Karmaveer Kakasaheb Wagh Education Society’s

K. K. WAGH COLLEGE OF AGRICULTRURE,

Saraswatinagar Nagar, Panchavati, Nasik-422003

Department Of Agronomy

Theory Notes of Ist Semester

Course No: AGRO-112

Course Title: Introductory Agro-Meteorology &
Climate Change
Credit: 2(1+1)
 Introduction to Meteorology

1.Aristotle [384-322B. C.] defined Meteorology as a study of lower atmosphere.

[Meteor- Lower atmosphere and logus- means science]

2. It is also defined as the science of atmosphere and its phenomena, especially those phenomena
which we call collectively as weather and climate.

3. Meteorology can be defined as the Science of atmosphere which deals with the
physics, chemistry and dynamics of atmosphere and also their direct and indirect effects upon
the earth surface, oceans and thereby Life in general

Study of weather elements comes under Meteorology and this Science and with Animals Science
also.

Climatology:-

It is defined as a scientific study of climate. It discovers, describes, & interprets the climate on
the basis of causes processes that generate them or
Climatology is the science which studies average condition of weather or the state behavior of
the atmosphere over a place or region for a long period of time.

Ecology:-

1. According to Taylor 1936. Ecology is the science of all relation of all organisms to all their
environment.

2. According to 1957. Ecology concerned itself with the inter relationship of Living organisms
and their environment.

3. In general ecology is a branch of biology that deals with the relation of living things to their
surroundings.

Agricultural Meteorology and Its Levels

1. J.W. Smith (1916) has defined Agricultural Meteorology as “Meteorology in its relation to
agriculture”

2. It can be defined as the science investigating Meteorology, climates and hydrologic condition,
which are significant to agriculture.

3. In short Agril. Meteorology is the applied branch of meteorology, which deals with the
relationship between climates, weather and life and growth of the cultivated plants and animals.
Levels of Study of Meteorology:

Study of meteorology is organized at three levels.

1. Micro scale:
A process operating within vegetation canopies near earth surface its size is in few cm and Life
span is few seconds.
2. Mesoscale:
The systems are approximately 10km in size and a lifetime is of few hours [up to5 hrs] eg.
Thunder storm.
3. Macro scale: It is divided into two scales.
A) Synoptic scale:
These systems have a diameter of few thousand km. and life time of about 5 days
Eg. Tropical storm, cyclones.
B) Planetary scale:
These systems have a diameter of 5000 to 10000 km and persist for several weeks
Eg. Waves in the atmosphere circulation.

Importance and Scope of Meteorology in Agriculture

Weather and climate is a resource and considered as basic input or resources in agricultural
planning, every plant process related with growth development and yield of a crop is affected by
weather.

Similarly every farm operation such as ploughing harrowing, land preparation, weeding,
irrigation, manuring, spraying, dusting, harvesting, threshing, storage and transport of farm
produce are affected by weather.
The scope of Agril Meteorology can be illustrated through the following few applications.

1. Characterization of agricultural climate for determining crop growing season: solar

radiation, air temperature, precipitation, wind, humidity etc. are important climatic factors on
which the growth, development and yield of a crop depends Agro-meteorology considers and
assess the suitability of these parameters in a given region for maximum crop production and
economical benefits.

2. Crop planning for stability in production:

To reduce risk of crop failure on climatic part, so as to get stabilized yields even under weather
adversity, suitable crops/cropping patterns/contingent cropping planning can be selected by
considering water requirements of crop, effective, rainfall and available soil moisture.

3. Crop management:
Management of crop involves various farm operations such as, sowing fertilizer application. Plat
protection, irrigation scheduling, harvesting etc. can be carried out on the basis of specially
tailored weather support. For this the use of operational forecasts, available from agro met
advisories is made
e.g. 1) Weeding harrowing, mulching etc are undertaken during dry spells forecasted.
2) Fertilizer application is advisable when rainfall is not heavy wind speed is<30 km/hr and soil
moisture is between 30 to 80%
3) Spraying/dusting is undertaken when there is no rainfall, soil moisture is 90% and wind speed
is<25km/hr.

4. Crop Monitoring:
To check crop health and growth performance of a crop, suitable meteorological tools such as
crop growth models. Water balance technique or remote sensing etc. Can be used.

5. Crop modeling and yield –climate relationship:

Suitable crop models, devised for the purpose can provide information or predict te results about
the growth and yield when the current and past weather data is used.

6. Research in crop –climate relationship:

Agro-meteorology can help to understand crop-climate relationship so as to resolve complexities
of plant process in relation to its micro climate.

7. Climate extremities:
Climatic extremities such a frost floods, droughts, hail storms, high winds can be forecasted and
crop can be protected.

8. Climate as a tool to diagnose soil moisture stress:

Soil moisture can be exactly determined from climatic water balance method, Which is used to
diagnose the soil moisture stress, drought and necessary protective measures such as irrigation,
mulching application of antitranspirant, defoliation, thinning etc. can be undertaken.

9. Livestock production:
Livestock production is a part of agriculture. The set of favorable and unfavorable weather
conditions for growth, development and production of livestock is livestock is studied in Agril.
Meteorology. Thus to optimize milk production poultry production, the climatic normal are
worked out and on the suitable breeds can be evolved or otherwise can provide the congenial
conditions for the existing breeds.

10. Soil formation:

Soil formation process depend on climatic factors like temperature, precipitation, humidity, wind
etc, thus climate is a major factor in soil formation and development.
 Climatic Controls

The value of weather elements are modified by the interference of the factors of determining
causes like latitude, altitude, etc. Such factors are called as climatic factors of climatic controls.

1Latitude:
The most important influence of latitude is on temperature of a place Temperature tends to
decrease with increase with increase in latitude. Places far away from the equator are colder than
those near it. This is because the angle of the Sun’s rays decreases as we go to higher latitudes
and also the rays have to pass through a greater distance of the atmosphere before they strike the
earth’s surface. They have therefore less heating effect than the rays falling on the equatorial
region.

2 Altitude: Pressure and temperature generally decreases with increase altitude, and the
capacity of the air to hold moisture also decreases.

3 Topography:
Wind velocity primarily changes with change in topography which may result in Change in
temperature

4. Mountains:
High mountain chains fact as a barrier to free flow of winds and divide one type of climatic zone
from another. For example moist monsoon current of the Indian sub-continent is not allowed by
the Himalayas to crossing into our country in winter.

5. Land and sea distribution:

Distribution of land and sea has a profound effect on climate. Places near the sea have moderate
climate. On the other hand places for away from the sea are very hot in summer and very cold in
winter. So they are said to have an extreme climate.

6. Oscan currents:
Ocean currents have a considerable influence on the climate of the coastal regions and Islands
near which they flow. The warm currents tend to raise the temperature of the place while the
cold currents make a place colder.
 Earth’s Atmosphere

Meaning:

The dynamic layer surrounding the earth above its surface containing various gases, moisture,
aerosols etc. is called atmosphere.

Definitions:

1. Atmosphere can be defined as the gaseous envelope surrounding the earth.

2. Atmosphere can be defined as a grand body from the earth surface to the outer space and
composed of number of gases.
The estimated mass of the atmosphere is 5.6 x 1014 metric tones. It extends over about 400 km
height and meteorological events and effects occur in it. The thickness of gaseous envelope is
equal to 1% of the earth’s mean radius.

Usefulness of the atmosphere:

1. It fulfils the biological oxygen demand (BOD) of the animal life.

2. It supplies the necessary precipitation or moisture.
3. It protects the biological life on the planet from harmful extraterrestrial radiations like UV, by
absorbing it though ozone.
4. It maintains the warmth of the plannet through its green house effect, avoiding the temperature
to fall to too extreme limits.(The earth’s temperature in the absence of atmosphere would have
been +950C (day),and -1450C (Night)
5. It provides the necessary CO2 which is basic input required to run photosynthesis process in
plants to build biomass.
6. It provides the necessary medium for the transport of pollens. Seeds spores and insets.
7. Many physical chemical and hydrological processes responsible for weather and climate occur
in atmosphere only.
8. Atmosphere is a big reservoir of nitrogen. Some plants and microbes can fix this nitrogen for
plant growth eg, Azolla pinara Azotobacter.
 Non Variable and Variable Components

1. Non-variable components:

Some gases of the atmosphere remain constant at surface of globe up to the height of 80 to 88
km. This is due to transportation of gases on continental* level, diffusion of gases, turbulent
mixing and convection.

These gases are called non-variable components.

They are Non-variable components (permanent constituents)
Constituents Symbol Percentage by volume
Sr.No
1 Nitrogen N2 78.084
2. Oxygen O2 20-946
3 Argon Ar 0.934
4 Carbon dioxide CO2 0.032
5 Neon Ne 18.18x10-4
6 Helium He 5.24x10-4
7 Crypt on Kr 1.14x10-4
8. Xenon Xe 0.087x10-4
9. Hydrogen H2 0.5x10-4
10. Methane CH4 1.5x10-4
11. Nitrous oxide N2O 0.5 x 10-4
12 Radon Rn 6 x 10-18

1. Variable components:

Some gases or components of the atmosphere ch anges with change with change in time, palce,
season etc, and these components are called as variable component they are-

Variable components or constituents:

S.N. Constituents Symbol Percentage by volume

1 Water vapour H2O <4
2. Ozone O3 <0.07x10-4
3. Sulphur dioxide SO2 <1x10-4
4. Nitrogen dioxide N02 <0.02x10-4
5. Ammonia NH3 1 race
6. Carbon monoxide CO ~0.2x10-4
7. Dust (Salt2 Soil) <10-3
8. Water (liquid & solid)
 Compositional layering of the Atmosphere

The atmosphere can be divided into two spheres on the basis of its chemical composition
occurring with height i.e. (1) Homosphere. (2) Hereto sphere.

Homosphere:
In the lower region up to the height of 88 km the various gases are thoroughly mixed and are
homogenous by the process of turbulent mixing, and diffusion. This sphere is called as
Homosphere. Herein the presence of gases is governed by the diffusion and the composition
remains normally.

Hydrosphere:
In hydrosphere gaseous composition changes and various gases form separate compositional
layering individually.
Satellite data have shown the presence of different chemosphere in follows:

1 Nitrogen and oxygen From 88 to 115 km

2 Automatic oxygen layer From 115 to 965 km
3 Helium layer From 965 to 2400 km
4 Hydrogen layer From 2400 to 10,000 km

The distribution of the gases is governed by the earth’s gravitational field. Thus heavier gases
sink downward while the lighter gases like hydrogen remain at higher altitude.

Physical Structure of Atmosphere

(Stratification of atmosphere or layering of atmosphere)

On the basis of the vertical temperature difference, the atmosphere can be divided into four
horizontal layers or shells, namely.

A) Lower Atmosphere: 1. Troposphere and 2. Stratosphere

B) Upper Atmosphere: 1. Mesosphere and 2. Thermosphere.

A) Lower Atmosphere:

1. Troposphere:
The altitude of the troposphere changes according to latitude. It has an elevation of about 16 km
at the equator and only 8 km at the poles. Its average altitude is about 11 km. It contains near
about 75% of the gaseous mass of the total atmosphere, water vapour and aerosols. It is the realm
of clouds, storm and convective motion, The outstanding characteristic of the troposphere is the
filmy uniform degree in temperature with increase in altitude until minimum temperature of 00C
to -600C is reached. The isothermal layer marking the end of temperature decrease is called
tropopause and it separates troposphere and stratosphere. Throughout the troposphere there is a
general decrease of temperature with increase in height at a minimum rate of about 6.50C/km or
3.60F/1000 ft.

2. Stratosphere:
This is the second atmospheric layer above tropopause which extends upwards about 50
km. The stratosphere contains much of the total atmospheric ozone. The density of ozone is
maximum at 22 to 24 km height approximately. The ozone at the upper layer of this sphere
absorbs the ultraviolet rays from the Sun and temperature may exceed 00C. In stratosphere the
temperature increases with increase in height.

B) Upper Atmosphere:

1. Mesosphere:
This is the third layer of atmosphere. A thin isothermal layer called a stratopause is the boundary
layer, which separates stratosphere and mesosphere. Above the warm stratopause, temperature
decreases with increase in height to a minimum of about-900C at about 80 km height Pressure in
this layer is very low and decreases from 1 Mb at about 50 km to about 0.01 mb at 80 km
nearly. The thin isothermal layer, which separates mesosphere from thermosphere, is called
mesopause.

2. Thermosphere:
Outermost shell is known as thermosphere. It lies above 80 km height . In this sphere the
atmospheric densities are extremely low. In this sphere temperature increases with increase in
height due to absorption of ultraviolet radiation from the Sun. probably it reaches to 9500C at
350 km to 17000 C at an underfined upper limit but these temperatures are essentially
theoretical. Such temperatures are not felt by the hands exposed by astronaut or the artificial
satellite because of rarefied air.
 Solar Radiation and Its Terms

Agriculture is the exploitation of solar energy under adequate supply of nutrients and water by
maintaining plant growth. So it is but natural that any efforts of thoroughly understanding of
solar radiation will be immense use for its fullest exploitation by the crop plants in terms of their
growth and yield.

The sun is the primary source of energy. Supplying about 99.9% out of total energy available at
the earth surface. The temperature of the Sun is 6000 K and gives out energy about 5.6 x10 27
cal per minute. The Sun radiates its energy in the form of wave lengths from 0.15 to 4.0 u and
are generally called as short wave lengths. On contrary after absorption of solar energy, earth
emits its energy between 4 to 100 u and is categorized as long wave length.

There are three methods of transfer of heat or energy that means there are three different ways by
which heat can flow from one point to another are:
1. Conduction
2. Convection
3. Radiation.

For conduction and convection of heat, material medium is necessary. But for radiation material
medium is not necessary, because radiation takes place in the form of Electro magnetic waves.

The ultimate source of all the energy for physical and biological processes occurring on the earth
is radiation received from the sun that is why it is commonly called solar radiation.

Some Terms (Definitions):

1. Radiation:
The transfer of heat energy in the form of electro magnetic waves with the speed of light is
known as radiation (Light speed is 3x105 km/second).

2. Solar insulation:
The heat energy received from the Sun is known as solar insulation.

3. Radiant flux density:

It is defined as the amount of energy received on a unit surface in a unit time.

8. Short wave radiation (SW):

Radiation with wave length range 0.15 to 0.76 u is known as short wave radiation.

9. Long wave radiation:

Radiation with wave length range 0.76 to 100 u is defined as long wave radiation.

Solar Constant: The solar constant is a measure of the rate at which solar wave radiation is
received at the top of the atmosphere on a unit surface unit time.
.

Albedo: The term albedo is usually defined as the fraction or percent of the reflected solar
radiation from the surface to incoming solar radiation.

Green House Effect

Out of the total solar radiation about 47% is absorbed at the earth surface. As a result the earth
becomes hot and starts re-radiating long wave radiation. The exchange between the sky and the
terrestrial radiation is largest governed by the atmospheric gases. The important principle is that
the short wave lengths of the radiation’s from the sun can penetrate the atmosphere without
being fully absorbed. These short radiation’s fall on ground, they heat it , and ground starts no-
radiating long wages, The long waves emitted by the earth are absorbed in the atmosphere by
water waves emitted by the earth are absorbed in the atmosphere by water vapour, CO2 and
Ozone. On absorption of earth radiations these gases become warmer and in turn they again
radiate the heat in still longer waves towards the earth This also increases the earth’s warmth.

The gases namely water vapour, CO2 and Ozone allows the solar radiations categorized as solar
short waves to pass through the atmosphere towards the earth and not allow to escape the long
waves radiations from the earth is known as green house effect.

This heat retaining behavior is similar to the roofed glass or green house used for experiment.

The atmospheric green house effect keeps the earth warm and does not allow its temperature to
fall. The mean temp, of the earth is 150C since long and is maintained by green house effect.

Significance Of Radiation In Agriculture

The importance of the radiation in crop production is as follow:

1. It provides the necessary energy for all the phenomena concerning biomass production.
2. Photo synthetically Active Radiations (PAR) are the real source of energy for photosynthesis
process. Plants are the efficient biological converters of solar energy into biomass. Radiation
defines the yield of crop in particular region.
3. It laso provides the energy for the physical processes taking place in plants, soil and
atmosphere.
4. It conditions the distribution of temperature and hence crop distribution on the earth surface.
 Atmospheric Temperature
Definition: The degree of hotness is known as temperature increases.

Temperature is a fundamental elopement of climate from many points of view, the most
important in controlling the distribution of life on the earth. Most of the weather elements are
dependent on it, directly or indirectly. Air of atmosphere receives the heat energy from the sun
and its temperature increases. Due to different amount of heat energy receipt at different places,
the air temperature at different places also vary. The variation in air temperature basically results
into air motion, so as to equalize the energy content of the different regions of the earth. Thus
temperature of air can be regarded as the basic cause for weather changes.

Qualification of Atmospheric Temperature:

Atmospheric temperature is continuously changing; it is never steady or constant for a long

time. Therefore quantification of atmospheric temperature is very important aspect. The
atmospheric temperature can be quantified in the following ways.

1. Maximum temperature:
It is the highest temperature attained by the atmosphere in diurnal variation.

2. Minimum temperature:
It is the lowest temperature attained by the at by the atmosphere in diurnal variation.

3. Average temperature:
It represents the average temperature condition of the atmosphere during 24 hours of the day.

Temperature Variation

Air temperature at any location is changes during a day, week, month, year or for any period. On
this basis it is classified as-

A Periodic variation.
1. Annual temperature variation or Annual temperature cycle.
2. Diurnal temperature variation or Daily temperature cycle.

B. Horizontal variation

C. Vertical Variation.

A) Periodic Temperature Variation:

The temperature continuously changes during a day, week, month, year or any period and this
change is called periodic temperature variation.Periodic temperature variations are -
1. Annual temperature variation or Annual temperature cycle:
The annual temperature Variation gives rise to seasons i.e. summer and winter. The annual
temperature range varies greatly from place to place. It reflects the daily increase in insulation
from mid-winter to mid-summer and decrease in the same from mid-summer to mid-winter
summer and decrease in the same from mid-summer to mid-winter usually there is a temperature
lag of 30 to 40 days after the period of maximum and minimum insulation

In the Northern hemisphere winter minimum occurs in January and summer maximum in July
and vice versa in the southern hemisphere. The smaller range occurs near equator and largest in
high latitudes. The difference between the highest and lowest temperature for a given period is
known as temperature range. In the northern hemisphere it is summer from 21st of March to
22nd of September and winter from 23rd September to 20th March and vice-versa in southern
hemisphere.

2. Diurnal Temperature Variation or Daily Temperature Cycle.

The Diurnal Temperature Variation give rise to daily maximum and Minimum temperatures.

From the sun-rise, sun energy continuously supplied and the Temperature continuously rises,
recording maximum at about 2.00 to 4.00.P. m. though the maximum amount of solar radiation is
received at the solar None (i.e. 12.00 hrs). This delay in occurrence of maximum temperature is
Caused by gradual heating of the air by convective heat transfer from the Ground which is
known as thermal lag or thermal inertia.
Similarly minimum air temperature occurs shortly offer sunrise due to lag in transfer of heat
form the surface to the air / space.

B Horizontal Temperature Variation:

The rate of change of change of temperature with a horizontal distance is known as Temperature
Gradient.

Maximum solar energy is received in equatorial region and therefore and Therefore highest
temperatures are observed in equatorial region. As the latitude Increases the solar energy
received on the earth correspondingly decreases and so also temperature decreases with increase
in latitude being lowest on the pole.
The Sum crosses the equator twice in a year therefore two maxima And two minima are
observed in annual cycle. Outside this zone only one Maxima and one minima is observed.
Isotherm:
Isotherm is defined as the line on the weather map joining the places of equal temperature.

C.Vertical Tempe ration Variation:

Vertical temperature variation does not show uniform behavior and The atmosphere can be
divided into four spheres.
1. Troposphere - Temperature decreases from 150 C at earth surface up to - 60 0C at 11 km
height.
2. Stratosphere - Temperature increases from -600C to 00C at 50km Height.
3. Mesosphere - Temperature fall and reaches about -900C at 80 km Height.
4. Thermosphere- Temp increases. Due to absorption of solar radiation by
Atomic oxygen, up to 9500C at 350 km height and 17000C at undefined upper Limit.
 Factors Affecting the Air Temperature

The distribution of temperature over the earth surface depends on following factors:
1. Latitude:
Highest temperatures are generally at the equator and the lowest at the poles. The temperature
decreases with the increase of latitude.

2. Altitude:
Temperature decreases with height in troposphere.

3. Season:
Coldest temperatures are in winter and highest temperatures are in summer seasons.

4. Distribution of land and water:

Water bodies are great moderators of temperature. Because of high Specific heat of water, so on
the oceans, the regularity in temperature is more as compared to continents.

5. Topogrtaphy:
Mountain ranges affect the temperature by acting as obstacles to the Flow of cold air cold air
near the surface and they often set conditions of warm winds.

6. Ocean currents:
Hot and cold ocean currents affect temperature e.g. Gulag Stream (Warm) in North Atlantic,
Benguela current (cold) along West coast of South Africa, Peru Current (cold) along West Coast
of South America.

7. Winds:
Various types of wind affect temperature.

8. Clouds and rains:

Clouds by obstructing the heat from the Sun and rains by cooling the Atmosphere, affect the
temperature.

9. Color of the soil:

Black color of soils absorbs more radiations and other types reflect them.

10. Slope of the soil:

Black color of soils absorbs more radiations and other types reflect them.

11. Forest and vegetation:

Due to Evapotranspiration and interception of sun – rays, temperatures are moderated.
 Soil Temperature And Its Importance

The Soil mantle of the earth is indispensable for the maintenance of plant life, affording
mechanical support and supplying nutrients and water.
Soil constitutes a major storage for heat acting as a sink of energy during the day and source to
the surface at night. In annual terms the soil stores energy during the warm season and releases
it to air during the cold portions of the year.

Importance of Soil Temperature:

1. In affects plant growth directly, that is all crops practically slow down their growth below the
soil temperature of about 90C and above the soil temperature of above 500C.
2. For germination of different seeds requires different ranges of soil temperature e.g. maize
begins to germinate at soil temp of 7 to 100C.
3. Most of the soil organisms function best at an optimum soil temperature of 25 to 350C
4. The optimum soil temperature for nitrification is about 320C.
5. It also influences soil moisture content, aeration and availability of plant nutrients.

Variations of Soil Temperature

There are two types of Soil Temperature:

1. Daily and Seasonal Variation of Soil Temperature.

a) There variations occur at the surface of the soil.

b) At 5 cm depth the change exceeds 100C At 20 cm the change is less and at 80 cm diurnal
changes are practically nil
c) On cooler days the changes are smaller due to increased best capacity as the soils become
wetter on these days.
d) On a clear sunny day a bare soil surface is hotter than the air temperature.
e) The time of the peak temperature of the soil reaches earlier than the air temperature due to the
lag of the air temperature.
f) At around 20 cm in the soil the temperature in the ground reaches peak after the surface
reaches its maximum due to more tune the heat takes to penetrate the soil. The rate of penetration
of heat wave within the soil takes around 3 hours to reach 10 cm depth.
g) The cooling period of the daily cycle of the soil surface temperature is almost double than
the warning period.
h) Undesirable daily temperature variations can be minimized by scheduling irrigation.

Seasonal variations of Soil Temperature:

a) Seasonal variations occur much deeper into the soil.

b) When the plant canopy is fully developed the seasonal variations are smaller.
c) In winter, the depth to which the soil freezes depends on the duration and severances of the
winter.
d) In summer the soil temperature variations are much more than winter in trophies and sub
trophies.

Factors Affecting The Soil Temperature And Its Control

1. Solar radiation:

The amount of heat from the Sun that reaches the earth is 2.0 cal/cm2 min -1 the amount of
radiation received by the soil depends on angles with which the soil faces the Sun.

2. Condensation:

Whenever water vapour from soil depths or atmosphere condenses in the soil, its heat increases
noticeably.

3. Evaporation:

The greater the rate of evaporation, the more the soil is cooled.

4. Rainfall:

Rainfall cools down the soil.

5. Vegetation:

A bare soil quickly absorbs heat and becomes very hot during the summer and become very cold
during the winter. Vegetation acts as a insulating agent. It does not allow the soil to become
either too hot during the summer and two cold during the winter.

6. Colour of the soil:

Black colored soils absorbs more heat than light closured soils Hence black color soils are
warmer than light colored soils.

7. Moisture content:

A soil with higher moisture content is cooler than dry soil.

8. Tillage:

The cultivated soil has greater temperature amplitude as compared to the uncultivated soil.

9. Soil texture:

Soil textures affect the thermal conductivity of soil. Thermal conductivity decreases with
reduction in particle size.
10. Organic matter content:

Organic matter reduces the heat capacity and thermal conductivity of soil, increases its water
holding capacity and has a dark color, which increases its heat absorbability.

11. Slope of land:

Solar radiation that reaches the land surface at an angle is scattered over a wider area than the
same amount of solar radiation reaching the surface of the land at right angles. Therefore, the
amount of solar radiation reaching per unit area of the land surface decreases as the slope of the
land is increases.

Soil temperature can be controlled by:

1. Regulating soil moisture.

2. Proper soil management practices so as to have good drainage.
3. Application /use of mulching.
4. Sufficient addition of organic matter.
 Air Pressure

Definition: Atmospheric pressure can be defined as the weight exerted by air column on units
surface of the earth.

Units of pressure:
1. Height of mercury column measured in inches, cm, and mm
2. Bar, bar is a force equal to 106 dynes/cm2
3. SI (standard International) unit or pressure is Pascal
1. Pascal = force of 1 Newton/m2
= 1 N m2

Standard atmospheric pressure:

The standard atmospheric pressure is given at mean sea level at 450K latitude and at temperature
of 2730K

Standard Atmospheric = 29.92 inches or 76 cm or 760 mm

Pressure = 1013.25 mb
= 101.325 kilo Pascal (Kpa)
= 14.7 lbs/inch2
1.014 x 106 dynes/cm2
Isobar:
Any line joining the places of equal atmospheric pressure on the weather map is known as
isobar. Where isobars are closely spaced, a rapid of steep change in reassure is indicated. When
isobars are widely speeding, a slow change in pressure is indicated. Two isobars are never cross
each other. Isobars are plotted on the map to show the distribution of pressure. The isobars are
drawn at pressure intervals of 2, 3,4 or 5 mb.

Pressure gradient:
The rate of change of atmospheric pressure per unit horizontal distance between two points at the
same elevation is known as pressure gradient or isobaric slope. This change tak3es place and is
measured in the direction perpendicular to the isobars preferably from high to low pressures.
This exerts a force on air particles and is important in determining the strength of wind. The
pressure gradient is expressed in decrease in pressure per unit horizontal distance as mb/100
meters.

Variation in Atmospheric Pressure

1) Variation with height or vertical variation:

The pressure depends on the density or mass of the air. The density of air depends on its
temperature. Its composition and force of gravity. I t is observed that the density of air decreases
with increase in height so the pressure also decreases with increase in eight.

The pressure at sea level is 1013.25 mb at 50 km height it becomes 0.93 mb and 80 km it is only
0.03 mb. This indicates how rapidly the atmospheric gas becomes thinner to decrease density
and so the pressure. The pressure decreases on an average at the rate of about 34 mb per every
300 meters height.

2) Horizontal variation of pressure:

The horizontal variation of atmospheric pressure depends on temperature, extent of water vapor,
latitude and land and water relationship.

i)The equatorial low pressure belt :

Along the equator lies a belt of low pressure known as the equatorial low or doldrums or calm.
This low pressure belt lies between 50 North and 50 South latitudes.

ii) Sub – tropical high pressure belt:

The high pressure belt are found between 24 – 300C latitudes in both the hemispheres.

iii) Low pressure belts near 600 latitudes:

The airs from this area get thrown outwards on account of the rotation of the earth and this is
how the low pressure belts are created.

iv) Polar high pressure belts:

The temperature is extremely low in the Polar Regions. The air being cold and heavy throughout
the year a high pressure belt is created in both Polar Regions.

3. Diurnal variation:
At a given station the pressure show the two high and two lows. On normal pressure day two
maxima i.e. one at 10 a.m. and another at 10 p.m. and two minimasi.e. one at 4 a.m. and another
at 4 p.m. are observed. Thus there is double oscillation caused by alternate heating and cooling
of atmosphere.

Factors affecting atmospheric pressure:

1. Temperature of air
2. Altitude
3. Water vapour in air
4. Revolution and gravitation of the earth.
 Wind And Its Importance

Definition: The air that moves parallel to any part of the earth surface is called wind or The air
moving horizontally on the surface of the earth is known as wind.

Air Current:
Vertically or nearly vertical movements of air resulting from convection ,turbulence or any other
cause is known as air current.

Importance or Role or Effects of Wind In Agriculture:

1. Wind increases the transpiration and intake of CO2

2. The turbulence created by wind increase CO2 supply and the increase in photosynthesis.
3. When wind is hot, desiccation of the plants takes place, because humid air in the inter cellular
places is replaced by dry air.
4. The hot and dry wind makes the cells expanding and early maturity, it results in the dwarfing
of plants.
5. Under the influence of strong wind the shoots are pressurized and get deformed.
6. Strong winds produces loading of crops.
7. The coastal area affected by strong wind bring salt and make the soil unsuitable for growing
plants.
8. Strong winds affect the plants life both mechanically and physiologically.

Wind Direction and Wind Speed

A wind is named according to the direction from which it blows e.g. a wind coming from west is
called west wind.

The direction from which wind blows is termed as windward direction and that to which it blows
is termed as leeward direction.

Earth’s general circulation system (Surface wind)

The earth’s surface wind system or earth’s general circulation of wind can be represented by a
simple model shown in fig. In this model, the earth surface been considered uniform, means
either all and or all water and the effects of local systems have been ignored, therefore the actual
wind system is much more complicated than the described in the model.

It is to be noted that unequal heating of the earth’s surface generate pressure gradient which give
rise to wind. There are three latitudinal circulations and there are also important longitudinal
variations around each hemisphere.
1. Trade winds:
The condition of greatest heating and expansion at the equator causes rising of air and creating
low pressure belt (50 N to 50S latitude) known as doldrums or equatorial low or calm. The rising
of air from equator causes increase in pressure at 350 N 350S which is known as sub-tropical high
or Horse latitude belt. The winds therefore flow from horse latitude belt. The winds therefore
flow from horse latitude to the equatorial region called “Trade winds” While moving these
winds, they are deflected by corollas force to the right in the and nor them hemisphere and to
left in southern hemisphere and become North – East trades and south –East Trades in northern
and southern hemispheres respectively.

The blow of air from equator and accumulation of air over 25-350 latitudes giving rise to high
pressure belt region of descending air is known as Hadley cell.

2. Westerlies wind:
There situated at about 600 – 650 latitudes a low pressure area in both the hemisphere is known as
sub-polar low or polar front. The winds that flow from sub-tropical high pressure area (Located
at 250 - 350 latitude in both the hemisphere ) to the low pressure area, situated at about 600 -
650 latitude in both the hemisphere, are known as Westerlies or prevailing wisterias ( anti trade
winds) In the upper atmosphere the reverse air movement takes place. This circulation is known
as feral cell. These winds instead flowing in straight line are deflected due to corollas force. In
northern hemisphere their direction is North – West and in southern hemisphere it is South-West.

3. Polar winds or Polar Easterlies winds :

Near the poles due to shrinkage of air and due to cooling, there exists permanent high pressure
on the poles. Therefore winds flow from the polar high to sub-polar low pressure area at about
60-650 latitude. The wind flow in North-East direction in northern hemisphere and in south-East
direction in southern hemisphere. These winds consist of cold air. The air circulation is known
as polar cell.

Local Winds:
These winds are generated due to local condition and hence influence over very small area,
therefore such winds are called local wind.

1. Land and sea breeze :

An interchange of air between the sea and coastal land due to unequal heating and cooling is
known as land and sea breezes. They are local in nature. During day time the coastal land and
sea breezes.. They are local in nature. During day time the coastal land as heated very fast as
compared to sea water causing low pressure over the land. Therefore the surface air blows from
sea to land and this is known as sea breeze. While during night time, the land cools faster than
the sea, causing high pressure area over land as compared to sea. Therefore air blows from land
to sea and this is known as land breeze

2. Mountain and valley breeze:

An interchange of air between the mountain and valley due to unequal heating and cooling of the
two places is known as mountain and valley breeze. During daytime, the valley breeze. During
daytime, the valley floors become more heated; the air over it expands and rises. This rising air
slides up the mountain slope and is known as valley breeze. During night reverse process takes
place. Due to cooling of the air in the valley contracts and consequently augmented by air from
the neighboring hills and mountains. The air on the mountain slopes also cools and slides down
into the valley which is known as mountain breeze.

3. Ketabatic winds:
A mass of cold air over an elevated plateau during the winter tends to become more dense
through radioactive cooling and then will drain down the slopes into the valleys below. The
resulting down slope, drainage type winds are called Ketabatic winds. Most are relatively gentle
breezes, not exceeding 4 to 5 m/s occasionally how ever, the cold dense air may be set in motion
by a migrating cyclone or anticyclone and the Ketabatic winds may then attain destructive
violence.

Monsoon Wind

An interchange of air between the land and oceans due to unequal heating and cooling of
continents and oceans is known as monsoon winds.

It has an annual period of occurrence. During summer, the land is heated Very much as
compared to the oceans which cause which oceans low pressure Over the land and the winds
blow from the oceans to the continents. During winter, land cools down faster than the oceans
causing high Pressure over continents and low pressure over the oceans and the wind Blows
from continents to oceans. The Indian monsoon is the best known Example of this alternating
circulation system. There are two types of Monsoons over India i.e. south – west monsoon and
North- East monsoon.

1.South – West Monsoon:

India is positional situated in North – East trade winds and should have N- E winds throughout
the year, but a low pressure through lies along the Ganges and upper India, due to which S.- W
winds predominate. During April to September a low pressure center is formed over N – W
India. The s-W trade winds of the Indian ocean blow to the equator and then turning to the right
under carioles force and move on a S – W winds Around the low pressure center over India.
[This monsoon blows from the African coast (150E)]. The moisture laden air while rising the
mountain of Asia cools, condense and precipitate. As a result the pressure is lowered to increase
the pressure gradient.

2. North – East Monsoon:

Complete reversal of the S – W monsoon winds takes place as the high pressure centre is located
in eastern Asian (1035 mb) and low in about 1010 mb. During this time from North to South the
cold season is established. This monsoon is active during October and November. The winds
flow in North- East direction. This wind is generally dry but gives rains to AP, TN states.
Monsoon winds also exist over west Africa, Brazil, eastern USA, Australia. Philippines etc.
 Forces Acting to Produce Wind

Wind is motion of air in response to unbalanced forces acting in horizontal direction. The
different forces involved in the flow of the wind are described below:-

1. Horizontal pressure gradient (PG) force:

The rate of change in atmospheric pressure between two points at the Same elevation is called
the pressure gradient or isobaric slope. It is proportional to the difference in pressure and is the
immediate Cause of horizontal air movement. The direction of air flow is from high to Low
pressure and the speed of flow is directly related to the pressure Gradient. The pressure gradient
is said tube steep when the rate of change Is great and the gradient the more rapid will be the
flow of air. The direction of the pressure gradient is perpendicular to the isobars and pointing
towards low pressure.

2. The earth’s rotational deflective force [Coriolis force]:

This force comes into play due to rotation of the earth on it axis. It has most potent influences
upon wind direction. The Coriolis force effect Causes all winds in the nor them hemisphere to
move or deflect toward the Right and those of the sot herm hemisphere to move to the left with
respect to the rotating earth. At the equator the effect has a value of zero and it increases
regularly towards the poles The Coriolis effect changes wind direction but does not change wind
speed.

3. Centrifugal force :
This force tends to throw the air particles outward from the centre of small circle path on which
the particle is moving. The centrifugal force works against the gravitational attraction directed
forwards the earth centre.

4. Frictional force:

The roughness of the surface provides frictional resistance to the air motion. It is the retarding
effect of trees, buildings and other irregularities in the topography; It is always opposed to the
direction of the air motion and therefore fends to decrease the wind speed. Friction causes a
movement of air across the isobars towards low pressure.

5. Geotropic winds:
When a wind flows in a straight line with no acceleration or frictional force on it, the only forces
acting are the carioles and pressure gradient foresee. The wind that blows under these conditions
is called as geotropic wind.

Cyclone and Anticyclone

It is the atmospheric disturbance in which the air pressure decreases at a particular location (Low
pressure at centre) and there is a wind movement towards centre.
A system of close isobars with the lowest pressure at the centre is called as cyclone.
The pressure gradient force and carioles force cause air, flow in cycle to be spurning convergent
system. In the northern hemisphere the direction of rotation of cyclone is antilock wise while in
the southern hemisphere it is clockwise. Cyclones are also known as Lows or press ions. The
velocity of wind in cyclone is more than 34 knots.

Anticyclone:
When there is a area of high pressure at centre, the flow of air starts from centre to outer side.
A system of closed isobars with highest pressure at centre is known as anticyclone.

The air flow has spiraling divergent system so that it moves obliquely across isobars away form
centre. The direction of rotation of antic clones in the northern hemisphere is clock wise while in
southern hemisphere it is antilock wise. These are known as “Hights.”
 Atmospheric Humidity

Humidity:
Water vapor present in the atmosphere is known as humidity.

Vapour pressure:
When water vapour mixes with other gases of atmosphere, it exerts a pressure in all directions as
do the other gases. This partial pressure exerted by vapour is known as the vapour pressure.

Saturated vapour pressure :

When air contains all the moisture that it can hold to its maximum limit, it is called saturated air
and the vapour pressure exerted by this air is called saturated vapour pressure.

Relative humidity (RH) :

Relative humidity represents the amount of water vapour actually present in air compared with
the maximum amount of vapour can be held b same air at a given temperature.

RH = Actual quality of water vapour present in a given volume of air

x 100

Maximum amount of water vapour the same volume of air can hold

Absolute humidity (AH):

Absolute humidity is defined as the ratio of actual mass of water vapour present to the total
volume of moist air in which it is contained.

It is measured in grams per cubic meter of air or in terms of partial pressure of water vapour in
air in mbor mm of mercury.

AH = Weight of water vapour

Volume of air

Specific humidity:
Specific humidity is defined as the ratio of mass of water vapour to the given mass of air
containing the moisture.

Thus it will be measure in grams of water vapour per kilogram of air.

Psychometric: Measurement of humidity with different instruments is called psychometric.

 Variation in Relative Humidity

Diurnal variation in RH

The diurnal variation in relative humidity is approximately inverse to that of temperature about
sunrise the RH is maximum and between 14 to 15 hrs minimum RH is observed.

Annual variation in RH: The annual variation of relative humidity is largely depends upon the
locality.
At regions where the rainy season is in summer and winter is dry, the maximum relative
humidity occurs in summer and minimum in winter and at other regions maximum RH occurs in
winter.
Over ocean RH reaches maximum in summer.

Variation of RH with latitude and altitude:

RH shows maximum at equation or about 80%. Therefore it decreases to 70% in the regions of
high pressure belts in 30-350 and afterwards increases again to 80 to 90% in the polar region.

The vertical variation of RH is not governed by any exact law. In or near the clouds it is 100%
but below or above it is different. The moist air masses carried out by advection at altitudes also
change relative humidity.
 Clouds, It's Types and Their Classification

Cloud:
Cloud can be defined as a mass of tiny water droplets ice crystals OR both condensed on
hygroscopic nuclei and suspending in the atmosphere.

Clouds and fogs are composed of water droplets or ice crystals or both of the order of size 20 to
60 microns (0.008-0.024 millimeter).

Isoneph:
Lime joining places of equal clouds cover on a map is known as isoneph.

Principles of cloud classification :

The great variety of cloud forms necessitates a classification of weather reporting. The
internationally adopted system is based upon (a) The general shape; structure and vertical extend
of the clouds and (b) their altitude.
Types of clouds: There are four basic types of clouds:

1. Cirrus (CI):
Meaning “cur” and is recognized by its veil, like fibrous or featery form. It is the highest type of
cloud, ranging from approximately 7-12 km in altitude. (20,000 to 35,000 feet).

2. Cumulus (Cu):
Meaning “heap”, is the wooly, bundly cloud with rounded top and flat base. It is the most
common in the summer season and in latitudes where high temperature prevail and it always
results from convection Its height is variable and depends on relative humidity of the air.

3. Stratus (St):
It is a sheet type cloud without any form to distinguish it. It is usually lower than cumulus.

4. Nimbus (Nb):
It is any dark and ragged cloud and from which precipitation occurs.
 Classification Of Clouds
Clouds have been classified according to their height and appearance by world Meteorological
organization (WHO) into 10 categories.

Cloud family and Name of cloud Composition Possible weather Description and
Height and abbreviation change appearance
1 2 3 4 5
Family A High 1 Cirrus (Ci) Ice crystals May Indicate It is wispy and feathery, sun
clouds 7 to 12 km storm showery shines without shadow. It
weather does not produce
precipitations

2. Cirrocumulus Ice crystals Possible storm Meekerel sky, often fore

(CC) renners of cyclone, look like
sippled sand
3. Cirrostratus(Cs) Ice crystals Possible storm Meekeral sky, often fore
runners of cyclone, look like
sippled sand.
Family B middle 4. Altocumulus Ice & water Steady rain or Looks like wool peak, sheep
clods 3 3-7 km (As) snow bulk clouds.
5. Atmostratus Water and ice Impending rain or Fibrous veil or sheet, grey or
(As) snow bluish, produce coronos,
usually ct.st shadow.
Family C low clouds 6. Stratocumulus Water Rain possible Long parallel rolls, pushed
from ground to km (Se) together or broken masses
which look soft and grey but
with darker parts, air is
smooth above but strong
updrafts occur below.
7. straus (St) Water May produce A low uniform layer,
drizzle resembling fog, but resting
not on the ground, chief
winter cloud.
8.Nimbostrauts Water or Ice Impending rain or Fibrous veil or sheet, grey,
snow grey or bluish produce
coronas, usually
Family D clouds with 9.Cumu-lus (Cu) Water Fair weather Looks like wool pack, dark
vertical development below due to shadow, may
from 0.5 to 16 km develop into cumulous –
Nimbus flat base.
10, Cumulous – Ice in upper level Violet winds rain, Thunder head, towering
Nimbus (Cb) and water in lower all possible anvil top, violet up and down
level. thunderstorm hail drafts, aviators avoid them,
lighting possible develop from cumulus, chief
precipitation makers.
10 cumulo- Ice in upper level Violet winds rain, Thunder head, towering
Nimbus(Cb) and water in lower all possible anvil top, violet up and down
level thunderstorm hail drafts, aviators avoid them,
lighting possible develop from cumulus chief
precipitation makers.
 Forms of Condensation
1. Dew:
The deposition of water vapour in the form of tiny droplets on the colder bodies by condensation
is known as dew. The clear sky, absence of wind. The object on which dew forms must be good
radiator and bad conductor are necessary conditions for formation of dew.

2. Front:
When the temperature of air falls below 00 C before the dew point is reached, the water vapour
is directly converted into crystals of ice, and this is called as frost. It is frequently called as a
form of sublimation, Forts is injurious to vegetation.

3. Fog:
Extremely small water droplets suspending in the atmosphere and reducing the horizontal
visibility is fog.

Conditions necessary for formation of Fog:

i) The ground should not get much heated during daytime.
ii) The air must not be very dry
iii) Wind velocity should be calm or less the 3 mph.
iv) Inversion, radiation or cold mass advection must takes place.

Classification of Fog:
A) Thick Fog : Restricts visibility up to 45 meters
B) Moderate Fog : Restricts visibility up to 450 meters
C) Thin Fog : Restricts visibility up to 900 meters.

4. Mist:
Mist is less dense fog. The suspended water droplets restrict Visibility between 1000 to 2000
meters or 4 on the coded scale (IMD) The obscurity is known as mist. Relative humidity is at
least 75% Mist disappears with rising sun.

5. Rime:
It is formed when wet fog having super cooled droplets immediately freeze on striking objects
like telegraph post having temperature below freezing point. White ice is formed on windward
side.

6. Smog:
The combined effect of smoke and fog droplets may reduce visibility and this phenomenon is
called smog.

7. Haze:
Some solid particles like dust, smoke from fire and industry restrict visibility is haze.
 Hydrological cycle

Water is essentially required for different life forms such as plants animals. Birds etc. For cell
building and other purposes. The main source for the water is ocean. The water from the oceans
is evaporated, clouds are formed and carried away by wind and they precipitate. The water
received from precipitation is lost to the ocean back by different processes such as run- off
evaporation from soil, lakes and ponds, streams, etc evapotranspiration from plants the water
which is absorbed in the ground is also lost by direct or indirect way to the ocean, for example,
some water which is absorbed in ground is utilized by plants and then evaporated, the ground
waer which is absorbed in ground is utilized by plants and then evaporated. The ground water
flows to the streams and the stretch finally lost in the oceans etc. Thus, we find that there is a
constant circulation of water from oceans to the air and back again to the oceans. This process
has not end beginning and therefore it is termed as hydrological cycle or water cycle.

Precipitation and Forms of Precipitation

It can be defined as earthward falling of water drops or ice particles that have formed by rapid
condensation in the atmosphere.

Forms Of Precipitation

B. Solid form C. Mixed form

A. Liquid form
1. Rain 1. Snow 1. Sleet
2. Drizzle 2. Hail 2 Hail
3.Shower

A. Liquid Form

1. Rain:
Rain is defined as precipitation of drops of liquid water. The clouds consists of minutes of
minutes droplets of water of about 0.02 mm diameter. When these minute water droplets in
clouds combine and form large drops that become so large that they can not remain suspended in
the air and they fall down as rain. The droplets are formed by repaid condensation. The rain
drops have diameters ranging from 0.05 to 0.06 cm (0.5 to 0,6 mm) The line joining the places of
equal rainfall called Isohyets.

Types of Rain:

I) Convectional rains:
Due to heating, the air near the ground becomes hot and light and starts upward movement (This
is known as convection.) as air moves upward it cools at the DALR (9.80C/km) and becomes
saturated(having RH 100%) and dew point is reached where the condensation. begins .
This level or height is known as condensation level. Above condensation level air cools at
SALR (5 0C/km) clouds are formed. Then further condensation results into precipitation. These
rains are known convectional rains.

II) Ographic or relief rains:

When the moist air coming from sea encounters mountain or relief barrier, it can not move
horizontally and has to overcome mountain. When this air rises upward, coolsdown, cloud is
formed and condensation starts and giving precipitation. These rains are known as or orographic
rains thus high rains are possible on the windward side of the mountain. After crossing the
mountain divide, when air descends downward, the air is compressed and it warmed up at
DALR. This warm air does not give any precipitation on the leeward region. This is known as
rain shadow region.

III) Cyclonic /Frontal and Convergent rains:

Frontal precipitation is produced when two opposing air currents with different temperature
meet, vertical lifting takes place which gives rise to condensation and precipitation. When the
humus and warm air mass meets the cold air mass, the colder air being denser tends to push
below the warmer air and replace it. The boundary zones along which two air masses meet are
called as fronts. When the mixing of warm and moist air with cold air mass takes place, the
temperature of the warm and air falls down, saturation occurs and may give precipitation and it
also responsible for cyclone formation and rains received from cyclones are called cyclonic
reins.

Thunder Storms:
It is the atmospheric disturbance which is always accompanied by thunder and lightening and
sometimes by hail. It is a local storm covering comparatively small area and often causing
damage. Its chief Characteristics are an immense cumulo-nimbus cloud accompanied by copious
precipitation, a marked drop in temperature and a more or less destructive out rushing squall
wind which precedes the rainfall. Thunder storms occur in every part of the world and their
frequency decreases with increase in latitude.

Storms are of two types:

1) Frontal or general thunderstorm:

This occurs over wide areas in connection with passing of a cyclonic disturbance.

2) Local thunder storm:

This forms as a result of strong local convection.

3) Drizzle:
It is more or less uniform precipitation of very small and numerous raindrops which are carried
away even by light wind. The drizzle drop is less than 0.5 mm in size, and precipitate at the rate
usually less than 1 mm per hour.
4) Shower:
Precipitation lasting for a short time with relatively clear intervals is called shower. This occurs
from the passing clouds.

B) Solid Form:
1. Snow:
Snow is defined as precipitation of water in the solid form of small Or large ice crystals. It
occurs only when the condensing medium has a temperature below freezing temperature, snow is
generally in the form of individual crystals or in flakes that are aggregates of many
crystals. Snow flakes are formed in high clouds. Snow is measured with snow gauge.

2. Hail:
Hail is a precipitation of solid ice. On a warm sunny day, a strong Connective column may cause
the formation of pellets having spherical Shape and concentric layers of ice. Such a formation is
known as hail.

C. Mixed Form:
1. Sleet
Simultaneous precipitation of the mixture of rain and snow is called as sleet.

2. Hailstorm:
Rainfall associated with hail stones is called hailstorm.

Mechanism or process of Rain formation or Process of Precipitation:

There are two methods by which rain drop is formed.

1. Bergeron mechanism:
There are two methods by which rain drop is formed.

1. Bergeron mechanism:
The cloud having cold temperature is cold temperature is cold cloud. In these clouds Ice particles
are formed due to very low temperature (-150C to -250C). These ice particles are grow rapidly
by deposition of water vapors (sublimation) developing in to hexagonal shaped ice crystals.
These ice crystals on collision form snow pellets and melt into water droplets when falling on
ground through warm atmosphere. This mechanism is suggested by Swedish Meteorologist
Bergeron in 1933. Artificial rain making is based on these mechanisms.

2. Collission and coalescence mechanism:

The cloud having slightly higher temperature is not cloud. In these Clouds fine water droplets
exist instead of ice particles. This fine water Droplets colloid and coalesce (combine) and grow
into the larger size and fall on earth as rain drop.
 Drought and Its Classification

Definition:
Drought is a period of inadequate or no rain fall over extended time creation soil moisture deficit
and hydrological imbalances.

Classification of Drought:

Drought on different basis is generally classified into three categories.

A) Based on source of B) Time of occurrence C) On the basis of

Water availability medium:

1.Meteorological 1. permanent drought 1. Soil drought

Drought

1. Slight drought
2. Moderate drought
3. Severe drought

2. Hydrological drought 2.Seasonal drought 2. Atmospheric drought

3. Agricultural drought. 3.Contigent drought

Drought classification

A. On the basis of source of water availability:

Drought is classified into three types on the basis of water availability.

1. Meteorological drought:
The meteorological droughts mainly indicate deficit rain of different quantum. The IMP
classified this drought as follows from the rainfall departure.

1. Slight drought : When rainfall is 11 to 25% less from the normal rainfall.
2. Moderate drought : When rainfall is 26 to 50% less than the normal rainfall.
3. Severe drought : When rainfall is more than 50% less than the normal rainfall

2. Hydrological drought:
It is defined as the situation of deficit rainfall when the hydrological sources like streams, rivers,
lakes, wells dry up and ground water level depletes. This affects industry and power generation.

3. Agricultural drought:
This is the situation resulted from inadequate rainfall, when soil moisture falls to short to meet
the water demands of the crop during growth. Thus affects crop may wilt due to soil moisture
stress resulting into reduction of yield.
B. On the basis of time of occurrence:
Drought differs in time and period of their occurrence and on this basis Thormathwite delineated
following three areas.

1. Permanent drought area:

This is the area generally of permanent dry, arid p desert regions. Crop production due to
inadequate rainfall is not possible without irrigation. In the these areas vegetation like cactus.
Thorny shrubs, xerophytes etc. are generally observed.

2. Seasonal drought:
It occurs in the regions with clearly defined as rainy (wet) and dry climates. Seasonal drought
may occur due to large scale seasonal circulation. This happens in monsoon areas.

3. Contingent drought:
This results due to irregular and variability in rainfall, especially in humid and sub humid
regions. The occurrence of such droughts may coincide with grand growth periods of the crops
when the water needs are critical and greatest resulting into severity of the effects i.e. yield
reduction.

C. on the basis of medium:

On the basis of medium in which drought occurs. Mexico (1929) has divided the drought into
two types.

1. Soil drought:
It is the condition when soil moisture depletes and falls short to meet potential
Evapotranspiration of the crop.

2. Atmospheric drought:
This results from low humidity, dry and hot winds and causes desiccation of plants. This may
occur even when the rainfall and moisture supply is adequate.

Strategy to mitigate drought OR How to overcome the drought:

1. Preventing and recycling of excess runoff
2. Deep tillage to absorb and hold maximum moisture.
3. Timely weed management to control water loss by ET.
4. Planning for suitable cropping system.
5. Selection of short duration and drought tolerant crops.
6. Contingency crop planning for abnormal weather situation.
7. Management of various inputs to suit the climate.
8. Conserving the soil moisture by agronomic practices like mulching use of antitranspirant on
the crops to reduce ET.
9. To apply irrigation.
10. Reduction of plant population to reduce ET.
11. Timing of foliage to reduce ET.
 Weather Forecasting And Its Classification

Weather forecast:
Means any advance information about the probable weather in future, which is obtained by
evaluating the present and past meteorological conditions of the atmosphere is called weather
forecast.
Agricultural weather forecast:
Forecasting of weather elements viz sunshine hours, occurrence of dew, relative humidity,
rainfall, temperature, winds etc. Which are important in agriculture and for farming operations is
known as agricultural forecast.
In weather forecasting the advance information of weather elements like distribution of rain fall,
warming for heavy rain fall, temperature change important special hazardous weather like if
thunderstorm hailstorm, show or frost, sky cover, winds, humidity, dew drought, evaporation rate
etc is provided.

Classification weather forecasting:

Weather forecasting on the basis of their validity periods or time scale is classified as follows :

1. Now casting:
It is based on synoptic situation prevailing at the time of forecasting and is valid up to 3 days on
72 hrs and is issued twice a day.

2. Short range forecast (SRF):

It is based on synoptic situation prevailing at the time of forecasting and is valid up to 3 days
or 72 hrs. and are issued twice a day.

3. Medium range forecast (MRF):

Forecasting of meteorological elements over different agro climatic zones for periods ranging
from3-10 days is known as medium range forecast.

4. Long range forecast (LRF):

The forecast valid for more than 10 days (i.e. a month or a season is knows as long range
forecast.

Importance or Significance of Weather Forecast in agriculture

1. The forecast of the weather events helps for suitable planning of farm.
2. It helps in to undertake or withheld the sowing operation
3. It helps in following farm operation:
I) To irrigate the crop or not
II) When to apply fertilizer or not.
III) Whether to start complete harvesting or to withhold it.
4. It also helps in to take measures to fight frost.
5. It helps in transportation and storage of food grains.
6. Helps in management of cultural operations like plugging harrowing hoeing etc.
7. It helps in measures to protect livestock.
WEATHER MODIFICATION – ARTIFICIAL RAIN MAKING AND CLOUD SEEDING

Weather modification refers to willful manipulation of the climate or local weather.

Research done in this field goes back to as far as the early 1940s when the US military experimented with
cloud seeding to stimulate rain. Today, private corporations have joined the weather modification
research effort to protect people, cities and assets from the damage extreme weather brings.
Principles of rainmaking
Clouds are classified into warm and cold clouds based on cloud top temperature. If the cloud temperature
is positive these clouds are called warm clouds and if it is negative they are called as cold clouds. The
nucleus needed for precipitation differs with type of clouds. Hygroscopic materials are necessary as
nucleus for warm clouds
History of Cloud Seeding
Cloud seeding experiments started with the work of a scientist from General Electric named Vincent
Schaefer who discovered that ice crystals can induce precipitation. Since ice crystals are difficult to
transport and spread over an area, silver iodide, a compound with similar properties, was used as a
substitute. The experiments continued until the 1970's when the program was shelved because of lack of
usable results.
Cloud seeding
Cloud seeding is one of the tools to mitigate the effects of drought. It is defined as a process in which the
precipitation is encouraged by injecting artificial condensation nuclei through aircrafts or suitable
mechanism to induce rain from rain bearing cloud. The raindrops are several times heavier than cloud
droplets. These mechanisms aredifferent for cold and warm clouds.
How it Works
Cloud seeding involves the use of water-absorbent materials to encourage the formation of clouds and
rain so that there could be increased crop production in areas where there's little water. This practice has
already been implemented in some areas like Texas and Utah, though not without its share of
controversies. The effectiveness of cloud seeding cannot be proven and some worry that it may actually
cause harm.

Seeding of cold clouds

This can be achieved by two ways (1. Dry ice seeding and 2. Silver Iodide seeding).
1. Dry ice seeding
• Dry ice (solid carbon-dioxide) has certain specific features. It remains as it is at –80oC and evaporates,
but does not melt. Dry ice is heavy and falls rapidly from top of cloud and has no persistent effects due to
cloud seeding.

• Aircrafts are commonly used for cloud seeding with dry ice.
• Aircraft flies across the top of a cloud and 0.5 – 1.0 cm dry ice pellets are released in a steady stream.
• While falling through the cloud a sheet of ice crystals is formed.
• From these ice crystals rain occurs.
• This method is not economical as 250 kg of dry ice is required for seeding one cloud. To carry the heavy
dry ice over the top of clouds special aircrafts are required, which is an expensive process.
2. Silver Iodide seeding
Minute crystals of silver iodide produced in the form of smoke acts as efficient ice-farming nuclei at
temperatures below –5 ｰ C. When these nuclei are produced from the ground generators, these particles
are fine enough to diffuse with air currents. Silveriodide is the most effective nucleating substance
because; its atomic arrangement is similar to that of ice. The time for silver iodide smoke released from
ground generator to reach the super cooled clouds was offer some hours, during which it would draft a
long way and decay under the sun light. The appropriate procedure for seeding cold clouds would be to
release silver iodide smoke into super cooled cloud from an aircraft. In seeding cold clouds silver iodide
technique is more useful than dry ice techniques, because, very much less of silver iodide is required per
cloud. There is no necessity to fly to the top of the cloud, if area to be covered is large.
Seeding of warm clouds
1) Water drop Technique
Coalescence process is mainly responsible for growth of rain drops in warm cloud. The basic assumption
is that the presence of comparatively large water droplets is necessary to initiate the coalescence process.
So, water droplets or large hygroscopic nuclei are introduced in to the cloud. Water drops of 25 mm are
sprayed from aircraft at the rate of 30 gallons per seeding on warm clouds.
2) Common salt technique
Common salt is a suitable seeding material for seeding warm clouds. It is used either in the form of 10 per
cent solution or solid. A mixture of salt and soap avoid practical problems. The spraying is done by power
sprayers and air compressors or even from ground generators. The balloon burst technique is also
beneficial. In this case gunpowder and sodium chloride are arranged to explode near cloud base
dispersing salt particles
 Weather Disasters Management – Rainfall, Heat And Cold Waves,
Windstorms, Thunder Storms And Dust Storms, Tornados, Defective
Insolation

Weather Disaster Management

1 Crops depend upon certain optimum weather conditions for their potential production,
although other variables such as fertilizers, insecticides., etc interact to certain extent in an
agricultural system
2 Daily, seasonal and long term variations in any or all the climatic elements alter the efficiency
of plant growth thereby the crop production.
3 The deviation of climatic factors considerably from their normal values is referred as "Adverse
weather" or "Adverse climate" depending on duration of such impact.
4 The following are the adverse weather conditions and the possible management strategies.
1. Rainfall
Rainfall is the major source of water which is essential for plant growth and development.
However, rainfall is considered under it is a. Excess rainfall b. Scanty rainfall and c Untimely.
The total amount of rainfall in a season is not the criteria. But, its well distribution over a large
area is desirable. Heavy rains with short frequencies will result in floods. If 125 mm of rain is
received in two and half hours it is called as heavy rain.
a Excess rainfall
1 Even though water in all its forms plays a fundamental role in the growth and production of all
crops excessive amounts of water in the soil alter various chemical and biophysical processes.
2 Free movement of oxygen is blocked and compounds toxic to the roots are formed, due to
drainage problem.
3 Soils with high rate of percolation are unsuitable for cultivation as plant nutrients can be
removed rapidly.
4 Heavy rains directly damage plants on impact or interfere with flowering and pollination.
5 Top soil layers are packed or hardened which delays or prevents emergence of tender
seedlings.
6 Snow and freezing rain are threats to winter plants. The sheer weight of ice and snow may be
sufficient to break limbs on trees and shurbs.

7 A thick ice cover on the ground tends to produce suffocation of crop plants such as winter
wheat.
8 Under excess rainfall conditions floods occur. In areas drained by large river systems.
9 Floods submerge crops; silt up fields; tank bunds and river embankments are washed off.
Management of excess rainfall (floods)
1 By constructing multipurpose projects such as irrigation and electric systems.
2 Planned afforestration.
3 Keeping the field drains open
4 By growing flood obstructing crop
b Scanty rainfall
This is a synonym with "Inadequate rainfall" or `Drought'. The influence of drought can be
observed not only on phenology but also on phenophases of crop plants
1 Water limitation from seedling emergence to maturity in all the cereals is very damaging.
2 Water stress/drought during flowering reduces the size of inflorescence, affect fertilization,
grain filling and reduce final yield.
3 Plants show wilting symptoms
4 Cell division and enlargement are very sensitive to drought stress, which results in stunted
growth.
5 Drought effect nutrient absorption, carbohydrate and protein metabolism and translocation of
ions and metabolites.
6 Abscission of leaves, fruits and seeds can be induced by plant water deficit during droughts.
7 Plant respiration is drastically reduced.
Management of drought / scanty rain
1 Application of sufficient irrigation water negates the condition of insufficient or scanty rain.
2 Discover amount of water needed at various stages and adjust the sowing dates.
3 Conserve water by suitable management of fallow and cropped field’s viz., breaking up the
surface to reduce runoff, removal of weeds, digging pits of small size which collects runoff
water, etc.
c Untimely rains
This refers to rainfall received too early or too late in the season with the result that
normal agricultural operations are upset

1 Too early rains do not permit proper preparation of seedbed due to heavy rains.
2 Too late rains delay sowings and pest attack cause collosal losses.
3 Wet spells during flowering and harvesting results in poor fertilization and subsequent loss in
yield.
Management of untimely rains
1 Farmers shall be advised to follow the weather forecasting by IMD for proper management of
their crops through crop-weather advisories.
2 Contingency crop plans shall be made available to the needy farmers.
2. Temperature
Temperature is essential for all plant physiological processes, gaseous exchange between plant
and environment, stability of plant enzymatic reactions etc (chapter 3). However, both cold and
heat waves and abnormal soil temperatures are adverse to crop growth and development.
(a) Cold Waves
During winter (December - February), temperature decreases generally over the Indian
subcontinent. It is lower in northern-India and higher in southern- India. This fall in temperature
may cause damage to the crops. If the temperature drops on freezing or below, a frost may occur
which causes severe damage to the crops/crop plants. Threat of frost is danger to crops.
1. Forst is a form of condensation that forms on cold objects when the dew point isbelow
freezing
2. Frosts are of two types.
a. Advection or air mass frost: Which results when the temperature at the surface in an airmass is
below freezing
b. Radiation frost: Which occurs on clear nights with a temperature inversion
3. There is a special case of frost caused by loss of heat by evaporation. This occurs when cold
rain showers wet the leaves and are then followed by dry wind.
Advection Frost
The usual effects of Advection frost are:
1 The injury and death caused by frost is due to the formation of ice crystals in and outside the
plant cells.
2 During dormancy, plants can withstand lower temperatures upto -20oC.
3 Once growth has commenced temperatures of few degrees below freezing point may be fatal.
4 The cell sap gets frozen below 0oC, as also between cells.
5 Extra cellular ice formation occurs followed by withdrawal of water from the cell.

6 The protoplasm may become dehydrated and brittle, resulting in mechanical damage, or the
cell may contract and damage the protoplasm.
Management of advection frost
For production of most field crops, the only satisfactory solution to the problem of advection
freezing is to avoid it as far as possible by planting after the damage is past and by selecting
varieties which will mature before the beginning of the hazard.
Radiation frost
The damage due to radiation frost differs from the above freeze damage in degree and its spotly
occurrence.
1 This radiation frost damage is critical during critical stages of growth.
2 Young seedlings may be killed.
3 Flowering stage is most prone.
4 Crops like potato, tomato and melons are vulnerable right upto maturity.
5 For most field crops and orchard crops flowering stage is most critical for frost damage.
6 Forsty nights followed by warm sunny days produce a sunclad on orchard fruits, considerably
reducing their production.
Management of radiation frost
The management of radiation frost can be grouped into a) Passive and b) Active
a. Passive methods
• Clean cultivation.
• Maintenance of soil moisture.
• Wrapping plants with insulating material and enclosing the basal part of the plant.
• Proper site selection.
• Choice of growing season.
• Breeding of cold resistant varieties.

The above methods can be followed even for advection frost also. These passivemethods do not
involve any modification of environment.
b. Active methods
The active methods of frost protection are many, like use of
• Heaters.
• Wind machines.
• Sprinkling water.

Following weather forecasst for better management of crops.

B. Heat Waves
These are very harmful during summer. These are experienced over Deccan and Central parts of
India during March to May. The harmful effects include shedding of fruits, plants drying of
water resources.
1 Loss of water by evaporation from irrigation channels.
2 Transpiration increases from plants beyond recouping levels
3 Plants tend to wilt and die owing to rapid desiccation.
4 Hot winds cause shrivelling effect at milk stage of all agricultural crops.
Management of heat waves
Adoption of specific agronomic practices like, shelterbelts, choice of varieties etc.
3. Wind
Wind has its most important effects on crop production indirectly through the transport of
moisture and heat. Vegetative growth at `Zero' wind, as experienced in glass houses or under low
glass cover is luxurient. But, there is typically a reduction in
vegetative growth as the wind increases to small values, viz., 1 or 2 metres per second.
Beneficial effects of winds
1 Moderate turbulence promotes the consumption of carbon-dioxide by
photosynthesis.
2 Prevent frost by disrupting a temperature inversion.
3 Wind dispersal of pollen and seeds is natural and necessary for certain agricultural crops and
natural vegetation also.
Harmful effects of winds
1 At sustained high speeds (12-15 metres per second) at plant height, plants assume a low, dwarf
like form, whilest the intermittent high wind speeds experienced in gales, hurricanes etc., results
in gross physical damage to bushes and trees.
2 At higher wind speeds, the shape of the orchard tree alters giving rise to the characteristic wind
shaping of trees in exposed positions.
3 Leaves become smaller and thicker.
4 Breakage occurs and bushes and trees subjected to natural (seasonal) pruning.
5 Direct mechanical effects are the breaking of plant structures, lodging of cereal crops, or
shattering of seed from panicles.
Management of high winds

1 The effects of wind on evaporation can be avoided by using proper method of irrigation.
2 The damaging effect of wind can be reduced over a limited area by the use of shelter belts
(rows of trees planted for wind protection) and wind breaks (any structure that reduce the wind
speed).
4. Thunderstorms, dust storms and hail storms
These storms are known as local severe storms. As many as 44,000 thunder storms occur daily
on earth.
1 These are formed in a situation where a great deal of the energy for their genesis and
development comes from the release of the latent heat of condensation in rising humid air.
2 These local storms cause severe damage to the standing crop by causing mechanical injury to
the plants.
3 In dust storms, the dust is rised by the wind covers small plants, which may cause stomata
closure and suffocation.
4 Hails cause direct damage to crops by lodging, shattering of seeds etc., depending on their
intensity.
Management of storms
1 Prevention of hails by hail suppression techniques.
2 Following forecasts of weather and protecting crops.
3 Spraying of salt on harvested paddy, to prevent the germination / sprouting of
harvested produce.
5. Excessive or defective insolation
Excessive solar radiation results in rise of soil and air temperatures. Defective insolation with
consistantly cloudy weather on one hand and consistantly bright and
high intensity sunshine on the otherhand causes enormous damage to crop plants.
1 Cloudy weather retard growth, affect pollination and cause disease and pest incidence.
2 High solar radiation intensity cause pollen burst or flower drop.

Management
Since, these are very rare, the location specific solutions like
1 Proper site selection.
2 Allowing air drainage.
3 Adequate water supply.
4 Pruning of orchard trees.
5 Spray of chemicals and plant harmones.
6 Covering plants with "hot caps" (covering plants with some standard and recommended
material) may prove beneficial.
6. Tornado
This is a violent, destructive storm of small horizontal dimensions. A cumulonimbus cloud forms
into a funnel shape with an vortex extending from the base of the storm to the surface. The
Whirl-wind encireles a small dimension of about 500 metres. These are capable of causing
severe structural and other damages. The violent winds associated with this abnormality are
strong upward air currents. The tornados occurring on water are known as “Water spouts”.
Management
1 Warning in advance
2 Precautions to protect the agricultural produce like transportation to safety places etc.
3 Quick removal of debris immediately after damage.