RASAKUMAR R M.Sc (agri)

1. Lec –11
Precipitation – Forms of Precipitation – Isohyte
– Monsoon – Different monsoons of India –
Rainfall Variability – Drought and Flood –
Impact on Crop production

2. TAMIL NADU
 Many districts of Tamil Nadu fall under Semi Arid
Tropics (SAT), these districts face higher incidence
of solar radiation, more air and soil temperature and
highly varying rainfall

3. Forms of precipitation

4. Rain
 Precipitation of water in liquid state.
 Drops of more than 0.5 mm diameter

5. Drizzle
1. Fine drops of water (diameter less than 0.5 mm), very
close to one another".
2. Very small and of uniform size drops, and seem to float in
the air, it is referred to as drizzle.
3. It gives out very small amount of water on the ground.
4. It is often associated with fog, and poor visibility.
5. In some places drizzle is called mist.

6. Fog
1. Very minute water droplets suspended in the air or
it consists of a mixture of smoke or fine dust
particles.
2. Reduces the horizontal range of visibility to less
than 1 km.
3. Deposits on the leaves of the crop plants.
4. The evapotranspiration losses are minimized as the
leaves remain wet for a longer period.

7. DEW
 Dew is water in the form of droplets that appears on
thin, exposed objects in the morning or evening.
 Winter months
 Cool nights
 Clear sky

8. Hail
1. Precipitation of small balls or pieces of ice (hailstones)
2. Diameter ranging from 5 to 50 mm
3. Most dreaded and destructive form of precipitation
4. Produced in violent thunder-storms or cumulo-nimbus clouds.
5. Structure like an onion.
6. March to May -ideal conditions for hailstorms.
7. Standing crops in the field can be devastated in just a few minutes'
by severe hailstorms which mean tremendous economic loss to the
farmers.
8. Very difficult to forecast, particularly the time and place of their
occurrence.

9. Snow
 Precipitation of white and opaque grains of ice.
 It is precipitation of solid water, mainly in the
form of branched hexagonal crystals or stars.

10. Sleet
 Precipitation - mixture of rain and snow
 Small pellets of transparent ice, 5 mm or less dia
 Frozen rain - rain falling to the earth passing
through a layer of cold air and freezes -
temperature is very low
 Not commonly seen in India expect high ranges

11. Precipitation processes
 Adiabatic cooling process - condensation & precipitation
 Not all condensation gives precipitation
 All clouds contain water, some produce precipitation
 Precipitated moisture does fall from the clouds
 Only cloud droplets, ice pellets or rice crystals grow large
size precipitation occur
Precipitable water
Amount of water available to fall as precipitation

12. Rainfall mechanisms
 Cloud droplets to join together to form large raindrops
capable of falling as precipitation
Ice –crystal theory of Bergeron
 Ice-crystal theory in 1933 by Tor Bergeron (Norway)
 Common in cold clouds - water droplets do not freeze at 0°C
 Ice nuclei in top of the clouds, vapour diffuses rapidly from
air to ice crystals due to difference in saturation vapour
pressure
 Ice crystals grow rapidly - generates crystals large - to fall

13. Collision – coalescence theory
 In warm clouds- equatorial and tropical regions
 Temperature in upper most a part of these clouds < 7°C
 Proposed by E.F.Bowen from Australia in 1940s
 Precipitation from warm clouds involves the coalescence of
cloud droplets of difference sizes
 Larger droplets grow in big size and fall on the ground on
its own weight in the form of precipitation
 Whatever process at work, continuous supply of moisture
is most important condition

14. Types of precipitation
1. Convectional precipitation
2. Orographic precipitation
3. Cyclonic or Frontal precipitation

15. Convectional precipitation
 Two conditions
1. Intense heating of the surface
2. Abundant supply of moisture
 Warm weather phenomenon, - thunder, lightning and
local winds.
 It is entirely in the form of rain
 Due to the heating of the earth's surface,
 summer months and in the warmer parts of the day.

16. Convectional precipitation

17. 1. Less effective for crop growth than the steady rain.
2. Much of it is drained off in the form of surface drainage
and little remains for entering into the soil.
3. Slope wash and gullying are a menace to loose soil.
4. Temperate regions, it is most effective in promoting the
growth of plants.
5. Gives the maximum rainfall with the minimum cloudiness.
6. Clouds - cumulo-nimbus
Convectional precipitation

18. Orographic precipitation
 When mountains or highlands acting as barriers to the
flow of air, force it to rise the air cools adiabatically
resulted in formation of clouds and precipitation.
 Greek: oros = a mountain).
 Windward sides of mountain ranges lying across the
path of prevailing terrestrial winds where those winds
pass from the relatively warmer ocean to the land, After
striking the high land, the air is forced to rise and
thereby cooled.
 Maximum precipitation - windward slope

19.  South-west monsoon gives copious rainfall on
the windward slope of the Western Ghats
whereas on the leeward side there arc extensive
rain shadow areas.
Orographic precipitation

21. Cyclonic or frontal precipitation

23.  caused mainly by the occurrence of low pressure areas
 It is formed when two air masses of different temperature, humidity
and density meet such as the meeting of a tropical maritime air
mass and a polar air mass.
 A zone called a front separates them.
 At the warm front, the lighter warm air rises gently over the heavier
cold air which remains close to the ground.
 As the warm air rises, it expands and cools and condenses to form
clouds (altostratus).
 The rain falls steadily for a few hours to several days.
Cyclonic or frontal precipitation

24. Rainfall Pattern in Tamil Nadu
 Average annual RF is 925 mm
 South West monsoon – 307.6mm(33.3%)
 North East monsoon - 438.7 (47.4%)
 Summer - 136.5 mm (14.8%)
 Cold Weather period - 42.2 mm(4.6%)

25.  Rainy days- 50 days per year
 Higher is 106.2 (Nilgiris)
 Lowest is 45.8 (Ramnad).
 High Rainfall Regions (More than 1000 mm)
The Nilgiris, the coastal belt of the Cuddalore, Kancheepuram districts and
Palani hills.
 Medium Rainfall Regions (800 to 1000 mm)
Western parts of the Cuddalore, Tiruvellore districts, whole of Vellore, Thiru-
vannamalai , eastern parts of the Salem, Western part of Thanjavur,
Nagapattinam , eastern and northern parts of Trichy, eastern part of Madurai,
Dindigul, northern part of Ramanathapuram, Sivaganga, Virudunagar,
Coimbatore and Salem
 Low Rainfall Regions (600 to 700 mm)
Central and Southern parts of Ramanathapuram, Sivaganga, Virudunagar,
Tuticorin and Tirunelveli districts and Central part of Coimbatore, Central and
Western parts of Madurai Dindiguland the Southern half of Tiruchirapalli.
Rainfall Pattern in Tamil Nadu

26.  Indian continent peculiar phenomenon known as
monsoon.
 It consists of series of cyclones that arise in India Ocean.
 These travel in northeast direction and enter the
Peninsular
 This is followed by a second rainy season from October
to December.
 A third and fourth rainy seasons occur from January to
February and from March to May respectively.
 Of the four rainy seasons, southwest monsoon is the
most important as it contribute 80 – 95% of the total
rainfall of the country.
Monsoon Rainfall Variability

27. Two types of monsoon systems are
a) South West Monsoon
b) North East Monsoon

28. South West monsoon
 June-September
 State average rainfall of 307.6 mm
 Chennai, Tiruvallore, Cuddalore, Villupuram, Vellore,
Tiruvannamalai, Salem, Dharmapuri, Pudukottai, Sivagangai,
Kanyakumari and The Nilgiris.
 Not sufficient to meet the Evapo-transpiration (ET)
 Western Ghats- Reduce the rain

29. North East monsoon
1. Rainy season for Tamil Nadu (October – December)
2. State average rainfall of 438.7 mm
3. Heavy rains are received along the Coromandal coast.
4. Chennai, Chengleput, Cuddalore, Villupuram, Thanjavur, Nagapattinam,
Tirunelveli, Tuticorin and Ramanathapuram districts (coastal districts) –
peak during November 184.7 to 346.7 mm
5. Other districts (central & interior) it occurred during October month and it
ranged between 165.9 and 251.6 mm, little lesser than coastal districts.
6. Salem, Dharmapuri, Coimbatore, Erode, Pudukottai, Vellore, Madurai,
Dindigul, Sivaganga, Virudhunagar and Tuticorin districts receive lesser
rainfall.
7. Tank-fed areas of North Eastern region of Tamil Nadu

30. • Middle of October is generally considered as the "setting in of
Northeast Monsoon".
• Normal date of onset of the northeast monsoon is around
20th October with a deviation of about a week on either side.
North East monsoon

31. Winter (Jan-Feb)
 Rainless, light drizzles now and then
 Important for NEM sown crops
 Chennai, Cuddalore, Villupuram, Pudukottai, Thanjavur,
Nagapattinam, Ramnad, Sivaganga, Virudhunagar, Tirunelveli,
Tuticorin, Kanyakumari and The Nilgiris districts receive
more rainfall than that of State average compared to other
districts.
 Benefit the rice fallow pulses and cotton,
 Interfere with the harvest of rice and sugarcane etc., and do
more harm than good.

32. Summer
 State average summer rain is 136.5 mm
 Weather gets hotter steadily from the beginning of March.
April and May are the hottest months of the year.
 Summer showers which often confined to the afternoons, with
thunder and lightening.
 Standing garden land crops are benefited.
 Summer rains are utilized for preparation of land by ploughing.
 Uncertain in quantity and time.

33. S.No. District SWM NEM Winter Summer Total
1. Chennai 443.5 753.1 37.3 64.2 1298.1
2. Kancheepuram 462.7 697.2 32.1 60.1 1252.1
3. Thiruvallur 449.5 604.1 33.5 65.7 1152.8
4. Cuddalore 373.6 716.5 56.4 89.3 1235.8
5. Villupuram 433.0 484.8 34.5 77.1 1029.4
6. Vellore 442.0 353.0 20.3 101.7 917.0
7. Thiruvannamalai 465.8 439.8 32.8 108.2 1046.6
8. Salem 380.0 347.0 21.3 149.7 898.0
9. Namakkal 317.0 291.0 18.1 150.4 776.5
10. Dharmapuri 361.0 316.7 18.5 156.9 853.1
11. Krishnagiri 403.6 290.9 14.3 154.7 863.5
12. Coimbatore 192.9 327.0 26.1 148.4 694.4
13. Erode 213.1 323.5 20.7 154.1 711.4
14. Tiruchirapalli 270.3 356.1 25.0 110.1 761.5
15. Karur 249.7 365.4 24.0 103.1 742.2
16. Perambalur 349.6 449.6 34.5 115.9 949.6
District wise seasonal rainfall in Tamil Nadu
Conti..

34. S.No. District SWM NEM Winter Summer Total
17. Pudukottai 350.7 418.0 38.2 114.6 921.5
18. Thanjavur 342.0 545.7 50.7 114.6 1053.0
19. Thiruvarur 301.8 665.4 57.9 104.8 1129.9
20. Nagapattinam 274.1 886.4 81.5 99.7 1341.7
21. Madurai 305.4 373.0 29.8 131.8 840.0
22. Theni 178.4 384.0 48.4 222.7 833.5
23. Dindigul 251.4 399.2 33.0 148.0 831.6
24. Ramanathapuram 136.1 507.4 53.9 123.8 821.2
25. Virudhunagar 181.8 431.2 42.0 174.6 829.6
26. Sivagangai 289.6 415.5 35.8 135.1 876.0
27. Tirunelveli 92.6 429.8 72.6 141.9 736.9
28. Thoothukudi 86.8 410.1 46.6 112.2 655.7
29. The Nilgiris 1060.0 367.7 30.8 237.2 1695.7
30. Kanyakumari 327.8 427.4 33.4 217.4 1006.0
State Average 332.8 459.2 36.8 129.6 958.5
District wise seasonal rainfall in Tamil Nadu

35. Drought
 Condition under which
crops fail to mature
because of insufficient
supply of water through
rains.
 Situation - amount of
water required for
transpiration and
evaporation - a defined
area exceeds the amount
of available moisture in the
soil
 A situation of no
precipitation - more than
15 days
 Also called - dry spells.

36. Classification of Drought
 3 categories based on nature of impact and spatial
extent.
Meteorological Drought
 If annual rainfall short (75 %) of expected normal rainfall
over a wide area
 In every state - receives certain amount of normal
rainfall.
 Basis for planning the cropping pattern of area.
Hydrological drought
 Hydrological resources like streams, rivers, reservoirs,
lakes, wells etc dry up - depletion of surface water.
 Ground water table depletes.
 Industry, power generation and other income generating
major sources - affected.
 Meteorological drought is significantly prolonged -
hydrological drought sets in.

37. Agricultural Drought
 Inadequate rainfall and
followed by soil moisture
deficit.
 Soil moisture falls short to
meet the demands of
crops
 Affects growth and finally
reduction of yield.
 Further classified as
Early season drought
Mid season drought
Late season drought.

38. Flood
 Years -actual rainfall is
above normal by twice
the mean deviation or
more excessive rainfall.
 Some flood years
characterized based on
the spatial damage due
to high and intense
rainfall in India
1878,1872,1917,1933,19
42,1956,1959,1961,1970,
1975,1983,1988.

39. Flood
 Years -actual rainfall is above normal by twice the mean
deviation or more excessive rainfall.
 Some flood years characterized based on the spatial
damage due to high and intense rainfall in India
1878,1872,1917,1933,1942,1956,1959,1961,1970,1975,
1983,1988.