RAINFALL AND SEASONS OF TAMIL NADU.ppt

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
ManydistrictsofTamilNadufallunderSemiArid
Tropics(SAT),thesedistrictsfacehigherincidence
ofsolarradiation,moreairandsoiltemperatureand
highlyvaryingrainfall

3. Forms of precipitation

4. Rain
Precipitationofwaterinliquidstate.
Dropsofmorethan0.5mmdiameter

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.Veryminutewaterdropletssuspendedintheairor
itconsistsofamixtureofsmokeorfinedust
particles.
2.Reducesthehorizontalrangeofvisibilitytoless
than1km.
3.Depositsontheleavesofthecropplants.
4.Theevapotranspirationlossesareminimizedasthe
leavesremainwetforalongerperiod.

7. DEW
Dewis 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.Precipitationofsmallballsorpiecesofice(hailstones)
2.Diameterrangingfrom5to50mm
3.Mostdreadedanddestructiveformofprecipitation
4.Producedinviolentthunder-stormsorcumulo-nimbusclouds.
5.Structurelikeanonion.
6.MarchtoMay-idealconditionsforhailstorms.
7.Standingcropsinthefieldcanbedevastatedinjustafewminutes'
byseverehailstormswhichmeantremendouseconomiclosstothe
farmers.
8.Verydifficulttoforecast,particularlythetimeandplaceoftheir
occurrence.

9. Snow
Precipitationofwhiteandopaquegrainsofice.
Itisprecipitationofsolidwater,mainlyinthe
formofbranchedhexagonalcrystalsorstars.

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, vapourdiffuses 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.Bowenfrom 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
Twoconditions
1.Intenseheatingofthesurface
2.Abundantsupplyofmoisture
Warmweatherphenomenon,-thunder,lightningand
localwinds.
Itisentirelyintheformofrain
Duetotheheatingoftheearth'ssurface,
summermonthsandinthewarmerpartsoftheday.

16. Convectional precipitation

17. 1.Lesseffectiveforcropgrowththanthesteadyrain.
2.Muchofitisdrainedoffintheformofsurfacedrainage
andlittleremainsforenteringintothesoil.
3.Slopewashandgullyingareamenacetoloosesoil.
4.Temperateregions,itismosteffectiveinpromotingthe
growthofplants.
5.Givesthemaximumrainfallwiththeminimumcloudiness.
6.Clouds-cumulo-nimbus
Convectional precipitation

18. Orographic precipitation
Whenmountainsorhighlandsactingasbarrierstothe
flowofair,forceittorisetheaircoolsadiabatically
resultedinformationofcloudsandprecipitation.
Greek:oros=amountain).
Windwardsidesofmountainrangeslyingacrossthe
pathofprevailingterrestrialwindswherethosewinds
passfromtherelativelywarmeroceantotheland,After
strikingthehighland,theairisforcedtoriseand
therebycooled.
Maximumprecipitation-windwardslope

19. South-westmonsoongivescopiousrainfallon
thewindwardslopeoftheWesternGhats
whereasontheleewardsidetherearcextensive
rainshadowareas.
Orographic precipitation

21. Cyclonic or frontal precipitation

23. causedmainlybytheoccurrenceoflowpressureareas
Itisformedwhentwoairmassesofdifferenttemperature,humidity
anddensitymeetsuchasthemeetingofatropicalmaritimeair
massandapolarairmass.
Azonecalledafrontseparatesthem.
Atthewarmfront,thelighterwarmairrisesgentlyovertheheavier
coldairwhichremainsclosetotheground.
Asthewarmairrises,itexpandsandcoolsandcondensestoform
clouds(altostratus).
Therainfallssteadilyforafewhourstoseveraldays.
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. Rainydays-50daysperyear
Higheris106.2(Nilgiris)
Lowestis45.8(Ramnad).
HighRainfallRegions(Morethan1000mm)
TheNilgiris,thecoastalbeltoftheCuddalore,Kancheepuramdistrictsand
Palanihills.
MediumRainfallRegions(800to1000mm)
WesternpartsoftheCuddalore,Tiruvelloredistricts,wholeofVellore,Thiru-
vannamalai,easternpartsoftheSalem,WesternpartofThanjavur,
Nagapattinam,easternandnorthernpartsofTrichy,easternpartofMadurai,
Dindigul,northernpartofRamanathapuram,Sivaganga,Virudunagar,
CoimbatoreandSalem
LowRainfallRegions(600to700mm)
CentralandSouthernpartsofRamanathapuram,Sivaganga,Virudunagar,
TuticorinandTirunelvelidistrictsandCentralpartofCoimbatore,Centraland
WesternpartsofMaduraiDindigulandtheSouthernhalfofTiruchirapalli.
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
Stateaveragerainfallof307.6mm
Chennai,Tiruvallore,Cuddalore,Villupuram,Vellore,
Tiruvannamalai,Salem,Dharmapuri,Pudukottai,Sivagangai,
KanyakumariandTheNilgiris.
NotsufficienttomeettheEvapo-transpiration(ET)
WesternGhats-Reducetherain

29. North East monsoon
1.RainyseasonforTamilNadu(October–December)
2.Stateaveragerainfallof438.7mm
3.HeavyrainsarereceivedalongtheCoromandalcoast.
4.Chennai,Chengleput,Cuddalore,Villupuram,Thanjavur,Nagapattinam,
Tirunelveli,TuticorinandRamanathapuramdistricts(coastaldistricts)–
peakduringNovember184.7to346.7mm
5.Otherdistricts(central&interior)itoccurredduringOctobermonthandit
rangedbetween165.9and251.6mm,littlelesserthancoastaldistricts.
6.Salem,Dharmapuri,Coimbatore,Erode,Pudukottai,Vellore,Madurai,
Dindigul,Sivaganga,VirudhunagarandTuticorindistrictsreceivelesser
rainfall.
7.Tank-fedareasofNorthEasternregionofTamilNadu

30. •MiddleofOctoberisgenerallyconsideredasthe"settinginof
NortheastMonsoon".
•Normaldateofonsetofthenortheastmonsoonisaround
20
th
Octoberwithadeviationofaboutaweekoneitherside.
North East monsoon

31. Winter (Jan-Feb)
Rainless,lightdrizzlesnowandthen
ImportantforNEMsowncrops
Chennai,Cuddalore,Villupuram,Pudukottai,Thanjavur,
Nagapattinam,Ramnad,Sivaganga,Virudhunagar,Tirunelveli,
Tuticorin,KanyakumariandTheNilgirisdistrictsreceive
morerainfallthanthatofStateaveragecomparedtoother
districts.
Benefitthericefallowpulsesandcotton,
Interferewiththeharvestofriceandsugarcaneetc.,anddo
moreharmthangood.

32. Summer
Stateaveragesummerrainis136.5mm
WeathergetshottersteadilyfromthebeginningofMarch.
AprilandMayarethehottestmonthsoftheyear.
Summershowerswhichoftenconfinedtotheafternoons,with
thunderandlightening.
Standinggardenlandcropsarebenefited.
Summerrainsareutilizedforpreparationoflandbyploughing.
Uncertaininquantityandtime.

33. S.No.District SWM NEM WinterSummer Total
1. Chennai 443.5753.1 37.3 64.2 1298.1
2. Kancheepuram 462.7697.2 32.1 60.1 1252.1
3. Thiruvallur 449.5604.1 33.5 65.7 1152.8
4. Cuddalore 373.6716.5 56.4 89.3 1235.8
5. Villupuram 433.0484.8 34.5 77.1 1029.4
6. Vellore 442.0353.0 20.3 101.7 917.0
7. Thiruvannamalai 465.8439.8 32.8 108.21046.6
8. Salem 380.0347.0 21.3 149.7 898.0
9. Namakkal 317.0291.0 18.1 150.4 776.5
10.Dharmapuri 361.0316.7 18.5 156.9 853.1
11.Krishnagiri 403.6290.9 14.3 154.7 863.5
12.Coimbatore 192.9327.0 26.1 148.4 694.4
13.Erode 213.1323.5 20.7 154.1 711.4
14.Tiruchirapalli 270.3356.1 25.0 110.1 761.5
15.Karur 249.7365.4 24.0 103.1 742.2
16.Perambalur 349.6449.6 34.5 115.9 949.6
District wise seasonal rainfall in Tamil Nadu
Conti..

34. S.No.District SWM NEM WinterSummer Total
17.Pudukottai 350.7418.038.2 114.6 921.5
18.Thanjavur 342.0545.750.7 114.61053.0
19.Thiruvarur 301.8665.457.9 104.81129.9
20.Nagapattinam 274.1886.481.5 99.7 1341.7
21.Madurai 305.4373.029.8 131.8 840.0
22.Theni 178.4384.048.4 222.7 833.5
23.Dindigul 251.4399.233.0 148.0 831.6
24.Ramanathapuram 136.1507.453.9 123.8 821.2
25.Virudhunagar 181.8431.242.0 174.6 829.6
26.Sivagangai 289.6415.535.8 135.1 876.0
27.Tirunelveli 92.6429.872.6 141.9 736.9
28.Thoothukudi 86.8410.146.6 112.2 655.7
29.The Nilgiris 1060.0367.730.8 237.21695.7
30.Kanyakumari 327.8427.433.4 217.41006.0
State Average 332.8459.236.8 129.6 958.5
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.