Agricultural Meteorology - Part - Ii

ANNAMALAI

UNIVERSITY

FACULTY OF AGRICULTURE DEPARTMENT OF AGRONOMY B.Sc. AGRICULTURE

AGR 121: AGRICULTURAL METEOROLOGY THEORY NOTES Chapter

Chapter Name

No.

Part - I

Page No.

1.

Introduction to Agricultural Meteorology

3.

Weather and Climate

18

Temperature

57

2. 4. 5.

Atmosphere

Solar Radiation and Light Part – II

Atmospheric Pressure

8.

Atmospheric Humidity (Moisture)

9.

10.

12

Clouds and Precipitation

33

Evaporation and Transpiration

Part - III

Precipitation

13.

Agroclimatic normals for field crops

14. 15.

2

Wind

11. 12.

6

39

6. 7.

2

27 39 2

Agroclimatic Zones

23

Weather Forecasting

30

Agricultural Seasons of India

26 35

1

Part – II Chapter –6 Atmospheric Pressure Pressure is defined as the force acting over any surface. Atmospheric pressure is the

weight of the air, which lies vertically above a unit area, centered at a point and expressed by the height of mercury in ‘inches’ or ‘millimeters’. Pressure mainly affects

temperature and precipitation. The weight of the air presses down the earth with the pressure of 1.034 g/cm2. The weight of air mass is over 56 trillion tons. (56x1014ton).

Weight of 1sq. Inch column of air from sea level to top of the atmosphere weighs nearly

15 1b. This weight is balanced by column of mercury 29.93 inches or 760 mm tall having the same cross sectional area. This is the pressure at sea level at latitude 450. Another unit of measurement millibar is widely adopted by national weather service of

2

the world. (Millibar = 1000 dynes / cm2). Dyne is a unit of force approximately equal to the weight of a milligram. Sea level pressure under this system is 1013.2 m.bars (mb). One tenth of an inch of mercury is approximately equal to 3.4 mb. Units of measurement:

Up to the year 1914 the unit of measurement of pressure was in inches or in m.m. At sea level the atmospheric pressure is 30” or 76” cm or 760 mm. At a temperature of 2730A. In the year 1914 a scientist by name Bjehkres derived a new unit called the “millibar” (mb). Normal pressure at sea level is roughly 30 inches or 760 mm. which corresponds

to 1013 MB. The conversion from units of length to unit of pressure is as follows.

Suppose the Hg column at M.S.L. is 76cm it is then multiplied by the density of mercury (13.595) and mass of Hg column is found out.

76 x 13.595 = 1033.22 gm. The acceleration of gravity (normal) in CGS units is 980.665. Multiplying the mass by gravitational force i.e. 1033.22 x 980.665 we obtain the pressure in CGS units (centimeter

gram second) is 10, 13,250 dynes/sq.cm. For convenient sake it is taken as 10, 00,000 dynes called a ‘bar”.

One thousandth of a bar is called a “millibar”. A millibar

= 1000 dynes/cm2

Approximately 760 mm = 1000 m.b. or 1 mb

33 m.b.

= 0.76 mm.

= 0,76 x 33

= 25.08 mm or = 1”.

The observed pressure is reduced to 320F or 00C at M.S.L. at 450 Latitude as the standard to facilitate the comparison of pressure of different stations.

3

Factors affecting the Atmospheric Air Pressure: 1. Altitude

It is the relative height of place above M.S.L. The pressure decreases for every increases of the altitude. At sea level the air column exerts its full pressure, but we when we go

up, pressure is reduced at high altitude. For every 900 ft of altitude 25 mm or 33 mb pressure is decreased by 1 mb for every 10 meters. 2. Latitude

When the latitude increases the pressure will increase. Temperature

When the temperature increases the pressure will decrease. The density of given volume of air vary with temperature. Thus when air is heated, it expands and becomes

less dense, so that column of warm, light air weight less than a column of cold, heavy

air both having the same height and cross sectional area. Changes in temperature produces changes in air density which setup vertical and horizontal movement resulting in differences in air pressure. Over a warm region when air is heated it

expands and overflows aloft to adjacent region when air is heated it expands and

overflows aloft to adjacent region when temperature is lower. As a result of this horizontal transfer, the weight of the air is reduced in the warm region with and

increased the adjacent cooler regions. Hence region with high temperature are likely to have lower air pressure than other regions where temperature is not so high. In other

words, high temperature tends to produce low sea level pressure while low temperature is conducive to high sea level pressure.

There is a rapid decrease in air weight or pressure with increasing altitude. The lower layers of atmosphere are densest because the weight of all layers above which rests up

on them. For the first few thousand feet above the sea level the rate of pressure decrease, is in neighborhood of 1” or 34 mb of pressure for each 900 to 1000’.

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Nearness to the sea

Places near to the sea are often subjected to cyclones due to low pressure. Water Vapour

Moist air of high temperature exerts less pressure. When compared to moist air of low temperature, because water vapour content is lighter in cold area than air, which is dry. Rotation of the earth Due to rotation of earth the pressure at 60 - 650N and S becomes low for the air to escape from these belts that move towards the horse latitude (30 – 350 N&S) and these belts absorb air from the sub-polar belts making the pressure high. Seasonal variation Pressure system changes according to the season. Season changes according to the position of the sun. When the sun moves to the tropic of cancer, pressure belts move to the North by 50 away from their normal. When sun moves to tropic of Capricorn, the pressure belt also moves south and sight by 50 away from their original position. This is known as “Swing of pressure belts”. Significance of pressure

The pressure are forms the cyclones. Whenever the atmospheric pressure of a place drops from the normal conditions, depression occurs and cyclone may be formed.

The barometer reading is the best indication of the possible occurrence of cyclone or storm as well as rain in area.

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Sea breeze and land breeze due to seasons

During summer horse latitudes receive the direct sunrays and an area of low pressure increases over the continent masses and they enlarge a small high-pressure center over the continents. But surrounding seas have a vast high-pressure area in summer the wind blows from sea (high pressure) towards the lands. (Low pressure) In winter

season, a major area of high pressure covers the landmasses. The sea areas are comparatively at low pressure. So winds start moving from the land towards the sea. Diurnal variations

To find out the mean daily change in air pressure, the average of hourly-observed pressure for a long period of time is calculated. The mean value of the daily pressure is

free from the temporary effect of atmospheric disturbances. There is a definite rhythm in the rise and fall of mercury. Insolational heating and radiational cooling are the principal reasons for diurnal variations of air pressure. In other words, pressure changes are mainly due to the expansion and contraction of the air. Seasonal or annual variation

This is clearly the effect of annual variation in the amount of insolation received in a particular region. Annual pressure variation in the tropical region is larger than other

regions of the world. The equatorial regions record the smallest amount of variation in their seasonal pressures, because there is practically no variation in the amount of insolation received at the equator throughout the year High pressure Low pressure

- cold season

- warm season

Pressure systems of the world

Pressure system differs greatly in both size and duration. Pressure System is of two types

i. High pressure system ii. Low pressure system

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Centers of low pressure are called as depression, cyclones or lows. Prolonged low pressure, centers are called troughs. The equatorial belt of low pressure is called

doldrums (50 N & 50 S of Equator) and it is because of the following factors viz of sun falling vertically all round the year, water vaporization being high and rising of air.

The doldrums belt is spread over Amazon, Congo, Passion and Guinea belt etc. The centers of high pressures are called anticyclones or highs. An elongated high pressure is

called as Ridge. Near 300N and 300S the pressure is always high because intensive hot air from the equator descends down in this belt and polar air from the sub-polar belts also descents here. Storm

A marked atmospheric disturbance characterized by a strong wind, usually

accompanied by rain, snow, sleet (rain that freezes as it falls-mixture of rain with snow or hail) or hail and often thunder and lighting. Thunder Strom

A storm invariably produced by a cumulonimbus cloud and always accompanied by thunder, usually accompanied by strong wind, gusts, heavy rain and sometimes hails. It is usually of short duration, seldom over 2 hour. -

Vertical motion is having many weather modifications.

Upward motion results due to expansion that it gets cooled and eventual condensation.

-

Cumulonimbus cloud types are closely related to the strength of the vertical motion.

-

A thunderstorm is as the name implies a storm accompanied by thunder and therefore lightning. As Benjamin Franklin demonstrated in 1750 lightning discharges giant electrical sparks.

7

-

Cumulonimbus clouds therefore are great electrical generators. The cloud produce ‘+’ and ‘-‘value charges by charged poles.

-

The lower part of the cloud is negatively charged and upper part is positively charged.

Hail

Precipitation in the form of balls or irregular lumps of ice is referred as hail. Hail Strom

Small round pieces of ice hail that sometimes fall during thunder storms (frozen rain drops, hail storms) is referred as hail storm and its features are -

Hails may be sometimes greater in size than a large marble. It falls from cumulonimbus clouds.

Hails are destructive to crops to crops that cause mechanical damage, structures.

8

Hurricane

A violent tropical cyclone with wind speed of 73 or more miles per hour or 134 and more km/h usually accompanied by torrential (very heavy fall) rain, originating usually in West Indian regions.

Tornado

Tornado is coined from a Spanish word – Torn as means, “to turn” and its characteristics are as follows. -

The smallest vortex (whirlpool, whirl or powerful eddy of air, whirl wind - a

whirling mass of water forming a vacuum at its center, into which anything caught in the motion are drawn). -

Eddy - current of air, water, etc., moving against the main current and worth circular motion.

-

But most powerful one.

The intense rotation is confined normally to diameter of kilometer or less. But its wind speed can reach even 300 km/h

9

Water spouts

The tornado occasionally forms over water and because of high moisture content of the air, the funnels are heavily laddened with water drops, so they look somewhat like a stream of water pouring from the base of the cloud. For this reason they are called waterspouts. Dust Devil

A whirlwind that frequently forms on very hot days especially over desert is the dust devil. Normally there are no clouds associated with it.

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Cyclone

It is a system of winds blowing around the center of low barometric pressure i.e., means

closed circulation about a low-pressure center, which is anti clockwise in the Northern hemisphere the characteristics are -

Cyclonic whirls are the “Storms” of middle latitude.

In the temperate latitude they produce much of the winter precipitation. Around the low-pressure centers.

Air circulates anti clockwise direction in Northern Hemisphere.

The air is heterogeneous in relation to temperature and moisture.

Anticyclone

It is a condition in which the atmospheric pressure distribution where central pressure

is high relative to the surroundings. Circulation is a clockwise in northern hemisphere and anti clockwise in Southern hemisphere. -

The whirling speed reduces @ 10-15 cm/sec. and fair weather generally prevail.

-

The air masses are homogenous with respect to temperature and moisture.

Typhoon

Any violent tropical cyclone originating in the western Pacific especially in the South China Sea

Plant growth

It is the resultant of all the environmental factors-climatic, physiographic, edaphic and

biotic factors. For a particular field it is primarily a function of climate with temperature and height being the most important factors. A very close relationship exists between plant phenology and both latitude and altitude.

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Chapter –7 Wind Wind is defined as the moving air of atmosphere parallel to earth’s surface air in horizontal motion. All other masses of air in motion (vertical) should be called as Air Currents. Wind is an invisible weather element but the effect of wind can be seen from

the movement of tree branches, dust particles and by feeling. The pattern and intensity of wind is affected by various factors.

Advantages of wind:

1. Fresh wind is useful for renewing the environment.

2. Wind is useful for effecting pollination in the crops. 3. It is useful for cleaning for agricultural produces.

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4. It is used as a force in certain machines such as windmills, winnowing machines etc.

Effect of Wind on crops

1. Increases transpiration under normal condition with increasing wind velocity. Layers of humid air adjacent to plant leaf surfaces are removed by wind and

become mixed with dry air above. This keeps RH low and increases transpiration rate. There is a greater increase in cuticular transpiration than stomatal transpiration witch cause moisture stress in plants.

2. Wind increases the rate if Photosynthesis. Wind increases turbulence in atmosphere thus raising the supply of Co2 to the plants and thereby increasing the rate of photosynthesis. However, the increase is only up to a certain wind speed.

3. When the wind is hot it accelerates the drying of the plants by replacing humid

air by dry air in the intercellular spaces. At the time of cell expansion, the hot dry wind affects the maturing cell and that result in dwarfing of plants.

4. Much damage is caused by hot dry winds at or near the time of flowering. The

internal water balance is upset, resulting in poor seed setting. Another form of injury is “blossom injury” caused by evaporation of secretions in the stigma.

5. Interfere pollination by insects. But mild wind will favour pollination by wind. 6. Deplete soil moisture.

7. Due to mechanical effect of wind the growth pattern and shape of trees ate changed lopsidal growth.

8. Uprooting of plants is common where as crops and trees with shallow roots are uprooted.

9. Cause fruit drops in plants. Example. Citrus fruit drop. Fruits and nuts are stripped off from trees.

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10. Soil erosion occurs when the plant cover is not thick, strong winds remove the dry soil exposing their roots and killing them. The eroded material from one place is deposited in another place causing hazard to small plants in that place. The deposited materials reduce the aeration around the roots and plants.

11. Salt deposition by wind is another phenomenon where wind from sea carries

salts as spray on coastal area and makes it impossible to grow crops which are sensitive to excess salts.

Disadvantages of wind

1. High-speed wind accelerates the drying of moisture from the soil and also it increases the rate of transpiration in plants thereby necessitating frequent irrigation.

2. High-speed wind results in lodjing of many crops such as Banana, Sugarcane and 3.

other fruit trees.

Heavy wind will affect the fruit set and also the available fruits to fall or to be

withered.

4. Heavy wind also results in soil erosion.

Effect of high wind can be prevented to some extent by means of the following practices.

1. Properly oriented and designed shelterbelts.

2. In temperature conditions wind breaks save plants from freezing and mechanical damage caused by cold winds.

3. Windbreaks also reduce soil erosion caused by wind.

4. Tall crops such as Corn, Sorghum, Sunflower etc are used as temporary wind barriers to protect crop viz., Sugar beat, Soy bean, Tomato, Brinjal etc.,

Causes for the formation of the wind:

The principal cause for wind is difference in pressure. Air always moves from areas of high pressure to those of low pressure.

1. Due to variation in the atmospheric temperature, pressure etc., i.e. when the

atmospheric temperature is very high the pressure will decrease correspondingly. 14

Due to fall of the atmospheric pressure the air moves from high-pressure area to low-pressure area and this gradient decides the direction of wind.

2. Due to deflection of atmosphere air over the earth surface while it revolves and this deflectional force is called as “Coriolis force”

Types of movement of air

5. Horizontal movement called wind

6. Vertical movement called Eddies, Convection currents, Convergent accents and Subsidence.

Wind force

The following are the wind forces and they are the factors affecting the wind motion. 1. Pressure force

The forces that move the air depend primarily at the distribution of pressure. Let us consider a vertical cross section through a cube of air with horizontal and vertical faces.

Since the atmospheric pressure decreases with elevation the pressure “P1” on the lower

face of the cube is greater a force that of ‘P2’ on the top face. This force is counteracted by the weight of air with in the cube or the gravity force. Usually there is balance between the two forces so that no vertical motion results. Rarely there will be in balance and vertical acceleration results and convective currents are created.

Large wind systems are mainly horizontal currents. The pressure also varies in the

horizontal direction and the pressure on the vertical force will exceed the other force and the difference in pressure is equivalent to a force to drive the cube horizontally from high to low pressure.

2. Pressure gradient force and Isobars

Suppose when we observe the atmospheric pressure in large number of places in a

horizontal surface and plot the pressures on a map and draw curves through the points

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that have identical pressure gradient which are called as isobars. The horizontal pressure gradient may be defined as the decrease in pressure/unit distance in the direction in which the pressure decreases most rapidly.

The isobars are the lines connecting different places of same pressure on chart or map of country or world. The lines can be drawn after reducing the readings to M.S.L. Such

lines or curves are called ‘isobars’. These lines are drawn every 5th of a millibar. Pressure distributive charts are constructed for sea level and for number of constant pressure surfaces in the atmosphere. 700 mb – at 10,000 ft. 500 mb – at 18,000 ft.

In sea level pressure chart all pressures at different elevation are reduced to pressure receiving to sea level.

There is rapid change in pressure in a direction at right angle to the isobars. The rate of change in atmospheric pressure between two points at the same elevation is called the

pressure gradient of isobaric slope. It is proportional to the difference in pressure, which causes the horizontal movement of air.

The change in atmospheric pressure during 3 hours preceding the observation is called “barometric tendency”. When the tendencies have been plotted on the map the lines connecting the points are called “isallobars”. They represent the pressure changes as

that of isobars but are drawn for each millibar. Usually the tropical regions are lowpressure belt due to high temperature in and around the equatorial line. The temperature regions are high-pressure belt (areas). Beyond temperate belt, the pressure

diminishes regularly in south but irregularly in North. (Alaska and ice lands have high pressure).

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Two important factors that exist between pressure gradient and winds are:

The direction of airflows is from regions of greater to those of less density i.e. from high to low pressure, which may be represented by a line drawn at night at night angles to the isobars.

The pressure gradient is:

1. Everywhere perpendicular to the isobars

2. Points from high to low values of pressure

3. Inversely proportional to the distance between the isobars, the more crowed the isobars the stronger is the pressure gradient.

3. Horizontal deflection force due to earth rotation

Surface winds do not flow directly down the barometric slope (right angles to the

isobars) but instead are deflected into oblique courses. Thus a west wind in the northern hemisphere becomes northwestern wind. The cause for the deviation of wind from the gradient direction is the deflective force of the earth’s rotation plus friction. This causes

all winds to be turned to the right in the northern hemisphere and to the left in the

Southern hemisphere (Farrel’s Law). This deflective force is called the “Coriolis” force. It is a resultant effect of the two motions.

1. Rotational movement of the earth.

2. The movement of the body relative to the surface of the earth.. This deflective force of the earth is minimum near the equator and it increases with

latitude and is maximum near the equator and it increases with latitude and is maximum at the poles. Therefore air moves rather directly across the isobars in low

latitudes and is greatly deflected in the Polar Regions. This deflective force also

increases with the wind velocity. The Coriolis force is directly proportional to the

moving mass of air and its velocity. It acts at right angles to the direction of the motion and has no influence on influence on the velocity of the wind. The broken arrow shows

the direction of the pressure gradient and the solid arrows shows the direction of wind 17

due to ‘Corolis’ force. Friction it’s next factor, which affects the wind motion. It modifies the effects of gravity and deflection.

Friction prevents the winds from attaining velocities and also from blowing parallel with the isobars.

4. Centrifugal force

The amount of deflection due to this force is dependent on the velocity of the wind. More the velocity greater will be the outward force and hence greater will be the deflection produced. Therefore in the northern hemisphere the rotational deviation is to the right and therefore the centrifugal force will enhance this deflection. This force is

negligible near the surface of the wind is low. If the path of the wind is curvilinear than it will be subjected to centrifugal force. Pressure belts

These are the regions of the high and low pressure formed on the earth as a result of 1. The differences in the rate of insolation

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2. Differences in the rate of absorption of heat by water and the different types of earths surfaces and

3. The rotation of the earth.

There are two types of pressure belts namely High and Low pressure belts. Low pressure belts

Cyclones and anti-cyclones are two special pressure and wind systems. A cyclone is a

system of very low pressure in the center surrounded by increasing high pressure

outwards. In a cyclone, the wind blows in a circular manner in a clockwise direction in the southern hemisphere and in an anticlockwise direction in the Northern hemisphere.

It is believed that most cyclones in the temperate regions occur due to the coming close

and imperfect mixing of two masses of air of contrasting temperature and humidity conditions. Cyclones of this types are also known as

Wave cyclones. On the other

hand cyclones in tropical areas result from the intense heating up of air in some regins causing great loss of life and property in coastal areas. These tropical depressions are

known as cyclones in the Indian ocean , Hurricanes in the west Indies, typhoons in the China Sea and willy-willes in northwest Australia. The equatorial strip and the polar

zones are low-pressure belts. As a result of intense heat at the equator, the air rise to the upper layers, producing a belt of low density and pressure of air and the lower layers near the surface of the earth called the doldrums. The air in the Polar Regions is swung

to the temperate regions by the rotation of the earth. The atmosphere above the Polar Regions is of low density and pressure and these are called “Polar calms”. In the following chapter a detail study on cyclones is attempted.

Anticyclones, which are the centers of high pressure, are the opposite of cyclones in all respect.

Tornadoes are very strong tropical cyclones of a smaller size. They are specially feared in some parts of southeastern United States. Sometimes, when they occur over sea, the

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funnel -shaped cloud formed by the whirling motion of the wind descends to the

surface and draws up the water forming a column of water known as a ‘waterspout’. The jet Stream

The jet stream is a system of upper-air westerlies. It gives rise to slowly moving upper-

air waves. In the upper-air waves are some narrow zones in which wind velocities of up

to 250 knots are observed in some air streams. This phenomenon is called the jet stream and is one of the systems affecting the distribution of pressure in the upper atmosphere.

The phenomenon of jet stream is believed to affect the onset and retreat of monsoons in India. Jet streams develop over areas of steep pressure gradient. High Pressure Belts:

The areas of high pressure relative to the surroundings are called high-pressure belts or

anticyclone. The wind circulation is clockwise around an anticyclone with a drift away from the center. Air currents at the upper layers from both the equator and the poles meet at latitudes 300 to 350 N and S called the horse latitudes and produces a belt of high

pressure. From these horse latitudes, winds blow towards the equator and the poles. These should take northerly and southerly courses but are deflected by the rotation of

the earth. Thus in the northern hemisphere N.E. wind blows towards the equator and S.W. winds towards the poles.

In the southern hemisphere S.E. blows towards the equator and N.W. winds towards the poles.

A trough of low pressure is an elongated area of relatively low pressure, which extends

from the center of a cyclone. The trough may have ‘U’ shaped ‘V’ shaped isobars. The

wind circulation around a trough is essentially of the cyclone type. A wedge of high pressure is an elongated area of high pressure that extends from the center of an anticyclone, and the wind circulation is anticyclonic.

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Around the equator there is a region of almost uniform pressure in which the winds are

light and variable and this belt is called the “doldrums”. The winds converge from the both the hemisphere into the doldrums. This convergence results in ascending air

currents, adiabatic cooling, condensation and precipitation. The doldrums are therefore characterized by frequent showers, thunderstorms and heavy rainfall. Further away,

from the equator are belt of high pressure with easterly winds on their equatorial sides and westerly winds on the pole ward side. These belts of high pressure are called the

subtropical anticyclones. The winds on this equatorial side are called the subtropical anticyclones. The winds on this equatorial side are called Trade winds. They blow

mainly from the east and have a component towards the equator; on the pole ward side

the winds have a pole ward component. The subtropical anticyclones are regions of descending air currents, low R.H. almost clear sky and deficit of rainfall. Most deserts are found in the region.

In the central portion of the subtropical anticyclone the winds are light and referred to by seamen as the “Horse Latitude”. The wind on the pole ward side of the high pressure are called prevailing westerlies. They increase in strength as the latitude increases.

Wind Systems There are three types of wind systems namely: 1. Primary wind system

2. Secondary wind system and 3. Special a type wind system.

The primary and secondary wind systems consist of Trade winds and monsoons (discussed in later chapter )respectively and special type consists of land and sea breezes(discussed in earlier chapter).

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1. Trade winds

Trade winds are the winds of primary wind systems that blow from subtropical centers towards the equatorial side low between 30 and 350 and the winds on the equatorial

side are called “Trade winds”. They are the most regular winds. Their steadiness has

earned their name trade winds. They blow with greater strength and constantly in winter than in summer. They are regular and steady over the oceans. They blow away

from the landmasses over continents. When the equatorial region gets heated, the air sizes from the surface and passes to the upper layers. The pressure of the atmosphere

near the surface decreases in due coarse. Air moves towards this low-pressure area from both north and south and this phenomenon continues right through the year.

The resulting wind takes the same course or track and is hence called “Trade winds or

Tropical Easserlies”. As the hot air arise to the upper layers over the equator, the pressure is raised there in due coarse and the surplus air moves northwards and

southwards in the lower layer. The movement is towards the equator form the north and south in the lower layers and from the equator towards north and south in the upper layers. The latter are called “Antitrade” winds. Relationship of wind and pressure -

Earth rotates from west to east along with atmosphere. Atmosphere is fixed to earth by gravitational equilibrium.

-

Wind therefore moves in addition to rotation.

Horizontal motion is greater than vertical motion.

Wind takes several days to cross the ocean but up and down movement is only in few minutes.

Seasonal and Local winds

The monsoons are the most important among seasonal winds. In this system, the

direction of the winds changes seasonally. They are experienced over parts of North America and much of South Asia, including the Indian subcontinent. These winds are

primarily a result of differential heating of land and sea. In summer, southern Asia 22

develops a low pressure and airflows landwards from the Indian Ocean. This is known

as the summer monsoon. In winter, the pressure over land is higher than over the sea and consequently the air starts flowing from land to sea. This is called the winter monsoon. The modern theories consider theories the monsoon a result of the shift in the pressure and wind belts.

According to the dynamic theory, monsoons are a result of the pole ward shift of the Inter Tropical Convergence (ITC) under the influence of the vertical sun during the

summer season. During summer in the northern hemisphere, in the months of May and June the sun shines vertically over the Tropic of Cancer and the ITC shifts north of

the equator. The ITC is the convergence zone of the trade winds bowing from northeast in the northern hemisphere and from the southeast in the southern hemisphere. As ITC

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Shifts northern of the equator, the southeast tread winds start blowing north of the equator to reach the ITC, and as they cross the equator, their direction is altered due to

the influence of the coriollis force, i.e., they are deflected towards their right and thus it gives rise to the formation of a belt of equatorial westerilies blowing between the

equator and the ITC. These westerlies in the months of May and June blow from the equator towards the ITC from the southwest to the northeast and they are called the southwest monsoon.

During the winter season the ITC again moves southwards and the areas north of the equator, which experienced the equatorial westerlies during the summer season, now

come under the influence of the northeast trade winds. These northeasterly winds are called the northeast monsoons.

During this very season the ITC shifts south of the equator and the northeast trades blowing towards the ITC, get deflected upon crossing he equator southward. Here they give rise to the equatorial westerlies blowing from the northwest to the southeast,

replacing the trade winds of the southern hemisphere between the ITC and the equator. Thus the areas situated in the tropical zone come under the influence of the trade winds

during the respective winter and the equatorial westerlies during the respective summer season. Thus the direction of the winds is reversed seasonally and it makes up the monsoon system of the region.

In certain regions, local winds are generated

as a result of the influence of the local

terrain. One example of this is the simple system of land and sea breeze experienced in coastal areas. Due to differential heating, the air moves from sea to land during the day

and from land to sea at night. Mountain and valley winds also follow daily alternation of direction. During the day air moves up along the valley slopes, as the slopes are very hot. When the slopes cool at night air moves valley wards.

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Measurement of wind speed

The speed or velocity of wind is expressed in meters per second, kilometers per second,

kilometers per hour, and miles per hour or knots per hour. The relationship between these units is

1 m/sec

=

3.6 km/hr

=

1.94 knots ph

=

2.24 mph

Wind speed is measured by Robinson’s cup anemometer.

In 1805 Admiral Francis Beaufort introduced a wind force (speed) scale, which was

based upon the response of certain objects to the wind. In applying Beaufort scale the extent to which smoke is carried horizontally or to which trees bend before the wind is

used as an index of speed. At sea, the condition of waves, swell and spray in addition to the response of sails and masts is the basis for wind speed estimates.

In modern method wind vane and anemometers are used for measuring the direction and wind speed respectively.

Table: The Beaufort scale of wind force with velocity equivalents

Beaufort Number 0 1

Beaufort

Descriptive Calm

Term

Light air

2

Light breeze

3

Gentle breeze

Velocity,

Land Criteria

miles/hour

Calm, smoke rises vertically

Less than 1

Direction shown by smoke drift, not by wind vans

1 to 3

Wind felt on face; leaves rustle; ordinary

4 to 7

Leaves and small twigs in motion; wind

3 to 12

vane moved by wind extend light flag.

25

4

Moderate breeze

Raises dust and loose paper, small,

13 to 18

5

Fresh breeze

Small trees in leaf being to away. Created

19 to 24

6

Strong breeze

Large branches in motion; whistling in

25 to 31

7

Moderate gale

8

Fresh gale

9

Strong gale

10

Whole gale

11

Strom

12

Hurricane

branches moved

wavelets form on inland waters

telegraph wires; umbrellas used with difficulty

Whole trees in motion; some difficulty

32 to 38

Breaks twigs off trees; progress generally

39 to 46

Slight structural damage occurs (chimney

47 to 54

Trees uprooted; considerable structural

55 to63

walking against wind impeded

pots and slate removed) damage inland

occurs;

seldom

experienced

Very rarely experienced; accompanied by widespread damage

63 to 75 Above 75

From Trewartha. An introduction to Climate. McGraw-Hill, N.Y., 1954.

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Chapter –8 Atmospheric Humidity (Moisture) Moisture present in the atmosphere plays a significant role in weather and climate of a region. There are three major components in the atmospheric moisture. i. Humidity

ii. Precipitation iii. Evaporation Humidity

The terminology related to humidity and concerned with gaseous form of water i.e., water vapour, several expressions of the amount of water vapour in the air is used.

27

Absolute humidity: It denotes the actual mass of water vapour in given volume of air. It may be expressed as the number of grams of water vapour in cubic meter of moist air or mass of water vapour per unit volume of air.

Specific humidity: It is defined as the moisture content of moist air as determined by

the ratio of the mass of water vapour to the mass of moist air in which the mass of water vapour id contained.

Relative humidity: Relative humidity is a common parameter for expressing water

vapour content of the air. It is the percentage of water vapour present in the air in comparison with saturated condition at a given temperature and pressure. The R.H. can be expressed as

RH

=

100r ---------rw

Where “r” is the mixing ratio of moist air at pressure (p) and temperature and “rw” is the saturation-mixing ratio at same temperature and pressure.

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Mixing ratio: The mass of water vapour per unit mass of dry air is a convenient

parameter to express the relative composition of the mixture. It is defined as the ratio of the mass of water vapour to the mass of dry air with which the water vapour is associated.

Dew point: The temperature at which saturation occurs in given mass of air. The dew point temperature is often compared with the temperature of free air and also used to predict the occurrence of fog, dew, frost or precipitation.

Vapour pressure: This is the amount of partial pressure created by water vapour in the

air expressed in the units of millibar (or) inches of mercury.

Vapour pressure deficit (VPD): It is the difference between saturated vapour pressures

and actual vapour pressure express as bar /Pascal. When the VPD is up to 1.5 Kpa the air is said to be humid and over and above 2.5 Kpa it is drier. It gives the rough estimate of drying power of air similar to RH. Rate of evaporation and transpiration are indicated by the magnitude of VPD.

Saturation point: When air contains all the vapour it can hold at that temperature air said to be saturated at the temperature reached saturation point.

Factors affecting humidity of the air: 1. Temperature

If the temperature of the atmospheric air is more, the water vapour present will be less.

But at the same time the high temperature will increase the capacity of the atmospheric air to absorb more water from the earth surface.

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2. Nearness of the place to the seacoast

The places near the seacoast are supposed to be cooler due to high deposition of water in vapour form in the atmospheric air from sea. 3. Climate

Based on the various climatological a factor such as temperature rainfall etc., a particular place is divided into various climatic periods like summer winter etc. Summer period is marked by high temperature, low rainfall and low humidity. Rainy

period is marked winter season is also marked by low temperature, but not with frequent rain and high humidity. Importance of humidity

It decides the dampness or dryness of the atmospheric air. Humidity has got the same effect as that of rain in deciding the water needs of the crops. The high humidity has also got some adverse effect on the crop growth. There will be high incidence of pest

and diseased under high humidity. The rate of evaporation and transportations entirely depends upon the saturated condition of the atmospheric air with water vapour. Measurement of humidity

The amount of vapour (water) in the atmospheric air is measured by gravimetric

method, and also by using wet and dry bulb thermometers, Assman Psyschrometer Hygrograph etc.

Effect of Relative Humidity on crops

RH directly influences the water relation of the plant and indirectly affects 1) Leaf

growth, 2) Photosynthesis 3) Pollination 4) Uptake and translocation of nutrients 5) Occurrence of pest and diseases 6) Economic yield of crop.

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Water relation:

RH affects the transpiration by modifying the vapor pressure gradient. In dry region RH will be low which causes severe water deficit in plants and reduce the leaf water potential, plants become dry and wilt. High RH lowers the ET.

1. Leaf growth: It is not only depends on photosynthesis and biochemical process but also depend on physical process of cell enlargement. Cell enlargement occurs as a

result of turgor pressure developed within the cell. Turgor pressure is high under

high RH due to less transpiration. Thus, leaf enlargement is high in humid region. E.g., cotton 40% RH recorded increased growth rate compared to 25 or 65% RH.

2. Photosynthesis: RH indirectly affects photosynthesis. When RH is reduced transpiration increases causes water deficit in plants. Water deficit causes partial or full closure of stomata and increases mesophyll resistant blocking the entry of CO2 thereby photosynthesis is affected.

3. Pollination: Moderately low air humidity is favorable for seed set in many crops

provided in soil moisture supply is adequate. For example, Seed set was higher in wheat at 60% RH compared to 80 % RH. When water availability in soil is not limiting, due to increase pollen germination. When RH is increased pollen may not disturbed from the anther. Low RH causes pollen sterility.

4. Uptake and translocation of nutrient: High RH decreases the transpiration, which

affects uptake of nutrients and causes deficiency. Uptake of P, K and Ca was higher at high RH the 60%. Increased the RH increases P uptake. RH 60% is effective for most of the crop growth by better nutrient uptake.

5. Pest and disease incidence: It increases with increased RH. Higher RH favors easy germination of fungus spores. For example, Blight disease of Potato and Tea. Jassid and aphid infestation will be more under high RH.

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6. Crop Yield: Very high or low RH is not ideal. In maize low yield was obtained due to high RH. Pest and disease incidence was observed at maturity stage and hence low RH is beneficial. 60-80% RH is ideal for most of the crops.

Diurnal variation in RH: The mean maximum RH occurs in the early morning hours and minimum in the early afternoon. The RH has its maximum at equator and decreases towards the poles up to 300 N and S due to subsiding and diver sing air

masses. From about 300 to poles the RH increase the result of decreasing temperature. This trend is known as Diurnal variation in RH. Effect of relative humidity on Plant Growth Increase in RH-decreases the temp. This phenomenon increases heat load of the leaves. Since transpiration is reduced not much heat energy used. Excessive heat due to closure of stomata entry of CO2 is reduced. Reduction in transpiration reduces the rate of food translocation and uptake of nutrients. Very high RH is beneficial to Harmful to

-

Maize, Sorghum, Sugarcane, (C4Plants) Sunflower, Tobacco.

Affect water requirement of crops: For almost all the crops it is always safe to have a moderate R.H. of above 40%. 60-80 % conducive for growth and development of plants.

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Chapter –9

Clouds and Precipitation Clouds are condensed moistening of droplets of water and ice crystals. The nuclei of those droplets are dust particles. Near the surface these drops forms as fogs and in the

free atmosphere, they form clouds. Clouds have been defined as a visible aggregation or minute water droplets and / or ice particles in the air, usually above the general ground level.

Air contains moisture and this is extremely important to the formation of clouds. Clouds are formed around microscopic particles such as dust, smoke, salt crystals & other materials that are present in the atmosphere. These materials are called “Cloud condensation Nucleus” (CCN). Without these no cloud formation will take place. Certain special types known as “ice nucleus” on which cloud depletes freeze or ice

crystals form directly for water vapour. Generally condensation nuclei are present in plenty in air. But there is scarcity for special ice forming nuclei. Generally clouds are made up of billion of these tiny water depletes of ice crystals or combination of both.

When a current of air rises upwards due to increased temperature it goes up, expands

and gets cooled. If the cooling continues till the saturation point is reached, the water vapour condenses and forms clouds. The condensation takes place on an nucleus of

dust particles. The water particles individually are very small and suspended in the air.

Only when the droplets coalesce to from a drop of sufficient weight, to overcome the resistance of air, they fall as rain. Clouds are considered essential and accurate tools for weather forecasting. Every feature of air masses (discontinuity, subsidence, instability and stability) is reflected by the shape, amount and structure of clouds.

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Classification of clouds

Clouds are usually classified according to their height and appearance. For convenience we list them in descending order as high clouds, middle clouds and low clouds. Since

for one do not fit in any of these categories. But fortunately their particular characteristics make them easily, identifiable as vertical development clouds. We must exercise some caution in relying on height data. There is some seasonal as well as

latitudinal variation and there is some overlapping from time to time. However, the appearances of clouds are quite distinctive for each height category.

‘

The main cloud genera are defined and described in the international cloud atlas of the WMO genera1957. That can be listed according to their heights as under.

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A. High Clouds (mean heights 5 to 13 km) (Mean lower level 20000 ft) i) Cirrus (ci) men height 9900 m. ii) Cirrocumulus (cc) 8300 m. iii) Stratus (Cs) 6500 m.

B. Middle Clouds (Mean height 2 to 7 km) (6500 to 20,000’) i) Altostratus (As) 4300 m.

ii) Altocumulus (Ac) 4300 m. C. Low Clouds (mean heights 0 to 2 km) (Close to earth’s surface to 6500’) i) Nimbostratus (Ns) 2000 m. ii) Stratocumuls (Sc) 500m. iii) Stratus (St) 900-1200 m. D. Vertical clouds

i) Cumulus (Cu) 1500-2000 m.

ii) Cumulonimbus (Cn) 3000-5000 m. Clouds with vertical development 1. Cirrus: Detached clouds in the form of white, delicate filaments or white or mostly

white patches of narrow bands. Those clouds have a fibrous (hair like) appearance or a delicate silky appearance or both. All the cirrus or cirro-type clouds are composed of ice

crystals. Cirrus clouds have brilliant colours of sunset sunrise. These clouds do not give precipitation.

2. Cirro-Status: Transparent whitish cloud veil of fibrous (hair like) or smooth appearance, totally or partly covering the sky and generally producing halo phenomena. This type of cloud is so thin it gives the sky a mild appearance

35

3. Cirro-cumulus: Thin, white flakes, sheet or layer of cloud without shading. Composed of very small elements in the form of grains, ripples etc. This type of cloud is

not common and is often connected with cirrus or cirrostratus. When arranged uniformly, it forms a “Mackerel sky”. Mackerel – Fish has greenish blue stripped back and silvery white belly.

4. Alto-stratus: A uniform sheet cloud of “Grayish or bluish cloud frequently showing a fibrous appearance, totally or partly covering the sky, and having parts this enough to reveal the sun at least wavely as through ground glass. Altostratus does not show halo

phenomena. This type of clouds a may cover all or large portions of the sky. Precipitation may fall either as fine drizzle or snow.

5. Alto-Cumulus: “white or grey, or both white and grey, patch, sheet or layer of cloud. They have devel shedding on their under surfaces. Sometimes referred to as “sheep clouds” or “Woolpack clouds”.

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6. Nimbo- Stratus: “Grey cloud layer, often dark, the appearances of which is rendered diffuse by more or less continuously falling rain or snow, which in most cases reaches

the ground. It is thick enough throughout to blot out the sun. It is a rain, snow or sleet cloud. It is never accompanied by lightening, thunder or hail. Streaks of water (rain) or snow falling from these clouds but not reaching the ground are called “Virga”. Wisps or

streaks of water or ice particles falling from base of a cloud but evaporating completely before reaching the ground. Wisps=bundle as of straw.

7. Strato-Cumulus: “Grey or whitish or both grey and whitish patch, sheet or layer of cloud which almost always has dark parts, composed of tessellation’s, rounded masses, rolls, etc.

8. Stratus: Generally grey cloud layer with a fairly uniform base, which may give

drizzle, ice prisms or snow grains, sky may be completely covered by this type of cloud. Sun is visible through this cloud.

9. Cumulus: “Detached clouds, generally dense and with sharp outlines, develop vertically in the form of rising mounds, domes of towers, of which the bulging upper parts often resembles a cauliflower. Cumulus is generally found in the dry time over land areas. They dissipate at night. They produce only light precipitation.

10.Cumulonimubs: “Heavy and dense cloud, with a considerable vertical extent in the form of a mountain or huge towers. This type of cloud is associated with heavy rainfall, thunder, lightening, hail or tornadoes. The fall of a real shower and sudden darkening of the sky easily recognize this type of clouds. Formation of Clouds:

Clouds are formed by condensation of moisture in the air by cooling.

1. It is due to direct cooling as they come in contact with cold surface. 2. By mixing of hot and cold air.

37

3. By expansion.

There are two rain forming process viz, 1. Warm rain process:

 Rains occur when the temp is above 00C never colder than 00C.

 When larger droplets collide and absorb smaller cloud droplets.  They grow larger and larger & become raindrops.  This process is known as “Coalescence” 2. Cold rain process

 Occurs when the cloud temperature is colder than 00C.

 Clouds are usually with ice crystals and liquid water droplets.

 These crystals grow rapidly drawing moisture from the surrounding cloud droplets until their weight causes them to fall.

 Falling ice crystals may melt and join with smaller liquid cloud droplets resulting in raindrops. If ice crystals do not melt, they may grow into large snowflakes and reach the ground as snow.

Conditions favorable for the occurrence of precipitation

 The cloud dimension (vertical –7 km horizontal 60-70km)  The lifetime of the cloud (at least 2-3 hrs.)

 The size and concentration of cloud droplets & ice particles.  RH should be 75%

 Wind velocity 20km.  Cloud seeding Cloud Seeding:

It is the process by which the conditions of the cloud (dimension, life time and size) are

modified by supplying with suitable nuclei us at proper time and place.

For

accelerating the warm rain process seeding with very large nuclei such as salt crystals

can be used. In the case of cold rain process, seeding with ice nuclei such as silver iodide are used to make good the deficiency in the clouds.

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Chapter –10

Evaporation and Transpiration Hydrologic cycle

Hydrologic

cycle

involves

four

major

steps

viz,

evaporation,

transpiration,

condensation and precipitation. Though the cycle has neither a beginning nor and end,

the concept of cycle begins with the water of the oceans, since it covers nearly ¾ of the earth’s surface. Radiation from the sun evaporates the water vapor from the oceans into

the atmosphere. The water vapour rises and collects to form clouds. Under certain conditions, the cloud moisture condenses and falls back to the earth as rain, snow, hail

etc., precipitation reaching the earth’s surface may be intercepted by vegetation, or enter

39

into the soil, may flow as run off or may evaporate. Evaporation may be from the surface of the ground of from free water surface. Transpiration may be from plants.

Evaporation: The change of state of water from solid and liquid to the vapour and its

diffusion into the atmosphere is referred to as evaporation. In agricultural Meteorology, evaporation is defined as the maximum possible loss of moisture form a wet, horizontal, flat surface exposed to weather parameters, which exist in the vicinity of plants.

Factors affecting Evaporation 1. Those affecting water supply at the evaporating surface. i.e., soil and plants including soil storage capacity, rainfall and irrigation.

2. Those affecting energy supply to the evaporation surface like solar radiation.

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Transpiration: Most of the water absorbed by plants is lost to the atmosphere. This loss

of water from living plants is called transpiration. It can be stomatal, cuticular or lenticular.

Factors affecting Transpiration: 1. Light,

2. Humidity,

3. Temperature, 4. Wind,

5. Root/shoot ratio,

6. Availability of water to plants, 7. Leaf characteristics.

Evapotranspiration (ET): As noted earlier, it is a combined loss of water through evaporation from the soil and transpiration from the plants.

41

Potential Evapotranspiration (ET):

It is defined as the amount of water which will be lost from an extensive water surface

or soil completely covered with vegetation where there is abundant moisture in the soil at all times.

Evapotranspiration is also called water use (WU) or consumptive use (CU). The

factors influencing evapotranspiration (ET) are climate and management practices. Evaporation

 One of the four components of the endless hydrological cycle (EvaporationTranspiration-Condensation-Precipitation).

 Most of the water vapor comes from ocean.

 It is also important in agriculture as it affects.  Soil Conditions.

 Plant growth crops.

 Water storage dams. Evaporation depends upon the

 Temperature of the water surface  Vapour pressure of the air

The pressure exerted by the water vapour in the air is known as “vapour pressure”. Evaporation is more when there in greater pressure difference between vapour pressure and saturation vapour pressure.

 Wind movement (Removes moisture) – Evaporation increases with wind velocity.

 Salinity – presence of dissolved minerals salts reduce evaporation from sea by 5% less than pure water.

42

Factors, which affect ET from plant & Soils, are i) Those affecting water supply  Soil storage capacity.  Rainfall.

 Irrigation. ii) Those affecting energy supply

 Light: Stomata open in light and close in the dark.

 Temperature: Humidity/ vapour pressure is function of temperature

 Relative Humidity: Less humidity higher temperature. Increases difference – incurred. Decrease temperature increase vapour pressure – reducing the

Wind

saturation deficit.

Saturated air is replaced by dry air around the plant and hence increased temperature was noticed. The cooling effect on leaves results in decreased in vapour pressure different.

Plant characters

1. Root: shoot

2. Leaf characteristics

3. More LAI results in higher transpiration high

4. Thick cuticle and presence of epidermal hair will lead to less transpiration. When Root shoot is more or equal then transpiration will be more. PET (Potential Evaporation) AE (Achal Evaporation)

AE is always less than PET

- denotes evaporation forms a free water surface. - Actual Evaporation.

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Factors affecting Evaporation Climatic Factors 1. Solar radiation 2. Relative Humidity 3. Temperature 4. Wind

Soil Factors: 1. Soil texture – a. Sandy soil, b. Clay soil. 2. Available soil moisture 3. Soil salinity 4. Hydraulic conductivity Plant characters: 1. Plant morphology a. Leaf size b. Thickness of the cuticle c. Stomata 2. Type of plant Other factors 1. Ploughed / unploughed field 2. Plant population and row pattern 3. Plant cover Eavapotranspiration and Crop production: 1. Working out ET or PET will be useful in scheduling the irrigation. (IW/CPE ratio method) 2. ET can also help in demarcating the drought prone areas. These will form the base for developing suitable soil and crop management practices, crop varieties, water conservation techniques, cropping pattern and ways to improve productivity of rain fed crops. 3. Water Use Efficiency (WUE) can be worked out. Condensation The physical process by which a vapour becomes a liquid or solid and it is a process opposite of evaporation.

44