METEOROLOGY

PHASE 1

Contents:

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REFERENC PAGE
DESCRIPTION
E NO. NO

Basic Knowledge of the characteristics

1.0
of the various weather systems

1.1 Ship-borne meteorological instruments 03

1.2 The atmosphere, its composition and physical properties 10

1.3 Cloud and precipitation 22

1.4 Atmospheric pressure 29

1.5 Wind 34

1.6 Visibility 47

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LESSON 1 - Basic Knowledge of the characteristics of the various weather
systems

1.1 Atmospheric Pressure

The pressure in the Earth's atmosphere is known as atmospheric pressure.

The atmospheric pressure is the force exerted at any given point on the Earth’s surface by
the weight of the air above that point. (weight of the column of air above a point)

Why Atm.Pressure Changes with height?

1. Length of the air column decreases. Therefore, weight of the air column decreases.

Rate of pressure drop?

 It varies with altitude

 when altitude increases, the amount of gas molecules in the atmosphere decreases
(air becomes less dense).

Rate of drop of atmospheric Pressure near the earth’s surface 1mb per 10m.

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Units of Atmospheric Pressure

The standard atmosphere is 1 atm

 1 atm = 1,013.25 hectopascals (hPa)
 1 atm = 760 millimeters of mercury (mmHg)
 1 atm = 1,013.25 millibars (mbar)

Instruments that measure Atmospheric Pressure

a) Aneroid Barometer
b) Precision Aneroid Barometer
c) Mercury Barometer

Basic principle of an aneroid barometer

a. The Aneroid barometer

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Principal :-

 A Partially evacuated capsule's shape changes as the current pressure changes.

Pressure Change Shape Change

 A lever system magnifies the capsule's movement.

 A metal spring holds a fine chain that connects this to the pointer spindle in tension.

Construction:-
 All of the components of the capsule spring lever and pointer mechanism are fastened to a
strong rear plate.
 Front panel is glass.
 Spring tension changes the reading.

Operation:-
 Normally installed on a vertical bulkhead, the aneroid barometer.
 Before reading, tap the glass screen to release any trapped fine chain air around the spindle
pulley.
 The majority of aneroid barometers are temperature compensated.

Corrections:-
1. Index error
2. Height correction (1mb for 10m)

Q1. Pressure reading on Aneroid Barometer placed on the bridge is 1022.5mb. Height of the
bridge from the sea level is 20m. Index error of the barometer is +0.1mb. Calculate the
atmospheric pressure at sea level.

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b. Precision Aneroid Barometer

 Better accuracy than Aneroid Barometer

 Readings down to 0.1 mb can be taken
 The PAB uses a lever system and an electronic circuit
 A cathode ray tube (CRT) is used to measure the pressure
 The operating knob, which operates the micrometer screw and the digital counter, is
fastened to gear wheels.

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Construction
 The instrument is placed inside a metal casing
 the instrument is placed on a vertical bulkhead. Bracket.
 the release of pressure caused by the CRT's (Cathode Ray Tube) creating and
stopping of the signal light.

Operation:-

 Pushing the operating button activates the electronic circuit.

 To continue "making & breaking" (joins and parts),

the operational knob is then adjusted in the appropriate detection until the CR indicator
appears. The reading is then taken from the digital counter.
Corrections:-
1.Height correction (1mb for 10 m)

c. The basic principle of a mercurial barometer

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  A portion of the mercury flows out of a one-meter-long, open-ended glass tube into the
container when it is placed upside down into another glass tube that is also loaded with
mercury.
 The mercury level stabilizes at around 76 cm from the mercury level in the container as a
"Torricellian vacuum" is created at the top of the glass tube.
 A height of this nature indicates the atmospheric pressure at the ambient level, according
to Torricelli's experiment. The idea behind a mercury barometer is to precisely measure
this height and use that information to calculate atmospheric pressure.

1.2 Temperature and Humidity

Temperature is a measure of the warmth or coldness of an object or substance with reference to

some standard value.

SI Unit of Temperature

The SI unit of temperature as per the International System of Units is Kelvin which is
represented by the symbol K. The Kelvin scale is widely accepted or used in the field of science
and engineering. However, in most parts of the world, Celsius or Fahrenheit scale is used for
measuring temperature.

Temperature Conversion

Celsius, Fahrenheit and Kelvin are the three common temperature scales. Each of the scales
has its uses, so it is likely that you will encounter them and would require you to convert between
them. In the table below, we have listed the conversion formula.

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RELATIVE HUMIDITY % = AMOUNT OF WATER VAPOUR IN THE ATMOSPHERE X 100
MAXIMUM AMOUNT IT CAN HOLD AT THAT TEMPERATURE

Humidity is the amount of water vapor in the air. If there is a lot of water vapor in the air, the humidity
will be high.

Humidity can be categorized in to

1.Absolute Humidity

2. Relative Humidity

1. Absolute Humidity

Absolute humidity is the measure of water vapor (moisture) in the air,

regardless of temperature. It is expressed as grams of moisture per cubic
meter of air (g/m3).

 The maximum absolute humidity of warm air at 30°C is approximately 30g of water vapor –
30g/m3.
 The maximum absolute humidity of cold air at 0°C is approximately 5g of water vapor –
5g/m3.

2. Relative Humidity

 Is a ratio, expressed in percent, of the amount of atmospheric moisture present relative to the

maximum amount that it could hold at that temperature.

 relative humidity is a function of both moisture content in the air and temperature.

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Instruments that measure Relative Humidity

a. Hygrometer & whirling’s Psychrometer

Construction:-

The "dry" and "wet" bulb thermometers are inserted into the hygrometer's wooden slatted screen,

 Wick and muslin, a very fine plain-weave cotton fabric, are used to keep a small layer of water
surrounding the moist bulb in a container.
 Typically, the screen is situated far from sources of heat from the outside.

Dry Bulb and Wet Bulb Thermometer

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 1. The Water evaporation from the thermometer wick will depend on the humidity in the
atmosphere. (ie High Humidity - less evaporation, Low humidity - more evaporation)
2. Evaporating water from the wet bulb wick will absorb heat from the thermometer thus
reducing the temperature reading on the wet bulb.

b. Whirling’s Psychrometer

Principal:-

Psychrometer:
Swivel instrument (rotate) at a steady rate for least 1 minute & get the reading.

Dew point

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The dew point is the temperature at which the water vapor in air at constant barometric
pressure condenses into liquid water at the same rate at which it evaporates. At
temperatures below the dew point, water will leave the air. The condensed water is called
dew when it forms on a solid surface.

Saturated air
The relative humidity rises as the unsaturated air is cooled. The temperature eventually rises
to the point where the relative humidity is 100%. then the air is said to as saturated. The dew
point is supposedly attained.
More cooling causes the extra water vapour to condense. Dew is the term for the
condensed moisture. This usually happens at night when radiation cools the earth's
atmosphere.
The relative humidity drops as the air temperature rises. This is due to the fact that there is
currently less water vapor in the air than what may be expected at this temperature.

1.3 Wind speed and Direction measurement

Anemometer:-

1. A direction arrow
2. Revolving cups
Thermometers that transmit data to analog instruments are 3 and 4.

An instrument used to gauge wind speed and direction is the anemometer.

Anemometer readings on a ship will only provide the "RELATIVE" wind

direction and speed if the V/L is making way; consequently, a triangle of forces
must be built to determine the actual direction and speed.

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The “rotating cups” generates a current which is measured by an analogue
instrument to obtain the speed.

The “direction Vane” is fitted with a sensing coil which transmits the information to
another dial.

Eg:- A vessel steaming on a course of 000 0 Tr. at 15 Kts. The anemometer shows a
relative wind speed of 22 kts., and a direction of 035 O Tr . Find true speed &
direction of the wind.

Work out / scale: 1Kt = 0.5 cm

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2.The atmosphere, its composition and physical properties

The atmosphere

The atmosphere is a layer of gases surrounding our planet, kept in place by its own weight under
gravity

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.

typical vertical temperature profile through the lower 100 km of the earth’s
atmosphere

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The main features of the troposphere

Troposphere:

The lowest layer of the atmosphere on Earth is called the troposphere, and it is where most
of the water vapor and aerosols are found.

 •In the middle latitudes, the troposphere is typically 11 km (7 miles) deep. It is deeper
in the poles (around 7 km (4 miles) in summer, indistinct in winter) and shallower in
the tropics (up to 20 km (12 miles)).
 As altitude rises, the troposphere's temperature often drops.
 The tropopause, which is defined by stable temperatures, is the border above the
troposphere..
 The troposphere is denser than the layers of the atmosphere above it (because of
the weight compressing it), and it contains up to 75% of the mass of the atmosphere.
It is primarily composed of nitrogen (78%) and oxygen (21%) with only small
concentrations of other trace gases.
 The troposphere is the layer where most of the world's weather takes place. Fake
 Air temperature then begins to rise in the stratosphere.

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Stratosphere: The atmosphere is steady between 10 and 20 kilometers. The tropopause is
the name given to this area. The stratosphere extends from 20 to around 50 kilometers. The
air here really becomes warmer the higher you go! This region of the atmosphere has a high
concentration of ozone, which absorbs UV radiation from the Sun. In comparison to the
lower stratosphere, more light is absorbed at higher altitudes, raising the temperature.

Mesosphere: However, at 50 kilometers, an area known as the stratopause causes the

temperature to stabilize once more. The mesosphere starts at a distance of around 55
kilometers. Because there is little ozone to warm the air in the mesosphere, the temperature
drops with height once more.

Thermosphere: The thermosphere, or region of the atmosphere above 90 km, is separated

from the mesosphere by the mesopause. The temperature rises once more in this area! This
time, the temperature increase is brought on by molecular oxygen (O2). Since there is so
little air in the thermosphere, the oxygen absorbs light from the Sun, and even a small amount of
absorption can have a significant impact.

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 3. Insolation
The solar radiation that reaches the surface of the earth is known as insolation.
It is measured by the quantity of solar energy received per square centimeter every minute
and is in short wave form. Temperature is affected by insolation. The temperature rises as
insolation increases. The strongest insolation is experienced at noon on any given day.

Factors affect insolation (without the effect of the atmosphere):

 Angle of the sun

The sun angle is the angle of incidence at which sunlight strikes the Earth at a particular
location, time of day, and season. The amount of thermal energy received at this location on
the planet is controlled by the rotation of the Earth and its orbit around the sun, which also
affects the climate.

The seasonal fluctuation in the angle of sunlight caused by the tilt of the Earth's axis is the
main factor that influences both the warmth of the weather and the length of the day.

 Duration of daylight

The longer the duration of daylight, the more the insolation received.

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What happens to the radiation from the sun

Assume that 100 solar energy units enter the upper atmosphere of the earth. Averagely, the
atmosphere and clouds absorb 19 units of solar radiation, while the earth, clouds, and
atmosphere all reflect and scatter 30 units back to space. This leaves 51 units of direct and
indirect (diffuse) solar radiation to be received at the planet's surface.

Long wave radiation that the planet emits back into space is known as terrestrial radiation.
Less than ten percent of it is radiated into space directly, with the majority being absorbed by
the water vapor in the atmosphere.

The term "greenhouse effect" describes situations in which the sun's short visible light
wavelengths pass through a transparent substance and are absorbed, The atmospheric
greenhouse effect occurs because water vapor, CO2, and other trace gases are selective
absorbers. They allow most of the sun’s radiation to reach the surface, but they absorb a
good portion of the earth’s outgoing infrared radiation, preventing it from escaping into space
.It is the atmospheric greenhouse effect, then, that keeps the temperature of our planet at a
level where life can survive. The greenhouse effect is not just a “good thing”; it is essential to
life on earth.

Earths Seasons

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One of the main divisions of an Earthly year is a season. A seasonal shift in the weather
occurs every year. The Earth's axis is tilted with respect to the Sun, which causes seasons.
The four seasons of the year—spring, summer, autumn (fall), and winter—are a result of this
tilting. Due to the tilt of the axis, different regions of the Earth are positioned to receive more
direct sunlight at certain times of the year.

Northern Hemisphere Southern Hemisphere

From To From To
Spring March 20th June 21st Sept.22nd Dec. 21st
Summer June 21st Sept.22nd Dec. 21st March 20th
Autumn Sept.22nd Dec. 21st March 20th June 21st
Winter Dec. 21st March 20th June 21st Sept.22nd

Atmospheric water vapor

 The gas phase of water is called water vapour.

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  Water vapour continuously evaporates and condenses in a typical atmosphere.
 On average, there are between 1 and 3 percent of water vapor in the atmosphere.

Transpiration - the process by which plants give off water vapor through their leaves.
Evaporation – It is the process by which an element or compound transitions from
its liquid state to its gaseous state below the temperature at which it boils; in particular, the
process by which liquid water enters the atmosphere as water vapour in the water cycle.
Sublimation - conversion of a substance from the solid to the gaseous state without its
becoming liquid.

The Hydrological Cycle

There are three main stages in the hydrological cycle and they are as follows.
1) Evaporation.
2) Condensation.
3) Precipitation.

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Evaporation
Water turns into water vapour during evaporation. If the air is warm and dry (has a low
relative humidity), more evaporation.
Huge amounts of water-vapour evaporate daily from sea surface.

Latent heat from the surrounding air and the water's surface is taken in during evaporation.
The energy needed for tropical thunderstorms and spinning storms is provided by this latent
heat.

Condensation
Water vapour turns into water during condensation. Evaporation is the opposite of it. Latent
heat is released to the atmosphere during condensation.
If air is cooled below its dew point, condensation will happen. Contact with a cold surface of
land or sea frequently causes this. When air rises, adiabatic cooling occurs. When the dew
point is reached condensation occurs.

On surface
If contact with cold land surfaces results in condensation, dew is created and left as a
deposit on the surface. However, if there is a light wind blowing over the chilly ground
surface or if condensation is brought on by any of the other factors.

Precipitation
The term "precipitation" refers to the water flakes that emerge from a cloud and fall to the
ground. The water drops may turn into soft ice (snow) or hard ice (hail) as they pass through
various layers of the atmosphere.
The water droplets in mist and fog do not fall but rather remain suspended in the air, they
are not classified as precipitation.

4. Clouds

When an air parcel rises the energy is released into the environment, which causes the air to
cool "adiabatically." The cooling of air makes the evaporated moisture to condense. The
condensed water droplets make clouds.

condensation nuclei

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The condensation of evaporated water droplets occurs around condensation nucleus.
Cloud condensation nuclei can come from both natural and man- made sources. In nature,
microorganisms, salt from sea spray, and volcanic dust all serve as sources of cloud
condensation nuclei. Burning fossil fuels and other industrial sources also contribute to the
emission of artificial chemicals into the atmosphere by humans. For instance, consider
photochemical haze.

a cloud can consist of ice crystals, super-cooled water droplets, water droplets or any
combination of these

Ice crystals and supercooled droplets coexist in "cold" clouds, which have a temperature
below zero degrees Celsius. When it comes to liquid water, the air in these so-called "mixed"
clouds is almost saturated, but when it comes to ice, it is super-saturated. As a result, ice
crystals grow much more quickly from the vapour phase than the adjacent droplets do in
mixed clouds.

The majority of clouds are made up of supercooled water droplets when the temperature is
between 0°C and -15°C.

Most clouds contain a mixture of ice crystals and supercooled water droplets when the
temperature is between -15°C and -40°C.

Nearly all clouds below -40°C are made completely of ice crystals.

The basic cloud types

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1. Cirrus: High, silvery clouds that resemble fibers or feathers. They always have a
background of blue sky since they are so far above, and at dusk they frequently
seem bright red or yellow. Cirrus can only be seen through its effect on the stars'
fading during the night.
2. Cirrostratus: A thin, lofty, whitish curtain that gives the sun or moon a watery
appearance. It is possible to use a sextant to measure the height because the sun
and moon's outlines are clearly visible. Cirrostratus clouds are frequently the cause
of haloes.

5. Cirrocumulus: A thick layer of white clouds that resemble little flakes or cauliflowers
and have no deep shadows between them.

6. Altostratus: A thin greyish or bluish cloud layer that gives the sun or moon the
appearance of being seen through frosted glass since it makes them appear very
dim. The sun's and moon's silhouettes are too blurry to use a sextant to determine
their altitude. Haloes are not produced by altostratus. Variations in the veil's
thickness may cause dark, shadow-like regions to be visible.
7. Altocumulus: Clouds in patch, layer or sheet form, white or grey or both in colour.
Have dark shadows in between and have the appearance of small flattened globules

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 or rolls or long bands or almonds, all of which may be in regular patterns aligned in
one, or sometimes two, directions.

8. Stratus: A low, even layer of dark grey cloud with light and dark patches. It has a dry
look and does not cause precipitation. It resembles fog, but is not experienced at sea
level. It can obscure the sun completely and can greatly weaken daylight. It should
not be confused with nimbostratus, which is described below. Small patches of
stratus, spread out, are referred to as fracto-stratus.

9. Nimbostratus: A low, even layer of dark-grey cloud generally uniform and threatening
in appearance with no light coloured patches. It has a wet look. If precipitation takes
place, it is continuous, not intermittent. Nimbostratus is usually formed by gradual
thickening and lowering of a layer of altostratus. It can completely obscure the sun
and greatly weaken daylight.

10. Stratocumulus: clouds consisting of a layer or patches of globular masses which

appear soft. They are grey in colour with dark shadows. The patches generally align
themselves in regular patterns in one, or sometimes two, directions. The patches
often join together and form an overcast sky, but are distinguishable from stratus by
its wavy or linear appearance.

11. Cumulus: Brilliant white, thick clouds with flat bases and rounded cauliflower-like
tops. Dark shadows are usually seen in them. The outline of each such cloud is very
clear cut. Cumulus clouds may be in small patches with ragged edges and little
vertical extent called fair weather cumulus or fracto-cumulus, or they may have very
great vertical extent called towering cumulus. Precipitation, if any, caused by even,
well developed cumulus, is light.

12. Cumulonimbus: Mass of grey, heavy cloud having its base in low cloud level, of great
vertical extent, with its top well in high cloud level. It has a threatening appearance
and is called a thundercloud. The top of a well-developed cumulonimbus cloud will

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 have attached to it, a cap of white cirrus cloud in the shape of an anvil. It is then
called anvil-shaped cumulonimbus. Its base resembles that of nimbostratus.

Rain: Water drops larger than 0.5 mm diameter. Termed heavy or light depending on
intensity of precipitation.
Clouds: Ns, As, Sc, Ac, Cu, Cb.( Nimbostratus , Altostratus ,
stratocumulus ,Altocumulus ,cumulus ,Cumulonimbus)

Drizzle: Fine drops of water, diameter less than 0.5 mm. Termed heavy or light depending
on intensity of precipitation.
Clouds: St, Sc.( Stratus , stratocumulus ,) – low clouds – between sea level to 2 km

Hail: Balls of hard ice, 0.5 to 50 mm diameter or more.

Clouds: Cb.

Snow flakes: Loose clusters of ice crystals, in very soft, small particles having branches.
Clouds: Ns, As, Sc, Cb.

Snow pellets: White opaque grains of ice, very soft and spherical or conical in shape,
diameter between 2 and 5mm. Clouds: Cb in cold weather.

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Sleet: Sometimes rain and snow fall together or partly melted snowflakes fall. This is called
sleet and is common in the U.K. Clouds: Same as for snowflakes.

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5. Wind

Wind is the horizontal movement of air. Wind occurs due to difference in atmospheric
pressures.

Atmospheric pressure is not equal on the earth’s surface. To learn about the difference in
atmospheric pressure we have to learn about isobars.

Isobar

A line drawn on a weather map connecting points of equal pressure is called an "isobar".
Isobars are generated from mean sea-level pressure reports and are given in mill bars.
Isobars are usually plotted in the metric system of mill bars.

The diagram below depicts a pair of isobars. At every point along the top isobar, the
pressure is 996 mb and at every point along the bottom isobar the pressure is 1000 mb.

Any point in between these two isobars will have a pressure somewhere between 996 mb
and 1000 mb. Point A, for example, has a pressure of 998 mb and is therefore located
between the 996 mb isobar and the 1000 mb isobar.

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29
The Pressure Gradient Force

Is the decrease in atmospheric pressure per unit of horizontal distance, shown on a synoptic
chart by the spacing of the isobars.

 The spacing of isobars, lines drawn on maps that connect locations of equal air
pressure, illustrates the pressure-gradient force, which is the main force that propels
wind and derives from pressure variations that occur over a specific distance.
 The distance between isobars represents the pressure gradient, or the amount of
pressure change that takes place over a specific distance.
 When isobars are closely spaced, there is a sharp pressure gradient and powerful
winds; when they are widely apart, there is a weak pressure gradient and low winds.
 In what is known as hydrostatic equilibrium, there is also an upward-directed vertical
pressure gradient that is typically balanced by gravity.
 Slow downward airflow occurs when the gravitational force only barely outweighs the
vertical pressure gradient force.
 --air flows from high pressure to low pressure
--wind speed depends on “steepness” of pressure gradient
--isobar spacing shows steepness of pressure gradient

The pressure gradient force is always directed at right angles to the isobars.

PGF = Pressure Difference

Distance

The Coriolis (Geostrophic) Force

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The Coriolis Force arises due to the fact that the earth is rotating...

Properties of the Coriolis Force:

 acts on objects not rigidly attached to the earth

 always acts to deflect an object to the right (left) of its direction of motion in the
northern (southern) hemisphere
 magnitude is zero at the equator, maximum at the poles
 if the earth were not rotating, the coriolis force would be zero
 the coriolis force increases with speed
 the coriolis force is not that large for slow-moving objects or for those moving over
short distances

The coriolis force is an "apparent" force that arises solely due to the fact that the earth is
rotating. Therefore, it can only change a parcel's direction, it CAN NOT affect its speed.

Centrifugal Force

The apparent force that a rotating body experiences that is equal to and opposes the
centripetal force and is brought on by the body's inertia. a phenomenon where a moving
object appears to be pushed away from the curve's center. The fact that a moving item has a
natural propensity to proceed in a straight line means that centrifugal force is not a genuine
force; rather, it is an effect of inertia.

Centri fugal force

Friction

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In terms of forces, friction is a crucial factor in meteorology. Friction modifies the wind's
direction in addition to reducing wind speed. The wind is slowed down by the frictional force.
The air/land/water interface is in close proximity to one another at the earth's surface.
Friction is caused by the air rubbing against the earth's surface. Given that land surfaces are
rougher and more vertically textured than water surfaces, friction is greater over land
surfaces. The effect of friction is only significant close to the Earth's surface. The effects of
friction become less noticeable as one ascends higher into the sky. Winds generally follow a
geostrophic pattern above 1500 meters, where they are balanced between the pressure
gradient and Coriolis Force.

The surface wind circulation around high- and low-pressure center

Air moves from a high pressure area to a low pressure area to create wind.
However, due to the Earth's rotation, the air is deflected to the right in the Northern
Hemisphere; to the left in the Southern Hemisphere, rather than directly from high to low
pressure.
As a result, the wind primarily blows around the high and low pressure areas.

For highly big and persistent pressure systems, the effect of the wind "feeling the Earth turn
underneath it" is crucial. The wind will move straight from high pressure to low pressure for
small, transient systems (like the chilly outflow of a thunderstorm).

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 The "pressure gradient" and winds increase in strength as the high and low pressure zones
come closer to one another. On weather maps, lines of constant pressure are depicted as
"isobars" (as in the aforementioned example). These isobars are often identified with their
millibar (mb) pressure value. The wind will be stronger the closer these lines are together.

The wind speed also depends on the curvature of the isobars. If the isobars are curled
anticyclonically (around the high pressure in the case above) with the same pressure
gradient (isobar spacing), the wind will be greater. The wind will be weaker if the isobars are
bent cyclonically (around the low pressure in the case above).

The ground's friction slows the wind down close to the Earth's surface. This effect is reduced
during the day when convective mixing is agitating the lower atmosphere. However, at night,
once convective mixing has ceased, the surface wind may significantly decrease or even
cease.
One way the atmosphere circulates more heat is through wind. Wind is primarily formed to
help move heat away from the Earth's surface, where sunlight causes an excess of energy
to build up, or from warm regions (often the tropics) to cooler regions (typically higher
latitudes).

Buys Ballot Law

In the Northern Hemisphere if you stand with your back to the wind • Low pressure is to the
left • High pressure is to the right • Opposite effect in the Southern Hemisphere.

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Beaufort Windscale
mph knots

0 0-1 0-1 Calm Sea like a mirror

1 1-3 1-3 Light air Ripples with the appearance of

scales are formed, but without
foam crests.

2 4-7 4-6 Light Breeze Small wavelets, still short,

but more pronounced. Crests
have a glassy appearance and
do not break.

3 8-12 7-10 Gentle Breeze Large wavelets. Crests begin

to break. Foam of glassy
appearance. Perhaps scattered
white horses.

4 13-18 11-16 Moderate Breeze Small waves, becoming larger;

fairly frequent white horses.

5 19-24 17-21 Fresh Breeze Moderate waves, taking a more

pronounced long form; many
white horses are formed.
Chance of some spray.

6 25-31 22-27 Strong Breeze Large waves begin to form; the

white foam crests are more
extensive everywhere.
Probably some spray.

7 32-38 28-33 Near Gale Sea heaps up and white foam

from breaking waves begins to
be blown in streaks along the
direction of the wind.

8 39-46 34-40 Gale Moderately high waves of greater

length; edges of crests begin to
breakinto spindrift. The foam is
blown in well-marked streaks
along the direction of the wind.

9 47-54 41-47 Severe Gale High waves. Dense streaks of

foam along the direction of the
wind. Crests of waves begin to
topple, tumble and roll over.
Spray may affect visibility.

10 55-63 48-55 Storm Very high waves with long over-

hanging crests. The resulting
foam, in great patches, is blown
in dense white streaks along the

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 direction of the wind. On the
whole the surface of the sea
takes on a white appearance.
The 'tumbling' of the sea becomes
heavy and shock-like. Visibility
affected.

11 64-72 56-63 Violent Storm Exceptionally high waves (small

and medium-size ships might be for
a time lost to view behind the
The sea is completely
covered with long white patches
of foam lying along the direction
of the wind. Everywhere the edges
of the wave crests are blown into
froth. Visibility affected.

12 73-83 64-71 Hurricane The air is filled with foam and

spray. Sea completely white with
driving spray; visibility very
seriously affected.

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The factors, other than the wind speed, which affect the appearance of the sea

surface

tides, barometric pressure, wind stress, temperature, salinity, etc.

The difference between apparent and true wind

The Apparent wind is the wind experienced by an observer in motion and is the relative
velocity of the wind in relation to the observer.

Apparent wind velocity is the vector sum of the true wind and the headwind an object would
experience in still air. The headwind velocity in still air is inverse of the object's velocity,
therefore the apparent wind can also be defined as a vector

True wind is the direction of actual wind with out any other factors. The direction, with
respect to true north, from which the wind is blowing.

The true wind velocity by using a vector diagram, given the apparent wind and
the ships course and speed

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m) Demonstrates the use of a geostrophic wind scale

Geostrophic wind scale

A Geostrophic wind scale is a graphical

device printed in synoptic weather charts.
It enables estimation of the Geostrophic
wind velocity by measuring the distance
between the isobars in the weather chart
and plotting this against the geographic
latitude in the wind scale.

step 1: determine the geographic latitude of the position you want to estimate the
geostrophic wind speed for.
step 2: measure the distance between the isobars shown on either side of that
position.
step 3: choose the correct latitude line in the geostrophic wind scale or interpolate in
the scale, using the result of step 1.
step 4: plot the distance measured in step 2 on that line.
step 5: read the estimated wind speed from the scale, using the curved lines.

NOTE that the geostrophic wind is only a theoretical wind flowing parallel to the
isobars in the chart. The true wind always is reduced by friction against the earth or
sea surface and will be deflected towards the centre of the low pressure system
which is circled by the isobars you used. Because of friction between the air and the
earth’s surface, the surface wind speed over land would be about half of the
geostrophic wind speed and over sea, about two thirds of the geostrophic wind
speed.

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 1.6 Visibility

a) States that visibility is reduced by the presence of particles in the

atmosphere, near the earth's surface

The maximum distance at which an item can be seen and differentiated in

normal daylight is known as visibility, which is the definition of which is the
transparency of the atmosphere.
In the following situations, liquid or solid airborne particles might limit visibility:

(a) Mist or fog.

(b) Precipitation.
(c) Spray.
(d) Smoke.
(e) Dust, volcanic ash, etc.
Therefore, visibility may change in various directions. Since it is a transparent gas,
water vapor does not impair visibility. The visibility through the atmosphere is around
130 nautical miles when there are no suspended particles.
b) Defines 'fog', 'mist' , 'haze '

MIST / FOG
When water droplets that have collected on dust, tiny salt flakes, or other airborne
particles are so small as to impede visibility, mist is said to exist. Fog is defined as
mist that has grown dense enough to impair visibility to one kilometer or less. With a
radius of less than 1 micron (one million microns is equal to one metre), mist can
form at relative humidity levels as low as 80%.Fog typically forms when there is a
relative humidity of 90% or more; the size of the water droplets is between 1 and 10
microns. Fog always comes before and after mist.

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HAZE
If visibility is reduced by solid particles such as dust, sand, volcanic ash, etc., in
suspension in the air, haze is said to exist. Haze can, in rare cases, reduce visibility
to 200 metres or less.

c) Applies the concept of processes leading to super saturating to a

classification of fogs as mixing, cooling or evaporation fogs.
d) Explains qualitatively the formation of radiation fog, mentioning areas,
seasons and reasons for its dispersal
Radiation fog is sometimes known as "Land Fog" since it only forms over land and
not over water. The terrain radiates heat quite rapidly at night. On nights without
clouds, heat radiation from the earth's surface into space occurs more quickly. As a
result, the air in contact with the ground cools, and if it does so below its dew point, a
significant amount of dew is deposited. However, if there is a light breeze, the
turbulence spreads the cold from the earth surface to the air a few meters above the
ground, creating a shallow fog known as "Ground fog." Even if the visibility above
this ground fog may be decent, in the fog, Radiation fog, which can form over land only,
may drift on to rivers, harbours, lakes and other coastal regions. For example: fog on the
Thames River, Dover Straits, the Sandheads of the Hooghly, etc.

Radiation fog forms over land because of the large diurnal range of air temperature over
land. It does not form over sea because of the very small diurnal range of air temperature
over sea.

Radiation fog reaches its maximum about half hour after sunrise because air temperature is
at its lowest at that time. It generally dissipates after the sun has shone for a few hours and
the land surface has warmed up.

Conditions favourable for radiation fog are:

a)Large moisture content in the lower layers of air.

b)Little or no cloud at night.
c) Light breeze at the surface.
d)Cold wet surface of land.

e) States the effect of pollution on the formation of radiation fog

The main mechanism for cooling nighttime air close to the ground is radiation.
Radiation fog is the term for the fog that results from the earth's radiation cooling. On
clear nights, it forms best when a thin layer of moist air close to the ground is
followed by a layer of drier air. More often than not, the fog that forms over the ocean
is thinner than the fog that occurs in the polluted city air. Typically, the fewer but
larger fog droplets are produced over the middle of the ocean by the smaller number

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of condensation nuclei. Numerous tiny fog droplets are produced by city air with a lot
of condensation nuclei, which considerably increases the thickness and decreases
visibility. Early in the 1950s, a dramatic instance of dense fog forming in air with lots
of nuclei happened in London, England. Radiation fog normally sustains its thickness
by the means of a temperature inversion existing above the fog. Smoke, dust or
other small particles may be trapped under the inversion and becomes difficult to
disperse. Water droplets may condensate on the particles trapping the dirt in
suspension in the air.

More ever, fog that forms in polluted air can turn acidic as the tiny liquid droplets
combine with gaseous impurities, such as oxides of sulphur and nitrogen. These
radiation fogs are then called acid fog. An acid fog poses a threat to human health,
especially to people with pre-existing respiratory problems.

f) Explains qualitatively the formation of advection fog, mentioning areas,

seasons and reasons for dispersal
g) Explains qualitatively the conditions leading to the formation of sea
smoke, and typical areas where sea smoke may be encountered
Advection fog is often known as "Sea fog" since it typically occurs over water. But it can also
form over land. It develops when a damp wind blows over a sea or land surface that is quite
chilly. The extra water vapour condenses into tiny droplets of water on dust or tiny salt
particles when the wet air is cooled below its dew point, creating advection fog. Advection
fog is created and spread by wind. Turbulence causes advection fog to form to great depths
when the wind is quite strong. Low clouds (of the stratus kind) and no fog, however, are the
result of extremely strong winds lifting the moisture too high.

Best examples of advection fog are:

a) On the Grand Banks of New Foundland where the warm, moist Westerlies, blowing
over the warm Gulf Stream, cross over the cold Labrador Current.
b) Off the east coast of Japan where the warm, moist Westerlies, blowing over the warm
Kuro Shio, cross over the cold Oya Shio.
c) The south coast of the UK in winter, whenever SW winds blow. These winds come
from lower latitudes and blow over the sea and are hence warm and moist, compared
to the cold land surface.

The possible time of occurrence of advection fog can sometimes be predicted by plotting the
temperature of the sea surface and the dew point temperature of the air as two separate
curves against ship's time as shown in the following figure.

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In the case illustrated, it is observed that the two curves appear to converge. By
extending the two lines as shown in dotted lines, it is noticed that the curves would
intersect at about 1400 hours. We can then expect to experience advection fog at
about 1400 hours.
OTHER TYPES OF FOG

1) SMOG
Smog is radiation fog mixed with industrial smoke.
Smoke + Fog = Smog
It is a thick, black, oppressive blanket, which not only wets all exposed surfaces but
also makes them black due to carbon particles in the smoke. Example: London,
Glasgow, Newcastle, Tokyo, Los Angeles, and Calcutta.

2) STEAM FOG OR ARCTIC SEA SMOKE

When extremely cold, dry air moves over a sea surface that is still somewhat warm,
water vapor swiftly condenses into water droplets, giving the impression that vertical
streaks of smoke are rising from the ocean. Since it frequently appears in the Arctic
Ocean, this is sometimes known as steam fog or arctic sea smoke.
h) Describes methods of estimating the visibility at sea, by day and by night,
and the difficulties involved
The assessment of the visibility in daylight is generally based on observation of
suitable objects at known distance.

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