NCERT 9 GEOGRAPHY CHAPTER 4

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   4. CLIMATE


  • Climate - sum total of weather conditions and variations over a large area for a long period of time (> 30 years)
  • Weather - state of the atmosphere over an area at any point of time
  • The elements of weather and climate are same (temperature, atmospheric pressure, wind, humidity and precipitation)
  • Indian climate - monsoon type (the word 'monsoon' - Arabic word 'mausim' meaning season) the seasonal reversal in the wind direction during a year
    CLIMATIC CONTROLS
  • Latitude - amount of solar energy received varies, the temperature decreases from the equator towards the poles 
  • Altitude - At higher altitudes, the atmosphere becomes less dense and temperature decreases So, hills are cooler during summers
  • Pressure and wind system - depend on latitude and altitude influences temperature and rainfall pattern
  • Distance from the sea - As distances increases, the moderating influence of sea decreases 
  • Continentality - North and Central India has continental climate (very hot during summers and very cold during winters)  
  • Peninsular India experiences equable climate
  • Ocean currents - ocean currents along with onshore winds affects the climate
  • Relief - causes precipitation (windward side) while leeward side of mountains remains dry (rain shadow region)
   FACTORS AFFECTING INDIA's CLIMATE
  • LATITUDE - India has tropical as well as sub-tropical climate (Tropic of cancer passes in the middle, half of the country lies in tropics and the rest in sub-tropics)
  • ALTITUDE - The Himalayas mountains(6000m) prevents the cold winds from Central Asia, hence India experiences comparatively milder winters than Central Asia
  • PRESSURE AND WINDS 
  • Trade winds : originate from subtropical high pressure belt; 
  • north easterly winds get deflected to the right and south easterly winds get deflected to the left (due to Coriolis force - apparent force caused by earth's rotation, also known as Ferrel's law)
  • ITCZ - The Inter Tropical Convergence Zone, belt of low pressure where the trade winds converge 
  •  Monsoon winds : 
  • winter - land to sea, from high pressure (north of Himalayas) to low pressure (oceans in the south) 
  • Summer - 'Southwest monsoon winds' reversal of the direction of winds(from sea to land)
  • Jet stream : narrow bands of high speed (110 km/h to 184km/h) strong winds in troposphere (above 12,000m)
  • India receives sub-tropical jet streams (27°-30°N)  
  • Subtropical westerly jet streams - blow south of the Himalayas except in summer
  • Subtropical easterly jet streams - blow over Peninsular India during summer
  • Western cyclonic disturbances: during winter - storm originating from the Mediterranean sea influence the north and north-western regions of India
  • Tropical cyclones : during October and November affect the coastal regions of the country

  THE MECHANISM OF INDIAN MONSOON
  • The differential heating and cooling of land and water
  • The shift of the position of ITCZ
  • High pressure on the east of Madagascar
  • Tibetan plateau gets intensely heated during summer
  • The movement of westerly jet streams to the north of the Himalayas and the presence of the tropical easterly jet stream over Indian Peninsula
  • Southern oscillation - Pressure conditions over the Southern ocean affects the monsoons
  • El Nino Southern Oscillation(ENSO) - replacement of warm ocean current in the Peruvian coast with the cold Peruvian current
    THE ONSET OF THE MONSOON AND WITHDRAWAL
  • Duration : 100-120 days (early June to mid September) pulsating in nature 
  • Burst of the monsoon - the normal rainfall increases suddenly
  • First week of June - arrives at southern tip of India
  • 10 July - the Arabian sea branch reaches Mumbai while the Bay of Bengal branch arrives Assam
  • Mid June - Arabian sea branch over the Saurashtra- Kuchchh and Central India
  • These branches merge over the northwest of Ganga Plains
  • End of June - Delhi receive monsoon showers
  • First week of July - covers Western Uttar Pradesh, Punjab, Haryana and Eastern Rajasthan
  • Mid July - reaches Himachal Pradesh and rest of the country
  • September - withdrawal begins from northwestern states of India
  • Mid October - completely from the northern half of Peninsula
  • Early December - withdrawn from the rest of the country
   THE SEASONS

  1.THE COLD WEATHER SEASON (WINTER)
  • Mid November to February
  • December and January - coldest months
  • Temperature - Chennai : 24° -25° C while northern plains : 10° and 15° C
  • Peninsular region does not have defined cold season
  • North east trade winds blow from land to sea with little moisture but Tamil Nadu receives rainfall as these winds blow from sea to land 
  • In Northern plains, the western cyclonic disturbance causes winter rains (locally called as 'mahawat' essential for cultivation of rabi crop)and snowfall 
   2.THE HOT WEATHER SEASON (SUMMER)
  • Due to the shifting of heat belt towards north resulted in the rise in temperature
  • March - 38° C over the Deccan plateau, April - 42° C over Gujarat and Madhya Pradesh and 45° C over the northwestern parts in May
  • Peninsular India - lower temperature (moderating influence of the oceans)
  • Loo - strong, gusty, hot, dry winds blow during the day ('Kaal Baisakhi' in West Bengal)
  • Pre-monsoon showers - 'mango showers' common in Kerala and Karnataka (help in the early ripening of mangoes)
   3.ADVANCING MONSOON (THE RAINY SEASON)
  • south east trade winds blow in a south westerly direction entering India as southwest monsoon winds
  • strong and blow (30 km/h)
  • The windward side of Western Ghats receives rainfall (> 250 cm), the Deccan Plateau and Madhya Pradesh receive some amount of rain (leeward side)
  • maximum rainfall in north-eastern region (Mawsynram receives the highest average rainfall in the world)
  • Rajasthan and Gujarat receives scanty rainfall
  • Monsoon Breaks - the monsoon rains take place for few days and interspersed with rainless intervals
  • causes floods in some part, may be responsible for droughts in other
   4.RETREATING/ POST MONSOONS (THE TRANSITION SEASON)
  • during October and November
  • When the southwest monsoon winds weaken, the low pressure over northwestern India transferred to Bay of Bengal causes cyclonic depressions in Andaman Sea
  • causes heavy rain and destruction to the deltas of the Godavari, the Krishna, the Kaveri and the Coromandel Coast
   DISTRIBUTION OF RAINFALL
  • Parts of western coast and northeastern India - get 400 cm per year
  • Western Rajasthan, Gujarat, Haryana, Punjab - less than 60 cm annually 
  • interior of Deccan plateau and east of sahyadris - equally low
  • low precipitation also in Leh 
  • rest of the country - moderate rainfall 
  • The Himalayas receives snowfall 
 
  

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