5.

EXERCISE

Determination of Vapour Pressure, Relative Humidity and Dew Point Temperature

Objective: To study about vapour pressure, relative humidity and dew point temperature.

The humidity in the atmosphere is of great physical as well as biological importance.

It influences the internal water potential of plants and the rate at which plants transpire into
the atmosphere. Humid conditions affect the growth and development of many pathogens
especially the fungal organisms.

What is Psychrometer?
The instrument which contains both dry bulb and wet bulb thermometers for the
measurement of dry bulb and wet bulb temperatures is called psychrometer. The depression
of wet bulb gives an idea about the relative humidity of the air at particular time.

What are the different measures of humidity parameters?

The atmospheric humidity is measured in various units. The important measures of humidity
are; vapour pressure, relative humidity, dew point temperature etc.

Vapour pressure: Air contains different gases and water vapour also behaves as a gas.
The pressure of air is the sum of the partial pressures of its component gases. The partial
pressure due to presence of water vapour in air is called vapour pressure. The vapour
pressure is expressed in c.g.s. Units of pressure viz. milli bars or milli meters of mercury
(mm of Hg). The M.K.S unit of pressure is Pascal. The pressure exerted by the water
vapour under saturated conditions is called as the saturation vapour pressure (S.V.P.).
The saturation vapour pressure depends on the temperature of the air. It increases nearly
exponentially with the air temperature. The pressure exerted by water vapour actually
present in the air is called as actual vapour pressure of the air or simply, the vapour
pressure of air.
Saturation deficit: It is the difference between saturated vapour pressure and the actual
vapour pressure present in the atmosphere.
Relative humidity: The ratio of actual vapour pressure to saturation vapour at the
prevailing air temperature is called as relative humidity. It is normally expressed in
percentage.
Dew point temperature: Dew point temperature is the temperature at which air would
become saturated if it is cooled at constant pressure without addition or removal of water
vapour. Thus, the actual vapour pressure is equal to the saturation vapour pressure at the
dew point temperature. If the dew point temperature is close to the air temperature it
means that the air is nearly saturated.
Dry bulb Thermometer:
 It is an instantaneous instrument, which gives the current temperature of the
air. It is mercury in glass type thermometer.
 It is just like an ordinary mercury thermometer ranging from -35°C to 55°C,
used for calculating the relative humidity, dew point temperature and vapour
pressure.
 The least count of the thermometer is 0.5°C but reading is recorded up to
0.1°C.

Wet bulb Thermometer:

 It is same as the dry bulb thermometer except that the bulb of this thermometer
acts as an evaporating surface.
 The bulb is enclosed with muslin, which is connected with a thread dipped in
distilled water container.
 The water flow is maintained through the thread and keeps the bulb of the
thermometer wet.
 This temperature is used for calculating dew point temperature, relative
humidity and vapour pressure.
As the temperature falls the alcohol contracts and end of alcohol column in stem moves
towards the bulb dragging the index along with it by the surface tension of the liquid. If
subsequently the temperature increases, the alcohol flows freely past the index without
displacing it. Thus the position of the end of the index farthest from the bulb indicates the
lowest temperature reached since the thermometer was last set. Dry bulb and Wet bulb
thermometers are kept vertically inside the Stevenson screen.

Relative humidity:

When the air is saturated no evaporation takes place and therefore there is no difference
between the temperature in the wet and dry bulb thermometers. The R.H. is therefore said to
be 100%. On the other hand, when the air is not saturated evaporation takes place in the wet
bulb thermometer. As this takes up heat energy from the mercury contained in the wet bulb
consequently the temperature goes down. The greater the evaporation, the lower will be the
temperature and hence the greater the difference between the temperature of the wet and the
dry bulb thermometer. The R.H. is obtained by calculation or by using ready- made tables
like Hygrometric and SVP tables from which one can find the relative humidity and dew
point temperature corresponding to wet and the dry bulb temperatures.

Hygrograph:
It is an instrument used for recording continuously the relative humidity of the air. Human
hair has a property when it is free from fat that its length varies with the relative humidity but
it varies very little with other elements. The length of human hair increases with increase in
humidity and decreases with its decrease, increase in length being ½-2 ½ % of original
length.
 Micro-climatic Pole (M.C.P):

It is a wooden pole of 12‟height fixed at the center of observatory at which

micrometeorological observations like temperature and humidity are taken with the help of
either a whirling or Assamann Psychrometer at 1‟, 2‟, 4‟, 8‟ and 12‟ height. In case of
Assamann Psychrometer extra reading at ground surface is also recorded. The pole is painted
white and height of 1‟, 2‟, 4‟, 8‟ and 12‟ from ground surface are marked on it. The side of
the pole places a wooden ladder 8 feet height.

Artificial ventilated psychrometers:

Whirling Psychrometer:
This instrument is used to measure the temperature and relative humidity of the air both in
open as well as inside crops at various heights.
In this Psychrometer whirling or rotating the thermometer provides the aspiration, which is
mounted side by side on a suitable wooden frame with a movable handle. To obtain desirable
air speed of about 5 meters/sec the psychrometer should be given about 4 revolutions per
second. Avoid direct sunlight falling on the instrument while taking observation.
Assamann Psychrometer:
This instrument is designed for accurately measuring the temperature and relative humidity
of the air, both in the open as well as inside crops.
In this psychrometer the aspiration is provided by means of a clockwork fan, by which air is
drawn at a speed higher than 10 feet per second. Each bulb is protected from external
radiation by two highly polished coaxial tubes so that instruments can be held even in strong
sunshine without risk of solar radiation affecting the readings.
By the reading of dry bulb and wet bulb thermometer of whirling or Assamann psychrometer,
vapour pressure and relative humidity is computed.

Hour of Observation: Observation of dry bulb and wet bulb thermometers are recorded
at 0700 and 1400 hours LMT.

Estimation of RH And VP

Relative humidity (RH)

e
RH = a x 100
es
Where, ea = AVP at dry bulb temperature or SVP at dew point temperature, mmHg
es = SVP at dry bulb temperature, mmHg

SVP at dew point temperature = ea = E’ – AP (Td- Tw)

Where, E‟ = SVP at wet bulb temperature in mmHg
A= Psychrometric constant (0.0008)
P= Atmospheric pressure (1013 mb or 760 mmHg)
AP = 0.6 mmHg or 0.8 mb

Vapour pressure (VP)

 ea = ew – 0.0006 P (Td – Tw) (1+0.00115 Tw)
Where, ea = Actual Vapour pressure (mb)
ew =Saturation vapour pressure at wet bulb temperature
P =Atmospheric pressure (mb),1013 mb
Td = Dry bulb temperature (°C)
Tw = Wet bulb temperature (°C)
0.480 (Td  Tw )
ea = ew = xP
610  TW

Solved example for given data :

Date Time Temperature (°C) Atmospheric SVP (es) SVP*
(hrs) pressure (ew)
(Td) (Tw) (mb) (mm)
24/01/03 0738 14.0 12.4 999.4 12.0 10.8
1438 18.4 16.4 992.7 15.9 14.0
* SVP values to be obtained from Hygrometric table

(i) Convert SVP into mb units to make the pressure value balanced, i.e. taking 1
mm = 1.333 mb
At 0738 hours es = 1.333 x 12 = 16.00 mb
ew = 1.333 x 10.8 = 14.4 mb
Calculation of VP at 0738 hrs
0.480 (14.0 12.4)
ea = 14.4 - x 999.4
610 12.4
= 14.4 – 1.28 = 13.12 mb
ea 13.12
RH (%) = x 100 = x 100 = 82 %
es 16.0

Dew point temperature

It is defined as the temperature at which air becomes saturated when it is cooled at

constant pressure without removal or addition of moisture.
It can be seen from hygrometric tables directly.
Calculation of dew point temperature and dew point depression
Ex.-1 : Calculate dew point temperature, if air temperature and relative humidity (RH) are
15.00 C and 58 % respectively.

Tdp = T- (100-RH)/5
Where Tdp= Dew point temperature (0C)
T= Air temperature(0C)
RH = Relative humidity (%)
Tdp = 15-(100-58)/5
=15-(42)/5
= 15-8.4
= 6.6 0C
 6. EXERCISE

Measurement of Rainfall and Evaporation Measuring Instruments

Objective: To study about precipitation and evaporation.

The terms precipitation and rainfall are used as synonyms with each other. Precipitation can
be defined as “earthward falling of water droplets or ice particles that have formed by rapid
condensation in the atmosphere and are too large to remain suspended in the atmosphere". In
condensation the water droplets is remain suspended in the atmosphere in different forms.
Condensation is a first step of precipitation. But in precipitation, those condensed droplets are
so big so that they cannot remain in the atmosphere but fall down to the earth surface.

6. 1 Name of instrument:

 Ordinary rain gauge with measuring cylinder

 Self-recording rain gauge

Ordinary Rain gauge: It is an instrument used for measuring the amount of

rainfall. It consists of five parts (1) Funnel (2) Receiver (3) Body (4) Base (5)
Measuring cylinder.
The funnel is provided with a brass rim, which is truly circular and exactly 5”
(127mm) in diameter. The rim of the rain gauge should be 12” above ground level and 10”
above cemented platform. The rain is collected in receiver and is measured by standard
measuring cylinder provided with the instrument.

Installation:

The rain gauge should be fixed on masonry or a concrete foundation of 60 cm X 60 cm X 60

cm sunk in the ground. The base of the gauge is cemented into the foundation so that the rim of
the rain gauge is exactly 30 cm above the ground level. The height is chosen in order to
minimize water splashing into the rain gauge. If the height of the rim is more, the rain water
collected would decrease because of the change in the wind structure near the rain gauge. The
top of the rim of the rain gauge should be perfectly horizontal. A rain gauge should be installed
on a level ground, not upon a slope or terrace or never on a wall or roof. In order to avoid the
loss of raindrops due to obstruction, the distance any object should be at least twice the height
of the object above the rim of the rain gauge.

Measurement of Rainfall:

The rain falling into the funnel of the rain gauge is collected in the receiver kept inside the
body and is measured by means of a special measuring glass cylinder graduated in milli
meters. Ten millimetres of rain means that if that rainfall is allowed to be collected on a flat
surface, the height of water collected would be 10 mm. In case, the special measuring glass
cylinder is not available, rainwater can be measured by commonly available measuring glass
graduated in ml. In such cases, 126.7 ml of water measured is equal to 10 mm of rainfall. This
conversion is applicable to a rainfall spell.
Self-recording Rain gauge: The instrument is designed to measure the duration, amount and rate
of rainfall. It consists of a float chamber containing a light hollow float. As the water collected by
the outer funnel is led into this chamber, the float rises along with the water level and the vertical
movement of the float is recorded on a pen on chart fixed on a rotating clock drum. This chart has a
range of 10 mm or 25 mm. As soon as 10 mm or 25 mm if rain falls, the pen reaches the top line of
the chart. But the instrument has a siphoning arrangement so, the water in the chamber gets
emptied and the pen and float come to the initial position immediately. If there is further rain, the
pen continues to rise and record the rainfall in the manner. If there is no rain the pen traces the
horizontal line from where it leaves off rising.

Intensity of Rainfall Spell:

The intensity of a rainfall spell is defined as the ratio of the total amount of rainfall recorded
during the spell to the total duration of the spell. It is expressed in mm per hr.

Measurement of Rate of Evaporation

Evaporation is measured by means of pan evaporimeter. This instrument is used to

measure the evaporation of water near the ground. The class A pan evaporimeter which is
commonly used in India, consists of a large cylindrical pan made of copper or tin with 120
cm diameter and 25cm depth. The pan is made of 20 gauge of copper sheet tinned inside and
painted outside. A still well is provided inside the pan so that there would undisturbed water
surface inside the well and ripples would be broken. It consists of a brass cylinder mounted
on a heavy circular base provided with three circular holes at the bottom. The reference
pointed is provided by a brass rods fixed at the centre of a still well.
For measuring evaporation, a graduated measuring cylinder made of brass is also provided
with the instrument. It has a scale of 0.20 cm engraved inside it along its height. The
reservoir of the Evaporimeter rests on a wooden platform120cmx120cm, placed on the
ground. The height of wooden platform is 10cm so that the rim of the reservoir is 40 cm
above the ground. The reservoir is covered with wire mesh to check water loss by birds etc

6.8 Part of Pan evaporimeter:

1. Class 'A' pan evaporimeter 2. Still well 3. Measuring cylinder

4. Thermometer 5. Wooden frame 6. Wire mesh

Procedure:

 Note the water temperature correct to 0.1 OC.

 If the water level below the tip of the rod, add sufficient water slowly with the help of
the measuring cylinder so that the water level again coincides with the reference level.
 Note the amount of water added by taking into account the number of cylinders of
water added and parts thereof.
 If rainfall has occurred during 24 hrs ending 08:30 hrs IST and still the water level has
fallen below the reference point and water has to be added to bring the water level to the
reference level, this amount of water should be added to the rainfall amount in mm to
 get the total evaporation for the day. If however, the rainfall has been heavy and water
level has gone above the reference point at the time of observation, remove water with
the help of the measuring cylinder in order to bring the water level back to the reference
point. Subtract from the rainfall the amount of water removed in order to get the total
evaporation for the day.
 If on any day, due to occurrence of very heavy rainfall, the water level has risen up to
the rim of the pan and some water has over flown. So, entry „over flown’ should be
made in the observation register.

Observations to be recorded:

 Temperature of water in the pan.

 Amount of water added or removed to bring back the water level to the reference
point.
 Amount of rainfall, of any, during the past 24 hours.

Installation:

The evaporimeter should be installed at an open sight with no obstruction casting shadow
on the pan. The pan should be placed on the wooden grill kept on a fixed foundation so that
the edge of the pan is on level and is exactly at 30 cm above the ground. The rate of
evaporation is measured daily at 08:30 hours IST.

Evaporigraph: This instrument is used for recording continuously the evaporating

power of the air.
Time of Observation
Ordinary rain gauge and self recording rainguage and Open pan Evaporimeter observations
are recorded and set at 0830hours IST (Indian Standard Time).

*****
Fig. 1: Ordinary Rain Gauge (Symon’s rain gauge )

Fig.2: U. S. W. B. Class ‘A’ pan evaporimeter

 7. EXERCISE

Analysis of Rainfall Data for Climatological Studies

Objective: To study about rainfall data

Study of rainfall over a long period is called rainfall climatology. It reveals the
general pattern and characteristics of rainfall of a particular place or region. It helps in
understanding the amount, intensity, and distribution of rainfall of a place. It helps in
classification of climate. Understanding the rainfall climatology can develop suitable and
efficient cropping systems. It helps in taking decisions on time of sowing, scheduling of
irrigation, time of harvesting, growing period etc. Only rainfall total or its mean over certain
period is of little use in agriculture. As the crops are affected by rainfall amount, distribution,
dry spells, wet spells, length of growing season etc. their determination in required. Also
some parameters giving probability, variability, and dependability are important in crop
planning and monitoring.

Standard Meteorological week

World Meteorological Organization (WMO) has divided a calendar year into 12
durations of 30 days each – called Periods and 52 weeks – called meteorological weeks of 7
days each. The first week starts from 1st January and ends on 7th January and so on. The 9th
(26th Feb. to 4th march ) week in a leap year and the 52nd week consists of 8 days.

Meteorological Seasons
The whole year can be divided into 4 seasons such as pre-monsoon or summer (March
to May), monsoon or Kharif season (June to September), Post-monsoon season (October to
November) and winter or Rabi season (December to February).

Methodology
 Central tendency of rainfall expressed through mean, mode, and
median.
 Dispersion of rainfall about mean expressed through mean deviation,
standard deviation and coefficient of variation.
 Dependability of rainfall through probability and coefficient of
variation.
 Rainfall trend through moving average.

Central tendency
Mean rainfall
Mean is the average value of rainfall for some years whereas decennial rainfall is the
mean of the total rainfall during the past 10 years and normal rainfall is the mean of
more than 30 years. The later represents the typical value of a whole distribution over
the region. It is given by the formula
 R =  r1 / n

Where R = mean rainfall, r1 = rainfall of ith year, n = total no. of years.

Median
The median of a rainfall series is the value, which divides the total frequency into
equal parts when the series is arranged in ascending or descending order.
For example,
1995 1996 1997 1998 1999 2000 2001
547 850 639 725 602 580 810
Arrange the rainfall in descending order irrespective of years

850, 810, 725, 639, 602, 580, 547.

ANS. 639 is the median value of the rainfall series.

Mode
Mode of a frequency distribution is defined as that value of the variable for which the
frequency is maximum or the amount of rainfall which occurs most frequently.
Range
It is the difference between the highest and the lowest values of the rainfall series.

Dispersion of Rainfall

Mean Deviation (MD)

The mean deviation is defined as the mean of the absolute values of the
deviations from the mean. It is a measure of variability. It shows the degree of scatter or
disperses from the average or central value.

MD =  ( X  X )
n-1
It does not take positive or negative signs unto consideration.

Standard Deviation (SD)

It is defined as the square root of the mean of the squares of deviations of the
rainfall values from the arithmetic mean of all such rainfalls. It is a measure of
variability or the scatter or the dispersion about the mean values. It is given by the
following formula.

 ( Xi - X )2
SD () =
n-1

Coefficient of Variation (CV % )

CV is defined as the standard deviation divided by the mean value of rainfall.
It shows the variability of rainfall in percentage. Higher the CV % lower is the
dependability and vice versa.
It is give by the formula
CV (%) = SD X 100
Mean
For weekly rainfall series
If CV % < 100 it is highly dependable or reliable.
If CV % is 100-150, it is dependable or reliable .
If CV % is > 150, it is not dependable or not reliable.
The coefficient of variation of rainfall in a humid region is quite lower than the CV of
rainfall in arid and semi-arid regions. If the annual variability is more than 20 percent
presents a great risk in rain fed faming.
Dependability of Rainfall
Dependable rainfall for a given period can be defined as a quality of rainfall
received at 75 % probability on long term basis. This is also known as assumed
rainfall. Generally following probability levels are considered.
 75 % level or a value of rainfall expected in 3 out of 4 years.
 80 % level or a value of rainfall expected in 4 out of 5 years.
 90 % level or a value of rainfall expected in 9 out of 10 yrars.
Dependable rainfall for a week or month or a season or a year is computed as
follows:-
Procedure:
 Use rainfall data for more than 30 years.
 Arrange the data in descending order.
 Assign a number to each entry value – called as rank number.
 Give 1 number to the highest value and than an increasing number to
decreasing values.
 Calculate probability for each rank by following formula:
Fa (m) = 100 m , where m = rank number
n+1 n = no. of observation / years
m = Fa X (n+1) Fa = level of percentage
100
 Preparation vertical scale and plot rainfall accounting to Fa position on log-
normal probability paper.
 Expected value of rainfall at 50, 75, 80 or 90 % probability can be obtained
from the the graph plotted or by finding rank number corresponding to the 50,
75, 80, and 90 probability level and finding corresponding rainfall.
Example: Annual rainfall

Year Rainfall Descending Rank No. Plotting

Order (m)
1990
1991
1992
1993

Rainfall characteristics
1. Mean rainfall
2. Median
3. Mode
4. Mean Deviation
5. Standard Deviation
6. CV %
7. Range
8. Rainfall at 75 % probability
9. Rainfall at 80 % probability
10. Rainfall at 90 % probability

*****
 8. EXERCISE

Measurement of Atmospheric Pressure and Analysis of Atmospheric Conditions

Objective: To study about atmospheric conditions.

Technically, pressure is defined as the force per unit area. But, the pressure exerted by
the atmosphere on the earth‟s surface is called atmospheric pressure. It is defined as the
pressure exerted by a column of air with a cross sectional area of a given unit extending from
the earth‟s surface to the upper most boundary of the atmosphere. The standard sea level
pressure is given as 1013 mb or 76 cm or 29.92” at a temperature of 15C and 45 north
latitude. Atmospheric pressure does not have direct influence on crop growth. It is however,
an important weather parameter in weather forecasting.

Instruments:

 Fortin‟s barometer
 Kew pattern barometer
 Aneroid barometer
 Barograph

The standard instruments for measuring atmospheric pressure are aneroid barometer
and barograph.

Fortin’s barometer

This barometer is standard and accurate instrument for measuring pressure. It consists
of a small cistern vessel containing mercury with a flexible leather bag and a screw at its
bottom. The mercury level can be raised or lowered with the help of the screw. In the cistern
vessel, a glass tube filled with mercury is kept inverted. In this vessel there is a pouted ivory
pointer. from the lower tip of this pointer, the zero of the scale starts and therefore while
taking reading, the mercury level in cistern vessel must touch the lower tip. There are two
scales on two sides of the tube, one in centimetres and the other in inches. Vernier caliperare
also attached for accurate reading. To take pressure reading the height of mercury column is
measured on main scale and then Vernier scale is read.

Atmospheric pressure = MSR + VSR X Vernier constant

The metal scale and the mercury expand differently at different temperatures. They
are, therefore transformed to one common temperature which is zero degree centigrade or
273 K. The gravitational pull changes according to latitude. Hence, the gravitational
correction is applied and all the readings are transformed into one common latitude i.e. 45
N. All the readings are transformed to sea level height. Thus, three corrections such as
temperature, gravity and latitude are applied.
 Kew pattern barometer

This is also similar to Fortin‟s barometer were the cistern vessel is fixed and has no
adjusting screw. The divisions are made unequal in order to allow rise or fall of mercury
column in the cistern. In this barometer initial adjustment of cistern is not required.

Aneroid barometer

This barometer does not contain any liquid. It consists of a evacuated box with a
corrugated sheet of metal lid held in position by means of a spring to avoid collapse of the top
and bottom. This box is called as siphon cell and is sensitive to change in pressure. When the
pressure increases the cell is compressed and when it decreases the cell is expanded. These
variations are magnified with the help of levers and are communicated through chain and
pulley to the pointer, which moves on graduated scale. This pointer gives direct pressure
reading. This is not an accurate instrument.

Barograph

This instrument is used for automatic and continuous record of atmospheric pressure;
it is a special type of aneroid barometer having recording system. It consists of several
vacuum boxes similar to aneroid barometer placed one above another. The combined motion
of these vacuum boxes becomes appreciable and is then communicated to a level system. The
changes are marked on a chart paper fixed on the clock driven rotating drum. The chart is
calibrated in cm or inches on one axis and hrs/days of week on another axis. Thus, a
continuous record of atmospheric pressure is obtained. Before use the instrument must be
standardized with the help of Fortin‟s barometer. This instrument does not give correct
pressure readings. However, it is helpful in recording the barometric tendencies.

Use of barometer

It is used for approximate forecasting, to measure atmospheric pressure and to

measure height of a given station above mean sea level.

Weather and pressure:

 Falling barometer indicates rain or storm (bad weather).

 Rising barometer indicates fair weather (clear and stable).
 Steady barometer indicates steady or settled weather.
 A continually rising pressure indicates fine and settled weather and a steadily falling
pressure indicates occurrence of unsettled and cloudy weather.
 Units of pressure

The pressure is measured in following units.

1 atmospheric pressure = 29.92” = 76 cms = 760 mm

= 1013 mill bar

= 101.32 kilopascal(Kpa)
= 14.7 lbs/inch2
= 1.014 X 106 dynes /cm2
= 1 bar

 Height of mercury column in inches or cms or mm

 Bar is force equal to 106 dynes /cm2. This is big unit and is

therefore divided into smaller units

1 bar = 1000 mb

 In standard international unit of pressure is Pascal

1 Pascal = force of 1 Newton/ sq.m.

Calculation Based on Air Density And Atmospheric Pressure

Ex.-1: Calculate the standard sea level air density, if the standard sea level pressure is
1013 hPa and temperature is 15.00C
Absolute temperature = C+273
= 15.0+273.0
= 288 0K
Air density = P/RT
Where P = Atmospheric pressure at msl
R = Gas constant = 2.87
T = Absolute temperature

Air density (kg/m3 ) = 1013/2.87*288

= 1013/826.56
= 1.225
Ex.-2: Calculate the mean sea level pressure ,if the air density and air temperature are
1.165 kg/m3 and 30.00C respectively.
Absolute temperature = C+273
= 30.0+273.0
= 303 0K
Air density (kg/m3 ) = 1013/2.87
1.165 = P/2.87
= 1013 hPa
 9. EXERCISE

Estimation of Heat Indices

Objective: To study about heat indices

The accumulated heat unit system or degree-day concept can be used for the
prediction of crop maturity dates in a region. The concept assumes that there is a direct
and linear relationship between growth and temperature. The assumption is that a crop
requires a definite amount of accumulated heat energy for completion of its life cycle.
Definition:
Phenology:

The periodic biological events and their dates of occurrence in the plant life in relation to the
influence of weather are called phenology. OR It is the branch of science which studies the
periodical biological events with respect to calendar days.
Growing Degree day (GDD):

The degree-day or heat unit is the departure from the mean daily temperature above the
minimum threshold or base temperature or critical temperature. OR It is the difference
between daily mean temperature and base temperature.
Base temperature (Tb):
The temperature below which growth does not take place is known as base
temperature. The value for majority of the plants ranges from 3.5 to 12.0oc.
Base temperature of some crops (in degree Celsius)
Crop Base Temperature (o C) Crop Base Temperature (o C)
Pea 1–2 Oats 4–5
Wheat 3.0 - 4.5 Groundnut 8 - 10
Barley 3.0 - 4.5 Tobacco 13-14
Sugar beet 4–5 Pumpkin 12
Rice 10-12 Lentils 4-5
Sorghum 8 - 10 Carrot 4-5
Maize 8 - 10
Photo-thermal unit (PTU):
The product of GDD and maximum bright sunshine hours of any day is called photo
thermal unit (PTU).
Helio-Thermal Unit (HTU):
The Product of GDD and the number of actual bright sunshine hours on the day is
called helio-thermal unit (HTU).
Hydrothermal unit (HYTU): The product of GDD and relative humidity is called
HYTU.
Materials:
Data on daily maximum temp; daily minimum temp., day length and daily number of
actual bright sunshine hours during the growing period of the crop and base temperature of
crop (Tb).

Methodology:

(Tmax +Tmin)
GDD = ∑ ------------------ - Tb Where, Tb is base temperature of crop
2

PTU = GDD x Day length (hours) or max. possible bright sunshine hours.

HTU= GDD x No. of actual bright sunshine hours

Trange = Tmax-Tmin

Heat Use Efficiency (HUE)= Yield (Kg/ha)

GDD(Degree day)

Radiation Use Efficiency (RUE) = Yield (Kg/ha)

PTU(Degree day hrs)

Heliothermal Use Efficiency = Yield (Kg/ha)

HTU(Degree day hrs)

Hydrothermal use efficiency (HYTUE) = Yeild (kg/has)

HYTU (oC day %)
 EXERCISE:10

Estimation of Potential Evapotranspiration

Objective: To study about evapotranspiration

The total water loss from soil surface through evaporation and that as water vapour
from plant canopies through transpiration together is estimated as evapotranspiration. The
concept of potential evapotranspiration (PET) is an attempt to characterize the climatic
environment in terms of its evaporative power the maximal evaporation rate which the
atmosphere is capable of extracting from a well watered field under a given meteorological
regime from a field of given surface condition, PET is a useful standard of reference for the
comparison of climatic regimes or seasons.
Methodology:
PET estimation by Thornthwaite method
E = 1.6 (10T / I) a
Where,
E = Unadjusted PET in cm, per month (30 day, 12 hours day)
T = Mean air temperature
12
I = Annual heat index = Σ i
I=1

1.514
i = Monthly heat index = ( T / 5)

a = 6.75×10-7.I3-7.71×10-5.I2×17.92×10-3.I + 0.49239

For calculation of daily value of PET

K X E X 10
PET = (mm day-1)
No. of days in month

K = Adjustment factor for which table value are given

PET by Modified Penman Method
Doorenbos and Pruitt (1975) proposed a modified Penman method as below for
estimating fairly accurately the reference crop ET and gave tables for necessary
computations.
ETC = C [W × Rn + (1-W) × F (U) × (ea-ed)]
Where, ETC = The reference crop ET in mmday-1 (not adjusted)
Rn = Net radiation (mmday-1)
ea = SVP in milibar at mean air temp (co)
ed = Mean AVP of the air in the milibar
= ea × RH min/100
f(U) = a wind related function
 F (U) = 0.27 (1+U/100) with U in kmday-1 measured at 2 mt, ht.
(1-W) = a temp. and elevation related weighing factor for the factor and effects of
wind and humidity on ETC.
W = a temp. and elevation related weighing factor for the factor and effects of
radiation on ETC.
C = adjustment factor for the ration U day/ U night for RN max and for Rs.
Rn = net radiation (mmday-1) or Rn = 0.75 Rs – Rnl
Rs is incoming short wave radiation (mmday-1) or obtained from Rs = (0.25
= 0.50 n/N) Ra.
Ra : is extra terrestrial radiation (mmday-1)
n : mean actual sunshine duration (hrday-1)
N : maximum possible duration and sunshine (hrday-1)
Rnl: Net long wave radiation (mmday-1) a function of f (T)of avp, fed) and
sunshine duration f (n/N). Rnl = f(T) × f(d) × f(n/N)

Example:
Latitude = 15.00C
Altitude = 200 m
T mean = 300C
Day wind = 15 km hr-1
Night wind velocity = 12 km hr-1
Mean sunshine (n) hr = 8 hr day-1
RHmax = 60 per cent
RHmin = 40 per cent

Solution
ea T = 300C Table 1 42.40 mb
ed ea × RHmin/100
42.4 × 40/100 Calc 16.96 mb
ea – ed 42.40 – 16.96 Calc 25.44 mb
f (U) 0.27 × (1 + U/100)
15 + 12
Umean = × 24
2
= 324 km day-1
0.27 × (1 + 324) Calc 1.15
100
0
Ra 15 N, June Table 6 15.8 mm day-1
N 150N, June Table 7 13.00 hr day-1
Rs (0.25 + 0.50 × n/N) × Ra
= (0.25 + 0.5 × 8/13) × 15.8 Calc 8.81 mm day-1
f(T) Temp. 300C Table 2 16.70
f(ed) 16.96 mb Table 3 0.16
f(n/N) 8/13 = 0.62 Table 4 0.66
Rnl f(T) × f (ed) × f (n/N)
= 16.7 × 0.16 × 0.66 Calc 1.76 mm day-1
 Rn (0.75 × Rs) - Rnl
= (0.75 × 8.81) – 1.76 Calc 4.85 mm day-1
0
W = Temp. 30 C, 200 m Table 8 0.78
C RH max 60%, Rs 8.81,
U day/U night 1.25 Table 5 1.05
ET0 = C × [W × Rn + (1 – W) × f(U) × (ea-ed)]
= 1.05 × [0.78 × 4.85 + (1 – 0.78) × 1.15 × (25.44)]
= 1.05 × [3.783 + (0.22 × 1.15 × 25.44)]

= 1.05 × (3.783 + 6.436)

= 1.05 × (10.219) Calc = 10.73 mm day-1

Modified Penman and Radiation method offer the best results for periods as short as 10days
followed by pan evaporation method.

Table 1: SVP (Ea) in mb as a function of Mean Air Temperature

Temperature (0C) 0 1 2 3 4 5 6 7 8 9 10
ea (mb) 6.1 6.6 7.1 7.6 8.1 8.7 9.6 10.0 10.7 11.5 12.3
Temperature 0C) 11 12 13 14 15 16 17 18 19 20 21
ea (mb) 13.1 14.0 15.0 16.1 17.0 18.2 19.4 20.6 22.0 23.4 24.9
Temperature(0C) 22 23 24 25 26 27 28 29 30 31 32
ea (mb) 26.4 28.1 29.8 31.7 33.6 35.7 37.8 40.1 42.4 44.9 47.6
Temperature(0C) 33 34 35 36 37 38 39 40
ea (mb) 50.3 53.2 56.2 56.4 62.8 66.3 69.9 73.6

Table 2: Effect Of Temperature [F(T)] On Long Wave Radiation (Rnl)

T0 C 0 2 4 6 8 10 12 14 16 18
f(T) 11.0 11.4 11.7 12.0 12.4 12.7 13.1 13.5 13.8 14.2
T0 C 20 22 24 26 28 30 32 34 36
f(T) 14.6 15.0 15.4 15.9 16.3 16.7 17.2 17.7 18.1

Table 3: Effect of vapour pressure [f(ed)] on long wave radiation (rnl)

ed(mb) 6 8 10 12 14 16 18 20 22 24
f (ed) 0.23 0.22 0.20 0.19 0.18 0.16 0.15 0.14 0.13 0.12
ed(mb) 26 28 30 32 34 36 38 40
f (ed) 0.12 0.11 0.10 0.09 0.08 0.08 0.07 0.06
Table 4: Effect of the ratio of actual and maximum bright sunshine hours (f(n/N)] onlong
wave radiation (rnl)

n/N 0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40

f(n/N) 0.10 0.15 0.19 0.24 0.28 0.33 0.37 0.42 0.46
n/N 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85
f(n/N) 0.51 0.55 0.60 0.64 0.69 0.73 0.78 0.82 0.87
n/N 0.90 0.95 1.0
f(n/N) 0.91 0.96 1.0
Table 5: adjustment factor (c) in Penman equation

Rs (mm day-1) RHmax= 30% RHmax = 60% RHmax=90%

U day ms-1) 3 6 9 12 3 6 9 12 3 6 9 12
U day/U night = 4.0
0 0.86 0.90 1.00 1.00 0.96 0.98 1.05 1.05 1.02 1.06 1.10 1.10
3 0.79 0.84 0.92 0.97 0.92 1.00 1.11 1.19 0.99 1.10 1.27 1.32
6 0.68 0.84 0.92 0.93 0.85 0.96 1.11 1.19 0.94 1.10 1.26 1.33
9 0.55 0.65 0.78 0.90 0.76 0.88 1.02 1.14 0.88 1.01 1.16 1.27
U day/U night = 3.0
0 0.86 0.90 1.00 1.00 0.96 0.98 1.05 1.05 1.02 1.06 1.10 1.10
3 0.76 0.81 0.88 0.94 0.87 0.96 1.06 1.12 0.94 1.04 1.18 1.28
6 0.61 0.88 0.81 0.88 0.77 0.88 1.02 1.10 0.86 1.01 1.15 1.22
9 0.46 0.56 0.72 0.82 0.67 0.79 0.88 1.05 0.78 0.92 1.06 1.18
U day/U night = 2.0
0 0.86 0.90 1.00 1.00 0.96 0.98 1.05 1.05 1.02 1.06 1.10 1.10
3 0.69 0.76 0.85 0.92 0.83 0.91 0.99 1.05 0.89 0.98 1.10 1.14
6 0.53 0.61 0.74 0.84 0.70 0.80 0.94 1.02 0.79 0.92 1.05 1.12
9 0.37 0.48 0.65 0.76 0.59 0.70 0.84 0.95 0.71 0.81 0.96 1.06
U day/U night = 1.0
0 0.86 0.90 1.00 1.00 0.96 0.98 1.05 1.05 1.02 1.06 1.10 1.10
3 0.64 0.71 0.82 0.89 0.78 0.86 0.94 0.99 0.85 0.92 1.01 1.05
6 0.43 0.53 0.68 0.79 0.62 0.78 0.84 0.93 0.72 0.82 0.95 1.00
9 0.27 0.41 0.59 0.70 0.50 0.60 0.75 0.76 0.62 0.72 0.87 0.96

Lat. Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.
Northern hemisphere
50 3.8 6.1 9.4 12.7 15.8 17.5 16.4 14.1 10.9 7.4 4.5 3.2
40 6.4 8.6 11.4 14.3 16.4 17.3 16.7 15.2 12.5 9.6 7.0 5.7
30 8.8 10.7 13.1 15.2 16.5 17.0 16.8 15.7 13.9 11.6 9.5 8.3
20 11.2 12.7 14.4 15.6 16.3 16.4 16.3 15.9 14.8 13.3 11.6 10.7
10 13.2 14.2 15.3 15.7 15.5 15.3 15.3 15.5 15.3 14.7 13.6 12.9
0 15.0 15.5 15.7 15.3 14.4 13.9 14.1 14.8 15.3 15.4 15.1 14.8
 Southern hemisphere
50 17.5 14.7 10.9 7.0 4.2 3.1 3.5 5.5 8.9 12.9 16.5 18.2
40 17.9 15.7 12.5 9.2 6.6 5.3 5.9 7.9 11.0 14.2 16.9 18.3
30 17.8 16.4 14.0 11.3 8.9 7.8 8.1 10.1 12.7 15.3 17.3 18.1
20 17.3 16.5 15.0 13.0 11.0 10.0 10.4 12.0 13.9 15.8 17.0 17.4
10 16.4 16.3 15.5 14.2 12.8 12.0 12.4 13.5 14.8 15.9 16.2 16.2
0 15.0 15.5 15.7 15.3 14.4 13.9 14.1 14.8 15.3 15.4 15.1 14.8
+Table 6: Extra terrestrial radiation (ra) expressed in equivalent evaporation (mm day-1) for
northern and southern hemisphere

Table 7: Mean daily duration of maximum possible sunshine hours (n) for different
months in north and south latitudes
N lat. Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec.
S lat. July Aug. Sep. Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June
0 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1 12.1
5 11.8 11.9 12.0 12.2 12.3 12.4 12.3 12.3 12.1 12.0 11.9 11.8
10 11.6 11.8 12.0 12.3 12.6 12.7 12.6 12.4 12.1 11.8 11.6 11.5
15 11.3 11.6 12.0 12.5 12.8 13.0 12.9 12.6 12.2 11.8 11.4 11.2
20 11.0 11.5 12.0 12.6 13.1 13.3 13.2 12.8 12.3 11.8 11.2 10.9
25 10.7 11.3 12.0 12.7 13.3 13.7 13.5 13.0 12.3 11.6 10.9 10.6
30 10.4 11.1 12.0 12.9 13.6 14.0 13.9 13.2 12.4 11.5 10.6 10.2
35 10.1 11.0 11.9 13.1 14.0 14.5 14.3 13.5 12.4 11.3 10.3 9.8
40 9.6 10.7 11.9 13.3 14.4 15.0 14.7 13.7 12.5 11.2 10.0 9.3
50 8.5 10.1 11.8 13.8 15.4 15.7 15.9 14.5 12.7 10.8 9.1 8.1

Table 8: Values of weightage factor (w) for the influence of radiation on et0 at different
temperatures and altitudes
Temp. Altitude
(0C) 0 500 1000 2000 3000 4000
2 0.43 0.45 0.46 0.49 0.52 0.55
6 0.49 0.51 0.52 0.55 0.58 0.61
10 0.55 0.57 0.58 0.61 0.64 0.66
14 0.61 0.62 0.64 0.66 0.69 0.71
18 0.66 0.67 0.69 0.71 0.73 0.76
22 0.71 0.72 0.73 0.75 0.77 0.79
26 0.75 0.76 0.77 0.79 0.81 0.83
30 0.78 0.79 0.80 0.82 0.84 0.85
34 0.82 0.82 0.83 0.85 0.86 0.88
36 0.83 0.84 0.85 0.86 0.87 0.89
38 0.84 0.85 0.86 0.87 0.88 0.90
40 0.85 0.86 0.87 0.88 0.89 0.90
27