Indian rainfall and Atmosphere

Course: B.Sc. Agricultural

Subject : Principle of Agronomy and Agricultural Meteorology

Unit: 5

1. Reservoir – Oceans, air (as watervapor), groundwater, lakes, evaporation,wind and precipitation (rain) move waterfrom oceans to land.

2. Assimilation – Plants absorb waterfrom the ground, animals drink water oreat other organisms which are composedmostly of water.

3. Release – Plants transpire, animalsbreathe and expel liquid wastes.

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 from soil,

evaporation from lakes, ponds,

streams and transpiration from plant.

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 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.

Hydrological Cycle

1

Evaporation and transpiration

Condensation and formation of cloud:

Rising air current all the vapor up into the atmosphere cool

formation of cloud cloud is made up of droplet of

water

Precipitation: Snow or rain fall

Run off and collection of under ground water

Infiltration and percolation

The hydrological cycle can be briefed by the following

equation.

P = ET + DST + S

The total amount of water present on the earth surface

remains constant but undergoes continuous transformation

from water vapor to liquid.

This equation is also called as water balance equation.

Where, P : Water received by precipitation,

ET : Loss by evapotranspiration,

DST : The gain on loss by storage in the soil

S : The surplus run-off of water

From this mathematical relation, we can find out the value

of other elements.

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

A. Liquid form B. Solid form C. Mixed form

1. Rain 1. Snow 1. Sleet

2. Drizzle 2. Hail 2 Hail storm

3. Shower

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.

•

A. Liquid Form

•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.

2

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 (dry adiabatic lapse rate) (9.8 °C/km) and

becomes saturated (having RH 100%) and dew point

is reached where the condensation. This level or

height is known as condensation level.

Above condensation level air cools at SALR

(Saturated Adiabatic Lapse Rate) (5 °C/km) clouds are

formed. Then further condensation results into

precipitation. These rains are known convectional

rains.

3

II) Orographic 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 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 (dry adiabatic lapse rate). This warm air does

not give any precipitation on the leeward region. This

is known as rain shadow region.

4

III) Cyclonic /Frontal and Convergent rains:

Frontal precipitation is produced when two opposing aircurrents with different temperature meet, vertical liftingtakes place which gives rise to condensation andprecipitation.

When the humus and warm air mass meets the cold airmass, the colder air being denser tends to push belowthe warmer air and replace it.

The boundary zones along which two air masses meetare called as fronts. When the mixing of warm andmoist air with cold air mass takes place, thetemperature of the warm and air falls down, saturationoccurs and may give precipitation and it alsoresponsible for cyclone formation and rains receivedfrom cyclones are called cyclonic reins.

5

2) 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.

6

3) Shower:

Precipitation lasting for a short time with relatively

clear intervals is called shower. This occurs from

the passing clouds.

7

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.

B) Solid Form:

8

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.

9

1. Sleet

Simultaneous precipitation of themixture of rain and snow is called assleet.

C. Mixed Form:

10

2. Hailstorm:

Rainfall associated with hail stones is

called hailstorm.

1211

The atmosphere is a cloud of gas and suspended

solids extending from the Earth's surface out

many thousands of miles, becoming increasingly

thinner with distance but always held by the

Earth's gravitational pull.

The atmosphere is made up of layers. it surrounds

the Earth and holds the air we breathe; it protects

us from outer space; and holds moisture (clouds),

gases, and tiny particles. In short, the atmosphere

is the protective bubble we live in.

This protective bubble consists of several

gases with the top four making up 99.998%

of all gases. of the dry composition of the

atmosphere nitrogen, by far, is the most

common.

Nitrogen dilutes oxygen and prevents rapid

burning at the Earth's surface. Living things

need it to make proteins.

Oxygen is used by all living things and is

essential for respiration.

Gas Name Chemical Formula Percent Volume

Nitrogen N2 78.08%

Oxygen O2 20.95%

*Water H2O 0 to 4%

Argon Ar 0.93%

*Carbon Dioxide CO2 0.0360%

Neon Ne 0.0018%

Helium He 0.0005%

*Methane CH4 0.00017%

Hydrogen H2 0.00005%

*Nitrous Oxide N2O 0.00003%

*Ozone O3 0.000004%

Thermosphere

13

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

2. Stratosphere

B) Upper Atmosphere:

1. Mesosphere

2. Thermosphere

14

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 vapor

and aerosols. It is the realm of clouds,

storm and convective motion.

A) Lower Atmosphere:

The outstanding characteristic of thetroposphere is the filmy uniform degree intemperature with increase in altitude untilminimum temperature of -50 to -60 °C isreached.

The isothermal layer marking the end oftemperature decrease is called tropopauseand it separates troposphere andstratosphere.

Through out the troposphere there is ageneral decrease of temperature withincrease in height at a minimum rate ofabout 6.5 °C/km or 3.6 °F/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 0 °C.

In stratosphere the temperature increases with

increase in height.

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 -90 °C 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.

B) Upper Atmosphere:

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 950 °C at 350

km to 1700 °C at an underfined upper limit but

these temperatures are essentially theoretical.

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.

Calm air

These are generally felt as an airy condition. It

is also called light wind, and it has a speed of about

1km/hr.

Calm air will be noticed by smoke rising

vertically from a chimney. They have a Beaufort

Number 0.

Gentle Breeze

Gentle breezes have speeds of about 12-

20km/hr. Leaves and small twigs would be constantly

moving with gently breezes. They have a Beaufort

Number 3.

Moderate winds

Moderate winds have speeds of about 20-

38km/hr. They are enough to fly a kite and keep it

going higher and higher. They can cause

moderate waves with whitecaps on the sea and

make trees whistle.

Strong winds

Strong winds have speeds of about 62-74km/hr.

They can cause high waves over the seas and

break off twigs and small branches from trees.

They have a Beaufort number of 8.

Gale winds

Gale winds have speeds of about 75km/hr and

more. They can be very destructive and they carry

lots of broken branches and twigs from trees.

They cause high tidal waves and rolling seas.

They can also rip off roofs of building.

15

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.

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).

16

2. Cumulus (Cu):

Meaning “heap”, is the wooly, bundly cloud withrounded top and flat base. It is the most common inthe summer season and in latitudes where hightemperature prevail and it always results fromconvection Its height is variable and depends onrelative humidity of the air.

17

3. Stratus (St):

It is a sheet type cloud without any form to

distinguish it. It is usually lower than cumulus.

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4. Nimbus (Nb):

It is any dark and ragged cloud and from which

precipitation occurs.

19

Cloud

family

and

Height

Name of

cloud and

abbreviatio

n

Composition Possible weather

change

Description and appearance

1 2 3 4 5

Family A

High

clouds

7 to 12

km

1 Cirrus (Ci) Ice crystalsMay Indicate storm

showery weather

It is wispy and feathery, sun shines

without shadow. It does not

produce precipitations

2.

Cirrocumulu

s (CC)

Ice crystals Possible stormMeekerel sky, often fore renners of

cyclone, look like sippled sand

3.

Cirrostratus

(Cs)

Ice crystals Possible stormMeekeral sky, often fore runners of

cyclone, look like sippled sand.

Family

B

middle

clods

3-7 km

4.

Altocumulus

(As)

Ice & waterSteady rain or

snow

Looks like wool peak, sheep bulk

clouds.

5.

Atmostratus

(As)

Water and iceImpending rain or

snow

Fibrous veil or sheet, grey or

bluish, produce coronos, usually

ct.st shadow.

Family C

low

clouds

from

ground

to 2-3 km

6.

Stratocumulus

(Se)

Water Rain possible

Long parallel rolls, pushed

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

drizzle

A low uniform layer, resembling

fog, but resting not on the ground,

chief winter cloud.

8.

Nimbostrauts

Water or

Ice

Impending rain

or snow

Fibrous veil or sheet, grey, grey

or bluish produce coronas,

usually

Family D

clouds

with

vertical

develop

ment

from

0.5 to 16

km

9.

Cumu-lus (Cu)Water Fair weather

Looks like wool pack, dark below due

to shadow, may develop into

cumulous –Nimbus flat base.

10.

Cumulous –

Nimbus (Cb)

Ice in upper

level and

water in

lower level.

Violet winds rain,

all possible

thunderstorm hail

lighting possible

Thunder head, towering anvil top,

violet up and down drafts, aviators

avoid them, develop from cumulus,

chief precipitation makers.

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- www. agriinfo.in

- ecourses.iasri.res.in

- Principles of Agronomy by T. Yellamanda

Reddy and G.H. Sankara Reddy

- Agrometeorology by SR Reddy and DS

Reddy