral
8. Stable
9. Unstable
10. Stable
11. Stable
(isothermal)
12. Stable (inversion)

102
 Chapter

8
Turbulence

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Windshear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Locations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
The Friction Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Thermal Turbulence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Mechanical Turbulence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106
Mountain Waves (MTW) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .106
Turbulence Effects of Mountain Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
Visual Recognition Features of Mountain Waves . . . . . . . . . . . . . . . . . . . . . . . . 107
Action to Avoid the Worst Effects of Mountain Waves . . . . . . . . . . . . . . . . . . . . . 107
Rotor Streaming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
Jet Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
Cumulonimbus Clouds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109
Turbulence around Upper Level Troughs and Ridges . . . . . . . . . . . . . . . . . . . . . . 109
Turbulence Reporting Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
Low Altitude Windshear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

103
 8 Turbulence
8
Turbulence

104
 Turbulence
8
Introduction
A dictionary definition of turbulence is a ‘disturbed state’ and so from the aviation point of
view this would mean disturbed or rough air. There are different ways in which this turbulence
is caused and also different parts of the atmosphere where it occurs.

Windshear
Windshear is the sudden change in speed and/or direction of the wind including vertical
currents. These changes affect the energy of the aircraft and that change in energy is felt
inside the aircraft as turbulence

Vertical Windshear: change in speed and/or direction with change of height, measured in

8
knots per 100 feet.

Turbulence
Horizontal Windshear: change in speed and/or direction in the horizontal plane, measured in
knots per 1000 feet.

Locations
Turbulence occurs:

• In the friction layer.

• In clouds - This will be discussed in detail in the chapters on clouds and thunderstorms.
• In clear air.

The Friction Layer

The friction layer is the lower part of the atmosphere extending from the surface to a height of
2000 ft to 3000 ft above the surface. The depth of the friction layer depends on:

• T he roughness of the terrain. The rougher the surface the greater the strength of the
(vertical) deflection and hence the greater the height to which it will penetrate.
• The wind speed. The higher the speed the greater will be the deflection.
• The stability of the layer. Stable conditions will resist vertical movement and hence limit the
depth.

Within the friction layer there are 2 sources of turbulence:

• Convection from thermal currents

• Frictional or mechanical turbulence

By day the presence of thermal currents will tend to reduce low level stability and hence
increase the depth of the friction layer, whereas at night there is only mechanical turbulence so
the stability will tend to increase because of the surface cooling and the depth of the friction
layer will reduce.

At night the surface cooling, particularly with clear skies, can lead to the formation of low
level inversions. Now vertical mixing is inhibited and the surface frictional effect is enhanced.
This means that below an inversion the wind speed will be light with a significantly different
direction to the much stronger wind above the inversion. Hence Windshear will occur at the
inversion. An aircraft climbing (or descending) through the inversion will experience a rapid

105
 8 Turbulence

change in speed and direction giving, possibly, moderate to severe turbulence. This and other
low level effects are discussed in the Aeronautical Information Circular (AIC) at the end of this
chapter.

Thermal Turbulence
Insolation gives rise to convection currents.
The intensity of these currents depends on
the heating of the surface. Surfaces like rock
and concrete heat rapidly and give rise to
strong vertical currents, whereas grass and
wooded areas will only heat slowly and create
weak convection currents. So flight within the
8

friction layer on a sunny day will be affected

by variable speed vertical currents and hence
Turbulence

windshear giving turbulence.

Figure 8.1 Thermal Turbulence

Thermal turbulence is greatest around

1500 hrs on clear sunny days. There is no
thermal turbulence over the sea.

Mechanical Turbulence
This is caused by physical obstructions to the
normal flow of air such as hills, mountains,
coasts, trees and buildings.

Mountain Waves (MTW) Figure 10.2.

Figure MechanicalTurbulence
8.2 Mechanical Turbulence.

Mountain waves may also be referred to as

standing waves or lee waves. These occur
when the following conditions exist:

• The wind direction is perpendicular to

the mountain range (+/-30°) without
significant change in direction as
altitude increases

• The wind speed at the summits is at

least 15 kt with speed increasing as
altitude increases
Figure 8.3 Conditions necessary for the formation of
• A marked layer of stability around mountain waves
the altitude of the summits, e.g. an
isothermal layer or inversion, with less
stable air above and below

106
 Turbulence
8
The resultant waves can extend for hundreds of miles downwind of the range if suitable
conditions prevail. The waves may extend well above the tropopause and the wave form may
be seen in cirrus clouds high in the troposphere and also in noctilucent clouds which occur at
altitudes around 250 000 ft in the upper mesosphere.

Turbulence Effects of Mountain Waves

Most severe turbulence can occur in the Rotor Zone lying beneath the crests of lee waves
and is often marked by Roll Clouds. The most powerful rotor lies beneath the first wave crest
(one wavelength downwind). Flight in waves can be smooth, but severe turbulence may occur.
Occasionally violent turbulence will occur, due to wave ‘breaking’.

Normal turbulence associated with flight across jet streams is frequently greatly increased

8
when the jet passes over mountainous areas, particularly when mountain waves are present.

Turbulence
It has been found that turbulence caused in the troposphere due to mountain waves may
continue well into the stratosphere. An aircraft flying close to its ceiling on these occasions
might find itself in serious difficulty.

Visual Recognition Features of Mountain Waves

Provided there is sufficient moisture in the atmosphere, distinctive clouds are formed with
mountain waves and these provide useful warning of the presence of such waves. The clouds
are:

• Lenticular, or lens shaped clouds which form on the crests of the waves. They may appear
above the mountain tops and in the crests of the waves downwind. They may be found up
to, and possibly above, the tropopause. Ragged edges indicate turbulence.

• Rotor, or roll-clouds occur under the crests of strong waves downwind of the ridge. The
strongest rotor is normally formed in the first wave downwind and will be level or slightly
above the ridge crest.

• Cap clouds form on the ridge and strong winds may sweep the cloud down the lee slopes.

Note:
• The characteristic clouds above may be obscured by other clouds and the presence of
standing waves may thus not be evidenced.

• If the air is dry, clouds may not form at all, even though mountain waves are present.

Action to Avoid the Worst Effects of Mountain Waves

• Read the Met. Forecast.

• Arrange to cross mountain ranges at 90 degrees.

• Fly at the recommended turbulence penetration speed.

• Do not fly parallel to and just downwind of the range at any altitude.

• Avoid flight through or near the rotor zone.

107
 8 Turbulence

• Avoid flight levels within 5000 ft of stable layer where severe turbulence is most likely.

• Allow a height clearance above highest ground at least equal to the height of that ground
above local terrain.

 void low altitude flight towards the mountain range from the lee side. Aircraft height
• A
variations will be out of phase with waves and downdraughts will be hazardous.

 void high altitude flight on the lee side of the mountain range downwind. Buffet margin
• A
at high level may be small, and speed of approaching standing waves will be high, with
subsequently greater loads applied to the airframe.

• Be prepared for icing in cloud.

8

Rotor Streaming
Turbulence

If the winds approaching a

mountain range are strong
only at lower levels and fall off
or reverse direction at higher
levels, Rotor Streaming may
result. This comprises violent
rotors moving downwind
from the ridge. Unlike the
stationary rotors described
above, these rotors travel
downwind after forming
on the lee slopes, Figure 8.4
shows rotor streaming.

Jet Streams Figure 8.4 Rotor Streaming

Figure 10.5. Rotor Streaming.
Jetstreams are narrow
fast moving currents of
air which occur just below
the tropopause and will be
discussed in detail in the
chapter on upper winds.
Generally the associated
turbulence is found on the
cold air side of the Jet Stream
just below the core where
the greatest windshear
occurs, with a secondary area
above the core extending
into the stratosphere as the
winds rapidly decrease in
strength. The turbulence
will be more severe with
Figure 8.5 A Vertical Cross-section Through a Jet stream
curved jets, developing and
rapidly moving jets and in
mountainous areas, particularly when mountain waves are present.

108
 Turbulence
8
If turbulence is encountered at high altitude as well as reducing speed to the rough air
penetration speed the pilot should also descend. At high altitude the margins to high and low
speed buffet may be quite small and the effect of the turbulence may put the aircraft into high
or low speed stall. Descent will increase these margins thus enhancing the aircraft safety.

Cumulonimbus Clouds
Cumulonimbus (CB) clouds
and their associated hazards
will be discussed in detail in
later chapters. Turbulence
will be found within CB with
strong vertical currents.

8
These vertical currents cause
air to be drawn in from

Turbulence
around the cloud creating
turbulence and the energy
of the air rising in the cloud
is transmitted to above the
cloud creating turbulence.
Below the cloud the inflow
and outflow of air creates
severe turbulence at low
altitude and the possibility of
Figure 8.6 Turbulence Surrounding Cumulonimbus Clouds
microbursts (or downbursts)
create a potentially fatal
hazard. The hazard of these microbursts is discussed in the AIC at the end of this chapter.

Turbulence around Upper Level Troughs and Ridges

Since upper level winds are stronger than those at the surface, the sharp changes in wind
direction at upper level troughs are likely to produce considerable horizontal windshear and
consequent disturbance which may be experienced as Clear Air Turbulence (CAT).

As upper level ridges tend to be more gently curved than troughs, the direction changes and
consequent turbulence will be less severe.

109
 8 Turbulence
8
Turbulence

Figure 8.7 Turbulence Produced at Upper Troughs and Ridges

Turbulence Reporting Criteria

Turbulence remains an important operational factor at all levels but particularly above FL150.
The best information on turbulence is obtained from pilots’ Special Aircraft Observations;
all pilots encountering turbulence are requested to report time, location, level, intensity
and aircraft type to the ATS Unit with whom they are in radio contact. High level turbulence
(normally above FL150 not associated with cumuliform cloud, including thunderstorms) should
be reported as TURB, preceded by the appropriate intensity or preceded by Light or Moderate
Chop. (Note: EASA still refer to clear air turbulence as CAT.)

Table 3.5.6.1 - TURB and other Turbulence Criteria Table

Incidence: Occasional - less than 1/3 to 2/3 Intermittent - 1/3 to 2/3 Continuous - more than 2/3

Intensity Aircraft Reaction (transport size aircraft) Reaction Inside Aircraft

Light Turbulence that momentarily causes Occupants may feel a slight strain against seat belts or
slight, erratic changes in altitude and/or shoulder straps. Unsecured objects may be displaced slightly.
attitude (pitch, roll, yaw). Food service may be conducted and little or no difficulty is
encountered in walking.
IAS fluctuates 5 - 15 kt. (<0.5 g at the
aircraft’s centre of gravity) Report as
‘Light Turbulence’. or;

turbulence that causes slight, rapid and

somewhat rhythmic bumpiness without
appreciable changes in altitude or
attitude. No IAS fluctuations. Report as
‘Light Chop’.

110
 Turbulence
8
Moderate Turbulence that is similar to light Occupants feel definite strains against seat belts or shoulder
Turbulence but of greater intensity. straps. Unsecured objects are dislodged. Food service and
Changes in altitude and/or attitude occur walking are difficult.
but the aircraft remains in positive control
at all times. IAS fluctuates 15 - 25 kt. (0.5
- 1.0g at the aircraft’s centre of gravity).
Report as ‘Moderate Turbulence’. or;

turbulence that is similar to Light Chop

but of greater intensity. It causes rapid
bumps or jolts without appreciable
changes in altitude or attitude. IAS may
fluctuate slightly. Report as ‘Moderate
Chop’.

Severe Turbulence that causes large, abrupt Occupants are forced violently against seat belts or shoulder
changes in altitude and/or attitude. straps. Unsecured objects are tossed about. Food service and
Aircraft may be momentarily out of walking impossible.

8
control. IAS fluctuates more than 25 kt.

Turbulence
(>1.0 g at the aircraft’s centre of gravity).
Report as ‘Severe Turbulence’.

Note 1: Pilots should report location(s), time(s) (UTC., incidence, intensity, whether in or near clouds, altitude(s)
and type of aircraft. All locations should be readily identifiable. Turbulence reports should be made when
moderate/severe turbulence is encountered, or on request. Example:

(a) Over Pole hill 1230 intermittent Severe Turbulence in cloud, FL 310, B747.

(b) From 50 miles north of Glasgow to 30 miles west of Heathrow 1210, occasional moderate Chop TURB, FL 330, MD80.

Note 2: The UK does not use the term ‘Extreme’ in relation to turbulence.

111
 8 Turbulence

Low Altitude Windshear

Vertical Windshear
Vertical windshear is change in wind velocity with height. It is typically measured in knots per
100 ft.
8
Turbulence

Figure 8.8 Vertical Windshear

Horizontal Windshear
Horizontal windshear is change in wind velocity with horizontal distance. It is typically measured
in knots per 1000 ft.

Figure 8.9 Horizontal Windshear

The remainder of this chapter consists of a UK Civil Aviation Authority Aeronautical Information
Circular covering low altitude windshear. Some of this material has already been covered, but
those parts which are new should be highlighted.

112
Turbulence
8

8
Turbulence

113
 8 Turbulence
8
Turbulence

114
Turbulence
8

8
Turbulence

115
 8 Turbulence
8
Turbulence

116
Turbulence
8

8
Turbulence

117
 8 Turbulence
8
Turbulence

118
 Questions
8
Questions

1. Maximum turbulence associated with the mountain waves is likely to be:

a. two wavelengths downwind and just above the surface

b. approximately one wavelength downwind of, and approximately level with,
the top of the ridge
c. just below the tropopause above the ridge
d. down the lee side of the ridge and along the surface

2. For the formation of mountain waves, the wind above the level of the ridge should:

a. decrease or even reverse direction

8
b. increase initially then decrease
c. increase with little change in direction

Questions
d. increase and then reverse in direction

3. When flying in IMC in a region close to a range of hills 2000 ft high, in stable air and
with wind direction at right angles to the axis of the range of hills, which of the
following is probably the most dangerous practice:

a. flying towards the hills, into the wind, at flight level 65

b. flying parallel to the hills on the downwind side at flight level 40
c. flying towards the hills downwind at flight level 55
d. flying parallel to the hills on the upwind side at flight level 40

4. Which of the following statements referring to jet streams is correct?

a. Turbulence associated with jet streams is probably associated with the rapid
windshear in the vicinity of the jet
b. The maximum wind speed in a jet stream increases with increase of height up
to the tropopause and remains constant thereafter
c. The core of a jet stream is usually located just below the tropopause in the
colder air mass
d. The rate of change of wind speed at any given level is usually greatest on the
warmer side of the jet

Continued overleaf

119
 8 Questions

Refer to the diagram (Appendix A) below, for questions 5-8, assuming mountain waves are
present.

Appendix A
8
Questions

5. The wind at square A3 is likely to be:

a. 35 kt
b. 50 kt
c. 25 kt
d. light

6. The wind at ABC 4 may be:

a. 50 kt
b. 40 kt
c. 35 kt
d. a jet stream

7. Flight conditions at B1 are likely to be:

a. smooth
b. turbulent
c. turbulent in breaking wave crests
d. turbulent due to marked up and down currents

8. The most extreme turbulence can occur:

a. at B1
b. at A2
c. at ABC 4
d. at B2, 3, 4 and at C2, 3, 4

120
 Questions
8
9. The significance of lenticular cloud is:

a. there may be mountain waves present and there will be severe turbulence
b. there are mountain waves present but they may not give severe turbulence
c. a Föhn wind can be expected with no turbulence
d. a katabatic wind is present which may lead to fog in the valleys

10. A mountain range is aligned in an east/west direction. Select the conditions from
the table below that will give rise to mountain waves:

2000 ft 5000 ft 10 000 ft

a. 020/40 020/30 020/50

b. 170/20 190/40 210/60
c. 270/15 270/20 270/40

8
d. 090/20 090/40 090/60

Questions
11. For mountain waves to form, the wind direction must be near perpendicular to a
ridge or range of mountains and the speed must:

a. decrease with height within a stable layer above the hill

b. increase with height within an unstable layer above the hill
c. decrease with height within an unstable layer above the hill
d. increase with height within a stable layer above the hill

12. A north/south mountain range, height 10 000 ft is producing marked mountain

waves. The greatest potential danger exists for an aircraft flying:

a. on the windward side of the ridge

b. at FL350 over and parallel to the ridge
c. towards the ridge from the lee side at FL140
d. above a line of clouds parallel to the ridge on the lee side at FL25

13. Clear air turbulence, in association with a polar front jet stream in the Northern
Hemisphere, is more severe:

a. underneath the jet core

b. in the centre of the jet core
c. looking downstream on the right hand side
d. looking downstream on the left hand side

14. Mountain waves can occur:

a. up to a maximum of 5000 ft above the mountains and 50 NM to 100 NM

downwind
b. up to mountain height only and 50 NM to 100 NM downwind
c. above the mountain and downwind up to a maximum height at the
tropopause and 50 NM to 100 NM downwind.
d. in the stratosphere and troposphere

121
 8 Questions

15. Clear air turbulence (CAT) should be reported whenever it is experienced. What
should be reported if crew and passengers feel a definite strain against their seat
or shoulder straps, food service and walking is difficult and loose objects become
dislodged?

a. Light TURB
b. Extreme TURB
c. Severe TURB
d. Moderate TURB
8
Questions

122
Questions
8

8
Questions

123
 8 Answers

Answers
1 2 3 4 5 6 7 8 9 10 11 12
b c b a b d d a b b d d

13 14 15
d d d
8
Answers

124
 Chapter

9
Altimetry

The Altimeter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

Altimeter Settings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
QFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
QNH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
Forecast QNH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
Altimeter Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
Altimeter Temperature Error Correction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
Minimum Safe Flight Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
Transition Altitude . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
Transition Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
Transition Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

125
 9 Altimetry
9
Altimetry

126
 Altimetry
9
The Altimeter
An altimeter is an instrument
which measures pressure
and causes a needle to move
across a dial. The dial is
calibrated in feet rather than
pressure as we know that
pressure decreases as altitude
increases.

The instrument is calibrated

in accordance with the
ICAO International Standard
Atmosphere so that all

9
altimeters will read the

Altimetry
same altitude for the same
pressure. (See previous notes
on the need for the ISA.)

In addition, altimeters have a

means of adjusting the needle
setting to take changes in the Figure 9.1 Simple altimeter
surface atmospheric pressure
into account.

Figure 9.1 shows how the

altimeter reading will change
with a change in pressure.

In Figure 9.2 section A, the

pressure at the airfield, which
is at sea level, is 1010 hPa. The
altimeter reads zero feet.

In section B, the pressure at

the airfield has fallen to 1000
hPa and the altimeter, rather
than showing a decrease in
pressure, shows an increase
in height.

Figure 9.2 The altimeter responding to changes in pressure

127
 9 Altimetry

• W
 hen flying at a constant indicated altitude, outside air pressure must remain the same.
To achieve this we must fly along a pressure level. However, when we fly to an area of
lower pressure, these pressure lines will dip, consequently our true altitude will decrease.
Conversely when flying into a region of higher pressure, the pressure lines will rise and our
true altitude will increase.
9
Altimetry

Figure 9.3

HIGHER PRESSURE; TRUE ALTITUDE > INDICATED ALTITUDE

LOWER PRESSURE; TRUE ALTITUDE < INDICATED ALTITUDE

• V
 arying temperatures within the atmosphere have significant effects on the pressure and
the shape of the pressure lines. Cold air will tend to compact and lower pressure lines whilst
warm air will expand and raise pressure lines. Using Figure 9.4 you can see that when flying
to a colder area at a constant indicated altitude your true altitude decreases. Conversely,
when flying into warmer region your true altitude will increase.

Figure 9.4

COLDER THAN ISA; TRUE ALTITUDE < INDICATED ALTITUDE

WARMER THAN ISA; TRUE ALTITUDE > INDICATED ALTITUDE

• T
 here is a need to be able to reset the altimeter to take account of the fall in pressure.
Consequently, if the altimeter is reset when the pressure changes, the altimeter will read
correctly. We may, by altering the altimeter subscale setting, set QFE, QNH or SPS for use
when we fly to ensure more accurate readings.

128
 Altimetry
9
Altimeter Settings
QFE 
The pressure measured at the aerodrome datum. With QFE set on the altimeter, the altimeter
will read zero when the aircraft is on the surface of the aerodrome. When airborne, with QFE
set, the altimeter reads the approximate height above the aerodrome. QFE is always rounded
down to the nearest hectopascal.

900 100

800 200

700 300

1010
600 400
500

9
Altimetry
900 100

800 200

700 300

1010
600 400
500

Figure 9.5 Airfield Pressure - QFE.

QNH
QFE converted to mean sea level using the ISA. With QNH set the altimeter will read aerodrome
elevation when on the surface of the aerodrome. When airborne it will read the approximate
altitude of the aircraft.

Note: QNH is always rounded down to the nearest integer.

900 100

800 200

700 300

1010
600 400
500

900 100

800 200

700 300

1010
600 400
500

Figure 9.6 Mean Sea Level Pressure - QNH.

129
 9 Altimetry

Forecast QNH
The lowest forecast QNH within an area, forecast for one hour ahead. The altimeter will be
in error, but as the setting is the lowest forecast, the actual pressure will always be higher,
or at least equal to the forecast QNH, and the altimeter will read low (or safe) or the correct
altitude. (See Figure 9.7).
9
Altimetry

Figure
Figure 6.7.9.7Altimeter
Altimeter Setting
SettingRegions
Regions.

FO UK 70 EGRR 110600
FO QNH
VALIDITY PERIOD 00708 01992 02995 03003 04007 05001 REGION NUMBER
07011 08011 09011 10014 11014 12019
13020 14015 15017 16987 17998 18989 R.P.S
19998 20004 21981 22987 23001 24011
25014

Note: The Cotswold area where Kidlington is situated is No.15 on the above decode table.
SPS  (Standard Pressure Setting) If the standard pressure of 1013 hPa is set on the altimeter,
the instrument will read what is known as pressure altitude height in the Standard
Atmosphere. This is the altimeter setting used when flying above the transition altitude.

130
 Altimetry
9
Terminology
Altitude Vertical distance above mean sea level.

Height Vertical distance of a level or point measured from a specific datum, e.g. above
aerodrome surface.

Elevation Vertical distance of a fixed object above mean sea level (e.g. aerodrome or
obstacle.

Flight Level Surface of constant atmospheric pressure measured from the 1013 hPa datum
used for vertical separation by specified pressure intervals (usually 500 or 1000
ft). Flight Level is measured in hundreds of feet.

e.g. FL350 = 35 000 ft.

9
Altimetry
hPa

hPa

Figure 9.8 Altimetry Terminology

131
 9 Altimetry

Altimeter Errors
Apart from instrument errors, there are two errors of interest meteorologically. They are:

• Barometric Error - Errors caused by setting a pressure on the subscale other than the correct
one. For calculations a height of 27 ft per hPa is used in all the meteorology syllabus altimetry
questions to determine the difference between indicated and true height/altitude.
9
Altimetry

Figure 9.9 Barometric Error

• Temperature error - The altimeter is calibrated in accordance with the ICAO ISA.
If the temperature is other than that in the ISA, the altimeter will be in error. Corrected
altitude is calculated by using a navigational computer, or a correction table. HI-LO-HI will
still apply.

Altimeter Temperature Error Correction

• Pressure altimeters are calibrated to indicate true altitude under ISA conditions. Any
deviation from ISA will result in erroneous readings, except that the altimeter will read
the correct elevation of the airfield regardless of temperature when the aircraft is on the
ground with QNH set.

 hen temperatures are lower than ISA an aircraft’s true altitude will be lower than the
• W
altimeter reading.

• T
 he error is proportional to the difference between actual and ISA temperature, and the
vertical distance of the aircraft above the altimeter setting datum.

• The
 height correction is 4 feet per degree Celsius deviation from ISA per 1000 feet.
Note: the calculation must be made over the indicated height difference from the datum
for the pressure setting.

• For example: When making an approach to an aerodrome at mean sea level in Siberia in
January the decision height is 200 ft. What is the true height when the indicated height is
200 ft if the temperature is -50°C?

132
 Altimetry
9
Error = 4×(-65) × 0.2 = -52 ft

Hence the true height is 148 ft!

This is clearly unacceptable so when carrying out an aerodrome or runway approach in

temperatures colder than standard the indicated decision height/altitude or minimum
descent height/altitude must be increased in accordance with the following table to ensure
safe operation.

ISA TEMP HEIGHT ABOVE TOUCHDOWN OR HEIGHT ABOVE AERODROME IN FEET

DEVIATION
°C
200 300 400 500 600 700 800 900 1000
-15 12 18 24 30 36 42 48 54 60

9
-25 20 30 40 50 60 70 80 90 100

Altimetry
-35 28 42 56 70 84 98 112 126 140
-45 36 54 72 90 108 126 144 162 180
-55 44 66 88 110 132 154 176 198 220
-65 52 78 104 130 156 182 208 234 260

With temperatures colder than standard consideration must be given to the effect of
temperature on terrain clearance.

For example:

A flight is planned at FL180 over Mont Blanc (elevation 15 782 ft). The mean sea level pressure
is 983 hPa, from an aerodrome at mean sea level, and the temperature of the air up to the
summit is 25°C colder than ISA. Determine the true altitude of the aircraft at Mont Blanc and
hence the terrain clearance. Figure 9.10.

Figure 9.10

133
 9 Altimetry

The indicated altitude is 18 000 ft above the 1013 hPa datum; the height correction for the 30
hPa pressure difference is:

30 × 27 = 810 feet so the corrected altitude is 17 190 ft. Figure 9.11.

9
Altimetry

Figure 9.11

The height correction for the temperature deviation from ISA is:

4 × (- 25) × 18 = -1800 ft

Hence the true altitude of the aircraft is 15 390 ft. Figure 9.12.

Figure 9.12

But Mont Blanc is 15 872 ft high so if we do not do something about it we will hit the mountain
392 feet below the summit. To simplify the calculation use the formula:

true altitude = indicated altitude + (indicated altitude/1000 × ISA deviation × 4) + 27(actual

pressure - pressure setting)

134
 Altimetry
9
So in winter, particularly if flying close to safety altitude, the effect of temperature on true
altitude must be taken into account.

For all of the following questions assume that 1 hPa = 27 ft.

1. A
 n aircraft is at an airfield with an elevation of 350 ft. The altimeter setting is 1002, but
the actual QNH is 993. What is the altimeter reading?

2. An aircraft is on an airfield, elevation 190 ft and has an altimeter reading of 70 ft with a
setting of 1005. What is the actual QNH?.

3. What is the altimeter reading if the setting is 978, the QNH 993 and the airfield
elevation 770 ft?

4. The regional pressure setting is 1012, the altimeter setting is 1022 and the indicated
altitude is 4100 ft. Ahead is some high ground shown on the map as being at 3700 ft.

9
Will the aircraft clear the high ground, and if so, by how much?

Altimetry
(Answers on page 137)

Minimum Safe Flight Level

Minimum safe flight level is the minimum indicated pressure altitude (using SPS 1013 hPa) that
will ensure the aircraft is not lower than the safety attitude for each section of the route.

When route planning we must ensure that on all sections of the route the selected flight level
is at or above the safety altitude for that section. This means that we have to take account of
both expected minimum pressure (QNH) and minimum temperature for each section of the
route.

For example: on a section of a route the safety altitude is 8300 ft, the forecast QNH is 983 hPa
and the temperature is ISA -30°, determine the minimum safe flight level for that section of the
route. Figure 9.13.

Figure 9.13

The correction for pressure difference is 30 × 27 = 810 ft, giving a minimum indicated pressure
altitude of 9110 ft. The temperature correction is 4 × (-30) × 9 = -1080 ft so the minimum
indicated pressure altitude required is 10 190 ft. Figure 9.14.

135
 9 Altimetry
9

Figure 9.14
Altimetry

This is now rounded up to 10 500 ft (FL105) or 11 000 ft (FL110) dependent of the status of the
flight and the type of airspace through which the flight is to be made.

Fill in the blank spaces in the following examples.

Assume 1 hPa = 27 ft

QNH ALTIMETER SETTING TRUE ALTITUDE ALTIMETER READING

1012 1010 4060
1015 1010 5000
1010 641 560
1020 1013 10 500
999 1013 8500
1015 46 125
1017 1027 3300
1012 270 0
993 405 0
1025 1015 4760

Transition Altitude
The altitude at or below which the vertical position of an aircraft is controlled by reference to
altitude (QNH).

Transition Level
The lowest flight level (1013) available for use above the transition altitude.

Transition Layer
The airspace between the transition altitude and the transition level.

136
 Altimetry
9
Answers to Questions on page 135 and page 136

1. 593 ft

2. 1010 hPa

3. 365 ft

4. Yes, by 130 ft

QNH ALTIMETER SETTING TRUE ALTITUDE ALTIMETER READING

1012 1010 4060 4006
1015 1010 5135 5000
1013 1010 641 560
1020 1013 10 689 10 500
999 1013 8122 8500

9
1015 1018 46 125

Altimetry
1017 1027 3300 3570
1012 1002 270 0
1008 993 405 0
1025 1015 4760 4490

137
 9 Questions

Questions
1. MSA given as 12 000 ft, flying over mountains in temperatures +9°C, QNH set as
1023 (obtained from a nearby airfield). What will the true altitude be when 12 000
ft is reached?

a. 11 940
b. 11 148
c. 12 210
d. 12 864

2. When flying at FL180 in the Southern Hemisphere you experience a left to right
crosswind. What is happening to your true altitude if indicated altitude is constant?

a. Remains the same

b. Increasing
9

c. Decreasing
d. Impossible to tell
Questions

3. Flying from Marseilles (QNH 1012) to Palma (QNH 1015) at FL100. You do not reset
the altimeter, why would true altitude be the same throughout the flight?

a. Not possible to tell

b. Air at Palma is warmer than air at Marseilles
c. Air at Marseilles is warmer than air at Palma
d. Blocked static vent

4. Which of these would cause your true altitude to decrease with a constant
indicated altitude?

a. Cold/Low
b. Hot/Low
c. Cold/High
d. Hot/High

5. An aircraft flying in the Alps on a very cold day, QNH 1013 set in the altimeter, flies
level with the summit of the mountains. Altitude from aneroid altimeter reads:

a. same as mountain elevation

b. lower than mountain elevation
c. higher than mountain elevation
d. impossible to determine

6. You are flying in an atmosphere which is warmer than ISA, what might you
expect?

a. True altitude to be the same as indicated altitude

b. True altitude to be lower than indicated altitude
c. True altitude to be the decreasing
d. True altitude to be higher than indicated altitude

138
 Questions
9
7. The QNH is 1030 hPa and at the transition level you set the SPS. What happens to
your indicated altitude (assume 27 ft per 1 hPa)?

a. Drops by 459 ft
b. Rises by 459 ft
c. No change
d. Rises

8. You are flying from Madrid (QNH 1012) to Paris (QNH 1015) at FL80. If your true
altitude and indicated altitude remain the same then:

a. the air at Madrid is warmer than Paris

b. the air at Paris is warmer than Madrid
c. the altimeters are incorrect
d. your indicated altitude must be changing

9
9. If you are flying on a QNH 1009 on very cold day and you circle the top of a peak in
the Alps, your altimeter will read:

Questions
a. the same as the elevation of the peak
b. lower than the elevation of the peak
c. higher than the elevation of the peak
d. not enough information to tell

10. How do you calculate the lowest usable flight level?

a. Lowest QNH and lowest negative temperature below ISA

b. Lowest QNH and highest negative temperature below ISA
c. Highest QNH and highest temperature above ISA
d. Highest QNH and lowest temperature

11. QNH is 1003. At FL100 true altitude is 10 000 ft. It is:

a. warmer than ISA

b. colder than ISA
c. same as ISA
d. cannot tell

12. How is QNH determined from QFE?

a. Using the temperature of the airfield and the elevation of the airfield
b. Using the temperature
c. Using the elevation
d. Using the temperature at MSL and the elevation of the airfield

139
 9 Questions

13. Using the diagram below you are on a flight from A to B at 1500 ft. Which
statement is true?

a. True altitude at A is greater than B

b. True altitude at B is greater than A
c. True altitude is the same
d. Cannot tell
9
Questions

14. QFE is 1000 hPa with an airfield elevation of 200 m AMSL. What is QNH? (use 8 m
per hPa).

a. 975 hPa
b. 1025 hPa
c. 1008 hPa
d. 992 hPa

15. Which of the following is true?

QNH is:

a. Always more than 1013.25 hPa

b. Always less than 1013.25 hPa
c. Never 1013.25 hPa
d. Can never be above or below 1013 hPa

16. Flying from Marseilles to Palma you discover your true altitude is increasing, but
oddly the QNH is identical at both places. What could be the reason?

a. Re-check the QNH

b. Re-check the radio altimeter
c. The air at Palma is warmer
d. Palma is lower than Marseilles

17. QNH is 1030. Aerodrome is 200 m AMSL. What is QFF?

a. Higher than 1030

b. Lower than 1030
c. Same
d. Not enough info

140
 Questions
9
18. If an aerodrome is 1500 ft AMSL on QNH 1038, what will the actual height AGL to
get to FL75 be?

a. 6675 ft
b. 8175 ft
c. 8325 ft
d. 5325 ft

19. Altimeter set to 1023 at aerodrome. On climb to altitude the SPS is set at transition
altitude. What will the indication on the altimeter do on resetting to QNH?

a. Dependent on temperature
b. Decrease
c. Increase
d. Same

9
20. What temperature and pressure conditions would be safest to ensure that your
flight level clears all the obstacles by the greatest margin?

Questions
a. Cold temp/low pressure
b. Warm temp/high pressure
c. Temp less than or equal to ISA and a QNH less than 1013
d. Temp more than or equal to ISA and a QNH greater than 1013

21. You are flying from Marseilles (QNH 1012 hPa) to Palma de Mallorca (QNH 1012
hPa) at FL100. You notice that the effective height above MSL (radio altitude)
increases constantly. Hence:

a. one of the QNH values must be wrong

b. you have the altimeters checked, as their indications are obviously wrong
c. the air mass above Palma is warmer than that above Marseilles
d. you have to adjust for a crosswind from the right

22. You are flying from Marseilles (QNH 1026 hPa) to Palma de Mallorca (QNH 1026
hPa) at FL100. You notice that the effective height above MSL (Radio Altitude)
decreases constantly. Hence:

a. one of the QNH values must be wrong

b. the air mass above Marseilles is warmer than that above Palma
c. you have the altimeters checked, as their indications are obviously wrong
d. you have to adjust for a crosswind from the right

23. Flying at FL135 above the sea, the radio altimeter indicates a true altitude of
13 500 ft. The local QNH is 1019 hPa. Hence the crossed air mass is, on average:

a. at ISA standard temperature

b. colder than ISA
c. warmer than ISA
d. there is insufficient information to determine the average temperature
deviation

141
 9 Questions

24. You are flying in the Alps at the same level as the summit on a hot day. What does
the altimeter read?

a. Same altitude as the summit

b. Higher altitude as the summit
c. Lower altitude as the summit
d. Impossible to tell

25. An airfield has an elevation of 540 ft with a QNH of 993 hPa. An aircraft descends
and lands at the airfield with 1013 hPa set. What will its altimeter read on landing?

a. 380 ft
b. 1080 ft
c. 0 ft
d. 540 ft
9

26. When is pressure altitude equal to true altitude?

Questions

a. In standard conditions
b. When surface pressure is 1013.25 hPa
c. When the temperature is standard
d. When the indicated altitude is equal to the pressure altitude

27. What is the relationship between QFE and QNH at an airport 50 ft below MSL?

a. QFE = QNH
b. QFE < QNH
c. QFE > QNH
d. There is no clear relationship

28. You are flying at FL160 with an OAT of -27°C. QNH is 1003 hPa. What is your true
altitude?

a. 15 540 ft
b. 15 090 ft
c. 16 330 ft
d. 15 730 ft

142
 Questions
9
29. Flying from A to B at a constant indicated altitude in the Northern Hemisphere.

a. True altitude increases

b. Wind is northerly
c. True altitude decreases
d. Wind is southerly

9
Questions
30. Up to FL180 ISA Deviation is ISA +10°C. What is the actual depth of the layer
between FL60 and FL120?

a. 6000 ft
b. 6240 ft
c. 5760 ft
d. 5700 ft

31. Up to FL 180 ISA Deviation is ISA -10°C. What is the actual depth of the layer
between FL60 and FL120?

a. 6000 ft
b. 6240 ft
c. 5760 ft
d. 5700 ft

32. What condition would cause your indicated altitude to be lower than that being
actually flown?

a. Pressure lower than standard

b. Pressure is standard
c. Temperature lower than standard
d. Temperature higher than standard

143
 9 Questions

33. You fly over the sea at FL90, your true altitude is 9100 ft and QNH is unknown.
What can be said about the atmosphere temperature?

a. QNH is lower than standard

b. It is colder than ISA
c. It is warmer than ISA
d. Nothing, insufficient information

34. You are flying at FL100 in an air mass that is 15°C colder than ISA. Local QNH is
983 hPa. What would the true altitude be?

a. 8590 ft
b. 11 410 ft
c. 10 000 ft
d. 10 210 ft
9

35. Which statement is true?

Questions

a. QFE is always lower than QNH

b. QNH is always lower than QFE
c. QNH can be equal to QFE
d. QFE can be equal to QFF only

36. You fly from east to west at the 500 hPa level in the Northern Hemisphere;

a. if the wind is from the north there will be a gain in altitude

b. if the wind is from the south there is again in altitude
c. if you encounter northerly drift, there is a gain in altitude
d. you fly towards an area of lower pressure, and therefore, experience a loss in
altitude

37. You have landed on an airport elevation 1240 ft and QNH 1008 hPa. Your altimeter
subscale is erroneously set to 1013 hPa. The indication on the altimeter will be:

a. 1200 ft
b. 1375 ft
c. 1105 ft
d. 1280 ft

38. You are cruising at FL200, OAT is -40°C, sea level pressure is 1033 hPa. Calculate the
true altitude.

a. 20660 ft
b. 21740 ft
c. 18260 ft
d. 19340 ft

144
Questions
9

9
Questions

145
 9 Answers

Answers
1 2 3 4 5 6 7 8 9 10 11 12
d b c a c d a a c a a c
13 14 15 16 17 18 19 20 21 22 23 24
b b c c d a c d c b b c
25 26 27 28 29 30 31 32 33 34 35 36
b a c b c b c d d a c a

37 38
b d
9
Answers

146
 Chapter

10
Winds

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
Gusts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
Squalls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
Gale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
Hurricane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
Measurement of Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
The Geostrophic Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Pressure Gradient Force (PGF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
Coriolis Force (CF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152
Geostrophic Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
Construction of the Geostrophic Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
Conditions Necessary for the Wind to Be Geostrophic . . . . . . . . . . . . . . . . . . . . . 155
The Gradient Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
Centrifugal Force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
Gradient Wind in a Depression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
Gradient Wind in a High . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
The Antitriptic Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
Winds below 2000 - 3000 ft (1 km). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
Rough Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
Diurnal Variation of the Surface Wind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
Diurnal Variation of 1500 ft and Surface Wind Velocity . . . . . . . . . . . . . . . . . . . . . 160
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
Land and Sea Breezes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
Practical Coastal Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
Valley or Ravine Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
Venturi Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

Continued Overleaf

147
 10 Winds

Katabatic Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

Anabatic Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
Föhn Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
10
Winds

148
 Winds
10
Introduction
Wind is air in horizontal motion. Wind Velocity (W/V) has both direction and speed.

Wind direction is always given as the direction from which the wind is blowing; this is illustrated
in Figure 10.1. It is normally given in degrees true, but wind direction given to a pilot by ATC will
be given in degrees magnetic.

10
Winds
Figure 10.1 Wind direction

Wind speed is usually given in knots, but

some countries give the speed in metres per
second (ms-1) and the Met. Office often work
internally in kilometres per hour, shown as
KMH on reports and forecasts.

On the wind vector the wind direction is from

the feathers to the point which indicates the
location of the wind. The illustrated wind
Figure 10.2 is 240° (true) at 125 kt. It should
be noted that, by convention, the feathers
always point towards the low pressure.
Figure 10.2

Veering is a change of wind direction in a

clockwise direction.

Backing is a change of wind direction in an

anticlockwise direction. This applies in both
hemispheres.

Figure 10.3 The wind veering and backing

149
 10 Winds

Gusts
A gust is a sudden increase in wind speed, often with a change in direction lasting less than one
minute and it is a local effect. A gust will only be reported or forecast if 10 kt or more above
the mean wind speed.

A lull is a sudden decrease in wind speed.

Squalls
A squall is a sudden increase in wind speed, often with a change in direction. Lasting for one
minute or more and can cover a wide area. It is often associated with cumulonimbus cloud
and cold fronts.

Gale
10

A gale exists when the sustained wind speed exceeds 33 kt, or gusts exceed 42 kt.
Winds

Hurricane
A hurricane force wind exists when sustained wind speed exceeds 63 kt.

Measurement of Winds
Surface wind is measured by a
wind vane which aligns itself
with the wind direction, and an
anemometer which measures
the speed. An anemometer
is a set of 3 hemispherical
cups which rotate on a shaft
with the effect of the wind.
The speed of rotation of the
shaft is directly proportional
to the wind speed. The
rotation is used to drive a
small generator, the output
of which is then displayed on
a gauge which is calibrated in
knots.

The ICAO requirement is

that the wind vane and
anemometer should be
positioned 10 m (33 ft) above Figure 10.4 A wind vane and anemometer
aerodrome level and located
clear of buildings and obstructions which could affect the airflow and hence accuracy. An
anemograph records wind speed and direction.

Upper winds are measured by GPS tracking of a radiosonde and by aircraft reports.

150
 Winds
10
Wind
Wind is generated by the pressure differences between high and low pressure systems which
give rise to what we call the pressure gradient force (PGF) the change of pressure over distance.
The PGF acts directly from high pressure to low pressure.

The spacing of the isobars determines the magnitude of the force, the closer together the
isobars the greater the pressure difference and hence the PGF and thus the wind speed.

Buys Ballot’s Law tells us that if we stand with our back to the wind in the Northern Hemisphere
low pressure is on the left (right in the Southern Hemisphere). This implies that the wind does
not flow directly from high pressure to low pressure but parallel to the isobars. Examination
of an analysis chart will show that the surface wind does indeed flow nearly parallel to the
isobars and we will see that above the friction layer the wind, generally does flow parallel to
the isobars. There are two winds that we need to consider:

• The Geostrophic Wind

10
• The Gradient Wind

Winds
The Geostrophic Wind
As with any theorized wind or model wind, a number of assumptions must be made to reduce
the complexity of reality and make the model more simplistic. These are as follows:

• The Geostrophic Wind is said to have only two forces.

• These must be working opposite from each other and in balance.

These two forces are:

Pressure Gradient Force (PGF)

• P
ressure Gradient Force,
(PGF), is the force that acts
from a high pressure to a
low pressure.

• W
 e can see the strength
of this force by studying
the spacing between
isobars. Closely spaced
isobars would indicate a
large pressure gradient
force. This is common
in low pressure systems.
Widely spaced isobars
indicate a small pressure
gradient force. This is
common in high pressure
systems. Figure 11.5. Pressure
Figure 10.5 PressureGradient Force
Gradient Force (P.G.F.).
(PGF)

151
 10 Winds

• T
 he Pressure Gradient Force, (PGF), controls the wind speed. A large pressure gradient
force would create strong winds, whereas a small pressure gradient force would create light
winds. Wind speed is directly proportional to the pressure gradient force.

• T
 he relationship between the isobar spacing, the pressure gradient force and the wind
speed can be seen in the Geostrophic Wind Scale (GWS). Using Figure 10.6, take the distance
between two isobars and reading from left to right, measure the geostrophic wind speed
using the geostrophic wind scale shown at the bottom of the diagram. You will notice the
wider the spacing of the isobars, the lighter the wind.
10
Winds

Figure 10.6 Geostrophic wind scale

Coriolis Force (CF)

• Coriolis Force, (CF), is the force caused by the rotation of the earth.

• It acts 90° to the wind direction causing air to turn to the right or veer in the Northern
Hemisphere and to the left or back in the Southern hemisphere. CF is maximum at the
poles and minimum at the Equator.

Figure 10.7 An illustration of the Coriolis force

152
 Winds
10
• T
 he Coriolis force is not a true force but is an explanation of the effect the rotation of the
earth has on a free moving body not in contact with the earth. It is the combination of 4
factors:

CF = 2 Ω ρ V sin θ, where:

Ω = angular rotation of the earth

ρ = density
V = wind speed
θ = latitude

• It should be noted that the CF is directly proportional to both wind speed and latitude. So
an increase in either will result in an increase in the CF.

Geostrophic Wind

10
• T
 he Geostrophic Wind blows parallel to straight isobars. Therefore the geostrophic wind
can only blow in a straight line. If the wind were to follow a curved path, it cannot be

Winds
considered as a geostrophic wind because there will be additional forces involved, namely
the centrifugal or centripetal forces. The gradient wind (which will be discussed later) uses
the pressure gradient force, the Coriolis force and the centrifugal force. This is the model for
wind which follows a curved path.

• H
 ow can we know the direction of the geostrophic wind along the isobar? If you remember
from earlier lessons, Buys Ballot’s Law told us that in the Northern Hemisphere with your
back to the wind, the low pressure is to your left. In the Southern Hemisphere with your
back to the wind, the low pressure is on your right. Looking at the diagram below and by
using Buys Ballot’s Law, we can see a geostrophic wind direction of 180°.

Figure 10.8 Geostrophic wind direction in the Northern Hemisphere

153
 10 Winds

• How can we know the speed of the geostrophic wind? If you remember from earlier, there
was a correlation between the isobar spacing, the pressure gradient force and the wind
speed. The geostrophic wind scale allowed us to quantify this relationship. Measure the
distance perpendicular between the isobars and use that distance on the geostrophic wind
scale, reading from left to right.
10
Winds

Figure 10.9 Latitude corrected geostrophic wind scale

• T
 he geostrophic wind only blows above the friction layer. Within the friction layer
the wind speed is reduced because of surface friction. Therefore the Coriolis force will
reduce, causing the two forces to be out of balance. Remember that the friction layer
varies depending upon the nature of the surface and the time of the day. Therefore, the
height of the geostrophic wind will vary. Generally though it is considered to be between
2000 - 3000 ft.

• W
 ith the geostrophic wind the pressure gradient force is equal to the Coriolis force. So,
for the same PGF (or isobar spacing) as latitude increases the Coriolis force will remain
constant so for the same PGF as latitude increases, sine of latitude also increases and
hence the wind speed will decrease. This can be deduced from the geostrophic wind
scale on the chart above where a certain isobar spacing at 40°N gives a wind speed of
25 kt. The same isobar spacing at 70°N gives a speed of only 15 kt. (Note, for this type
of question the key is “same PGF“ or “same isobar spacing”).

PGF
V=
2 Ω ρ sinθ
So the effect of latitude must be accounted for when using the geostrophic wind scale. The
diagram below shows the geostrophic wind scale for latitude between 40° and 70°. Notice
that the same spacing between the isobars at high latitude gives a slower wind speed when
compared to lower latitude. Within 5 degrees of the Equator the CF is close to zero. Within 15
degrees the CF is very small, so that the geostrophic formula is no longer valid.

154
 Winds
10
Construction of the Geostrophic Wind
Look at the diagram below for the Northern Hemisphere. Air is being accelerated towards the
low pressure but in doing so, the strength of the Coriolis force is increasing. The wind is being
deflected to the right until the two forces are acting opposite from each other and the wind
now blows parallel to the isobar. With your back to the wind, the low pressure is on your left.

10
Winds
Figure 10.10 The geostrophic wind

Conditions Necessary for the Wind to Be Geostrophic

For the wind to be geostrophic, it has to occur:

• Above the friction layer.

• At a latitude greater than 15 degrees.
• When the pressure situation is not changing rapidly.
• With the isobars straight and parallel.

The geostrophic wind can apply at all heights above the friction layer. However, with an
increase in height, the wind speed should increase due to the reduction in density assuming all
other factors are unchanged.

The Gradient Wind

The gradient wind occurs when the isobars are curved. This brings into play a force which
makes the wind follow a curved path parallel to the isobars. The gradient wind then is the wind
which blows parallel to curved isobars due to a combination of 3 forces:

• PGF
• CF
• Centrifugal Force

155
 10 Winds

Centrifugal Force
Centrifugal force is the force acting perpendicular to the direction of rotation and away from
the centre of rotation.
10
Winds

Figure 10.11

Gradient Wind in a Depression

If air is moving steadily around a depression, then the centrifugal force opposes the PGF and
therefore reduces the wind speed.

Figure 10.12 Gradient wind speed around a depression (Northern Hemisphere)

The gradient wind speed around a depression is less than the geostrophic wind for the same
isobar interval. Hence if the Geostrophic Wind Scale (GWS) is used, it will overread.

156
 Winds
10
Gradient Wind in a High
In an anticyclone the centrifugal force is acting in the same direction as the PGF so increases the
magnitude of the PGF. Hence the wind speed will be greater than the equivalent geostrophic
wind speed.

The gradient wind speed around an anticyclone

is greater than the geostrophic wind for the
same isobar interval. Hence if the Geostrophic
Wind Scale (GWS) is used, it will underread.

As an example in a system where the radius

of curvature of the isobars is 500 NM and the
geostrophic wind speed is 40 kt, the speed
in a cyclonic system will be 34 kt and in an
anticyclonic system 58 kt.

10
It should be noted that when discussing the

Winds
gradient wind we are making a comparison
of the wind in a low pressure system to Figure 10.13 Gradient wind speed around a high
the equivalent geostrophic wind and, as a (Northern Hemisphere)
separate argument, comparing the wind in a
high pressure system with the equivalent geostrophic wind.

We are not comparing the wind speed in a low pressure system with the wind speed in a
high pressure system.

The Antitriptic Wind

The wind which blows in low latitudes where the CF is very small is called the antitriptic wind.

Winds below 2000 - 3000 ft (1 km).

Friction between moving air and the land surface will reduce wind speed near the ground. This
reduction also reduces the CF. This will cause the two forces in the geostrophic wind to be out
of balance since now CF is less than PGF. The wind is now called a surface wind.

Since surface friction has reduced the wind

velocity, resulting in a reduction in the Coriolis
force, the PGF is now more dominant. This
causes the wind to blow across the isobars
towards the low.

Figure 10.14 The surface winds in the Northern

Hemisphere

157
 10 Winds

Rough Rules
• O
 n average in the Northern Hemisphere the surface wind over land is backed by 30 degrees
from the geostrophic, or gradient wind direction and its speed is reduced by 50%. In the
Southern Hemisphere, because of the opposite effect of the Coriolis force, the surface wind
is veered from the 2000 ft wind, but the numerical values are the same.
10
Winds

Figure 10.15 An example of rough rules over land in the Northern

Hemisphere

• O
 ver the sea friction is very much less and the surface winds are closer to geostrophic values.
Surface wind over the sea, in the Northern Hemisphere, is backed by 10 degrees from the
geostrophic or gradient wind direction and speed reduced to 70% (surface winds will veer
in the Southern Hemisphere).

Figure 10.16 An example of rough rules over sea in the Northern Hemisphere

158
 Winds
10
Diurnal Variation of the Surface Wind
There can be a regular change in the surface wind in each 24 hour period. It veers and increases
by day reaching maximum strength about 1500 hrs. It backs and decreases thereafter with
minimum strength around 30 minutes after sunrise.

This diurnal variation is due to thermal turbulence which mixes the air at the surface with
air moving freely above. It is therefore most marked on clear sunny days, and particularly in
unstable air masses, with sunny days and clear nights.

10
Winds

Figure 10.17 Diurnal variation of the surface wind

159
 10 Winds

Diurnal Variation of 1500 ft and Surface Wind Velocity

• Figure 10.17 and Figure 10.18 show the effect of diurnal temperature variation on both the
1500 ft W/V and the surface W/V.
10
Winds

Figure 10.18 Diurnal variation of 1500 ft wind velocity

• By Day. Thermal currents are greater on sunny days and at 1500 hours. They will cause
interaction between the surface and the top of the friction layer. The 2000 ft W/V will with
descent be increasingly affected by the surface friction and will therefore steadily reduce in
speed and turn towards the low pressure. (Back in Northern Hemisphere or veer in Southern
Hemisphere).

• By Night. Thermal currents cease. The top of the friction layer effectively drops below
1500 ft where the W/V will assume 2000 ft direction and speed thus becoming faster and
veering (NH). The surface W/V no longer has interaction with the stronger wind above
and will therefore decrease and back (NH). Thus a marked windshear can occur between
1500 ft and the surface, affecting handling for example on an approach.

160
 Winds
10

Figure 10.19 Diurnal variation of 1500’ and surface wind velocities

10
• DV of surface wind aids the formation of radiation fog at night and early morning, and its
dispersal by day.

Winds
• Diurnal effect over the sea is small because DV of sea temperature is small.

Summary

DAY NIGHT

DECREASES INCREASES
1500 ft
BACKS VEERS

INCREASES DECREASES
SURFACE
VEERS BACKS

Figure 10.20 Summary of diurnal variation and surface

wind velocities in the Northern Hemisphere

Land and Sea Breezes

Sea breezes. On a sunny day, particularly in an anticyclone with a light PGF, the land will heat
quickly.

The air in contact will be warmed and will rise and expand so that pressure at about 1000 ft will
be higher than pressure at the same level over the sea. This will cause a drift of air from over
the land to over the sea at about 1000 ft. The drift of air will cause the surface pressure over
the land to fall, and the surface pressure over the sea to rise.

As a result there will be a flow of air from sea to land - a sea breeze.

161
 10 Winds

On average, sea breezes extend 8 to 14 NM either side of the coast and the speed is about
10 kt. In the tropics speed is 15 kt or more and the inland extent is greater.

The direction of the sea breeze is more or less at right angles to the coast, but after some time
it will veer under the influence of the Coriolis Force.

HIGH
1000 FEET LOW

W C
A O
R L
M D
LOW
10 - 15 KNOTS HIGH
10
Winds

10 -15 NAUTICAL MILES

Figure 10.21 The sea breeze

Figure 10.22 The influence of the Coriolis force on sea breezes over time (Northern
Hemisphere)

162
 Winds
10
Land breezes. From mid-afternoon the land is starting to cool and this process will accelerate
after sunset. Overnight the situation will reverse and pressure will now be higher on land than
over the sea as the temperature reduces. This will give rise to a wind now blowing from land
to sea, the land breeze. The land breeze can be expected within about 5 NM of the coastline
and with a maximum speed of about 5 kt.

LOW
HIGH
C
O W
L A
D R
M
HIGH

10
LOW
5 KNOTS

Winds
5 NAUTICAL MILES

Figure 10.23 The land breeze

Practical Coastal Effects

• T
 he direction of take-off and landing can be reversed with the change from sea to land
breeze. This is shown in Figure 10.24.

SEA BREEZE

NIGHT TIME

APPROACH
DIRECTION

LAND BREEZE

Figure 10.24 Reversal of direction of take-off and landing

163
 10 Winds

• F og at sea can be blown inland by day to affect coastal airfields. This is illustrated in Figure
10.25.

F
O
G SEA
B BREEZE
A
N
K
10

S
Winds

Figure 10.25 Fog being blown inland by the sea breeze

• T
 he lifting of air over land with the sea breeze can cause small clouds to form as shown in
Figure 10.26. These are a good navigational feature of coastline.

SEA BREEZE

Figure 10.26 Cloud formation over a coastline

164
 Winds
10
Valley or Ravine Winds
A wind blowing against a mountain is impeded. If the barrier is broken by a gap or valley, the
wind will blow along the valley at an increased speed due to the restriction. This is illustrated
in Figure 10.27.

10
Winds
Figure 10.27 A valley or ravine wind

With a valley wind, if there is a relatively small change in the general direction, it is possible for
the valley wind to reverse completely as shown in Figure 10.28. The combination of high wind
speed and rough terrain is likely to give rise to considerable turbulence at low level, landing at
airfields in such areas may be difficult.

Figure 10.28 Wind direction reversal in a valley or ravine

Examples of valley winds are the Mistral (Rhone Valley), (see Chapter 21) Genovese (Po Valley),
Kosava (Danube) and Vardarac (Thessalonika). Valley winds also occur in fjords.

165
 10 Winds

Venturi Effect
The increase in speed as the wind flows through a valley will cause the Venturi Effect with
the consequent reduction in pressure which will result in the true altitude being less than the
indicated altitude. The same effect may be experienced above a mountain range as the wind
blows over the range, particularly in stable conditions.

Katabatic Winds
A katabatic wind is caused by a flow of cold air down a hill or mountain side at night.

If the side of the mountain is cooled by radiation, the air in contact is also cooled, it will thus be
denser and heavier than the surrounding air and it will therefore flow down the mountain side.

The katabatic effect is most marked if the mountain side is snow covered, if the sky is clear to
assist radiation and if the PG is slack. Speeds average 10 kt and the flow of cold air into the
10

valley helps frost and fog to form. Another effect is that with the sinking of cold air down the
slope, the air at higher levels will be warmer and an inversion results. The katabatic effect can
Winds

also occur by day when relatively warm air comes into contact with snow covered slopes. A
katabatic wind is shown in Figure 10.29.

Figure 10.29 Katabatic wind formation

An example of a katabatic wind is the Bora in the Northern Adriatic (see Chapter 21).

166
 Winds
10
Anabatic Winds
On a warm sunny day, the slope of a hill will become heated by insolation, particularly if it is a
south facing slope.

The air in contact with the ground will be heated by conduction and will rise up the hill. Free
cold air will replace the lifted air and so a light wind will blow up the hillside. An anabatic wind
is a light wind of around 5 kt which blows up a hill or mountain by day as illustrated in Figure
10.30.

CLOUDS

AIR WARMED BY
CONDUCTION

10
Winds
COLD FREE
AIR
REPLACES
LIFTED AIR

Figure 10.30 Anabatic wind formation

Föhn Winds
The Föhn Wind is a warm dry wind which blows on the downwind side of a mountain range.
It is a local wind in the Alps. A similar wind on the east of the Rocky Mountains in Canada is
called the Chinook (see Chapter 21). There is also the Zonda to the east of the Andes in South
America.

When moist air is forced to rise up a mountain in stable conditions it will cool adiabatically at
the DALR until saturated when it will continue cooling at the SALR. Precipitation will occur
removing water from the air so the dew point will decrease.

When the air descends on the leeward (downwind) side the cloud base will be higher so the air
will warm at the DALR over a greater height than it cooled at the SALR on the windward side.
Consequently the temperature at the base of the mountain will be greater on the downwind
side than it was on the upwind side.

So, on the windward side we can expect low cloud and precipitation whilst on the leeward side
we will see clear turbulent conditions.

167
 10 Winds

The result is a warm, dry wind blowing on the downside of the mountain. Temperature
increases in excess of 10°C may occur. The presence of a Föhn wind could also indicate the
presence of mountain waves.

Föhn winds can occur over the east and west coasts of Scotland when moist winds come over
the highlands off the Atlantic Ocean or North Sea.

STABLE AIR
8000'
-0.8°
PRECIPITATION CLOUD BASE
+1° +1°
7000'
+2.8° 6000' +2.8° DEW POINT
+4°
10

CLOUD BASE +4.6° 5000' +5.8°

Winds

+6.4° 4000' +8.8°

+8.2° 3000' +11.8°

DEW POINT +10°
+10° 2000' +14.8°

+13° 1000' +17.8°

+16° +20.8°
GROUND LEVEL

Figure 10.31 The Föhn effect

168
 Questions
10
Questions
1. In central Europe, where are the greatest wind speeds?

a. Tropopause level
b. 5500 m
c. Where the air converges
d. Above the Alps

2. Standing in the Northern Hemisphere, north of a polar frontal depression travelling

west to east, the wind will:

a. continually veer
b. continually back
c. back then veer
d. veer then back

10
3. ATC will only report wind as gusting if:

Questions
a. gust speeds exceeds mean speed by >15 kt
b. gusts to over 25 kt
c. gusts exceeds mean speed by 10 kt
d. gusts to over 25 kt

4. What is a land breeze?

a. From land over water at night

b. From land over sea by day
c. From sea over land by night
d. From sea over land by day

5. When heading south in the Southern Hemisphere you experience starboard drift:

a. you are flying towards a lower temperature

b. you are flying away from a lower temperature
c. you are flying towards a low pressure
d. you are flying out of a high

6. What are the factors affecting the geostrophic wind?

a. PGF,r, q, Ω
b. r, q, Ω
c. r, q, PGF
d. r, PGF, Ω

7. What is the Bora?

a. Cold katabatic wind over the Adriatic

b. Northerly wind blowing from the Mediterranean
c. Warm anabatic wind blowing to the Mediterranean
d. An anabatic wind in the Rockies

169
 10 Questions

8. Flying away from an area of low pressure in the Southern Hemisphere at low
altitudes, where is the wind coming from?

a. Right and slightly on the nose

b. Left and slightly on the tail
c. Left and slightly on the nose
d. Right and slightly on the tail

9. What causes the geostrophic wind to be stronger than the gradient wind around a
low?

a. Centrifugal force adds to the gradient force

b. Centrifugal force opposes the gradient force
c. Coriolis force adds to the gradient force
d. Coriolis force opposes the centrifugal force

10. A METAR for Paris gave the surface wind at 260/20. Wind at 2000 ft is most likely to
be:
10

a. 260/15
Questions

b. 210/30
c. 290/40
d. 175/15

11. A large pressure gradient is shown by:

a. closely spaced isobars - low temperature

b. distant spaced isobars - high temperature
c. close spaced isobars - strong winds
d. close spaced isobars - light winds

12. Where would you expect to find the strongest wind on the ground in temperate
latitudes?

a. In an area of Low pressure

b. In an area of High pressure
c. In the warm air between two fronts
d. In a weak anticyclone

13. At a coastal airfield, with the runway parallel to the coastline, you are downwind
over the sea with the runway to your right. On a warm summer afternoon, what
would you expect the wind to be on finals?

a. Crosswind from the right

b. Headwind
c. Tailwind
d. Crosswind from the left

14. What causes wind at low levels?

a. Difference in pressure
b. Rotation of the earth
c. Frontal systems
d. Difference in temperature

170
 Questions
10
15. If flying in the Alps with a Föhn effect from the south:

a. clouds will be covering the southern passes of the Alps

b. CAT on the northern side
c. wind veering and gusting on the northern side
d. convective weather on the southern passes of the Alps

16. Comparing the surface wind to the 3000 ft wind:

a. surface wind veers and is less then the 3000 ft wind

b. surface wind blows along the isobars and is less than the 3000 ft wind
c. surface wind blows across the isobars and is less than the 3000 ft wind
d. both are the same

17. 90 km/h wind in kt is approximately:

a. 70
b. 60

10
c. 50
d. 30

Questions
18. The geostrophic wind blows at your flight level in Northern Hemisphere and the
true altitude and indicated altitude remain constant. The crosswind is:

a. from the left

b. from the right
c. no crosswind
d. impossible to determine

19. With all other things being equal with a high and a low having constantly spaced
circular isobars, where is the wind the fastest?

a. Anticyclonic
b. Cyclonic
c. Where the isobars are closest together
d. Wherever the PGF is greatest

20. Föhn winds are:

a. warm katabatic
b. cold katabatic
c. warm descending winds
d. warm anabatic

21. What is the effect of a mountain valley wind?

a. It blows down a mountain to a valley at night

b. It blows down a mountain to a valley during the day
c. It blows from a valley up a mountain by day
d. It blows from a valley up a mountain at night

171
 10 Questions

22. What is the difference between gradient and geostrophic winds?

a. Difference in temperatures
b. A lot of friction
c. Curved isobars and straight isobars
d. Different latitudes and densities

23. What prevents air from flowing directly from a high to a low pressure?

a. Centripetal force
b. Centrifugal force
c. Pressure force
d. Coriolis force

24. What is the relationship between the 5000 ft wind and the surface wind in the
Southern Hemisphere?

a. Surface winds are veered from the 5000 ft and have the same speed
10

b. Surface winds are backed from the 5000 ft and have a slower speed
c. Surface winds are veered from the 5000 ft and have a slower speed
Questions

d. Surface winds are backed from the 5000 ft and have a faster speed

25. What is the relationship between the 2000 ft wind and the surface wind in the
Northern Hemisphere?

a. surface winds blow across isobars towards a high

b. surface winds blow parallel to isobars
c. surface winds blow across isobars towards a low
d. surface winds have laminar flow

26. Wind is caused by:

a. mixing of fronts
b. horizontal pressure difference
c. earth rotation
d. surface friction

27. For the same pressure gradient at 50N, 60N and 40N, the geostrophic wind speed
is:

a. greatest at 60N
b. least at 50N
c. greatest at 40N
d. the same at all latitudes

28. The wind in the Northern Hemisphere at the surface and above the friction layer at
2000 ft would be:

a. veered at the surface, veered above the friction layer

b. backed at the surface, veered above the friction layer
c. veered at the surface, backed above the friction layer
d. backed at the surface, backed above the friction layer

172
 Questions
10
29. Where are easterly and westerly jets found?

a. Northern Hemisphere only

b. Southern Hemisphere only
c. Northern and southern Hemisphere
d. There are no easterly jets

30. In high pressure systems:

a. the winds tend to be stronger in the morning

b. the angle between the isobars and the wind direction is greatest in the
afternoon
c. the winds tend to be stronger at night
d. the winds tend to be stronger in early afternoon

31. An aircraft is flying East to West in the Northern Hemisphere. What is happening to
its altitude?

10
a. Flying into a headwind will decrease altitude
b. If the wind is from the south, it will gain altitude

Questions
c. If the wind is from the north, it will gain altitude
d. Tailwind will increase altitude

32. Where would an anemometer be placed?

a. Close to station, 2 m above ground

b. On the roof of the station
c. 10 m above aerodrome elevation on a mast
d. Next to the runway, 1 m above ground

33. Which of the following is an example of a Föhn wind?

a. Bora
b. Harmattan
c. Chinook
d. Ghibli

34. Wind at altitude is usually given as …….. in ……..

a. true, m/s
b. magnetic, m/s
c. true, kt
d. magnetic, kt

35. If you fly with left drift in the Northern Hemisphere, what is happening to surface
pressure?

a. Increases
b. Decreases
c. Stays the same
d. Cannot tell

173
 10 Answers

Answers
1 2 3 4 5 6 7 8 9 10 11 12
a b c a b a a c b c c a
13 14 15 16 17 18 19 20 21 22 23 24
a a a c c c a c a c d c
25 26 27 28 29 30 31 32 33 34 35
c b c b a d c c c c a
10
Answers

174