Points to Remember
1. Climate It refers to the sum total of weather conditions and variations over a large area for a long period of time (more than 30 years).
2. Weather It refers to the state of the atmosphere over an area at any point of time.
(i) The elements of weather and climate are same i.e.,temperature, atmospheric pressure, wind, humidity and precipitation.
(ii) The climate of India is described as the ‘monsoon’ type. In Asia, this type of climate is found mainly in the south and south-east.
(iii) The word monsoon is derived from the Arabic word‘mausim’, which literally means season.
(iv) Monsoon refers to the seasonal reversal in wind direction during a year.
(v) Variations in climatic conditions are found in our country especially relating to two important elements–temperature and precipitation. They vary from place to place, and season to season.
3. Precipitation
(i) There are variations not only in the form and types of precipitation but also in its amount and seasonal distribution.
(ii) Precipitation is mostly in the form of snowfall in the upper parts of Himalayas and rains over the rest of the country.
(iii) The annual precipitation varies from over 400 cm in Meghalaya to less than 10 cm in Ladakh and western Rajasthan.
(iv) Most parts of the country receive rainfall from June to September. However, some parts like the Tamil Nadu coast get most of its rain during October and November.
(v) In general, coastal areas experience less contrasts in temperature conditions. Seasonal contrasts are more inthe interior of the country.
4. Climatic Controls There are six major controls of the climate of any place. They are latitude, altitude, pressure and wind system, distance from the sea (continentality), ocean currents and relief features.
5. Latitude
(i) Due to the curvature of the Earth, the amount of solar energy received varies according to latitude.
(ii) Air temperature generally decreases from the equator towards the poles.
(iii) The Tropic of Cancer passes through the middle of the country from the Rann of Kachchh in the west to Mizoram in the east.
(iv) Almost half of the country lying south of the Tropic of Cancer belongs to the tropical areas. All the remaining area north of this lies in the sub-tropics.
(v) Therefore, India’s climate has characteristics of tropical as well as subtropical climates.
6. Altitude
(i) As one goes from the surface of the Earth to higher altitudes, the atmosphere becomes less dense and temperature decreases. The hills are therefore cooler during the summers.
(ii) India has mountains to the north, which have an average height of about 6000 metres. The elevation in coastal areas is a maximum of 30 metres.
(iii) The Himalayas prevent the entry of cold winds from Centrol Asia. It is because of these mountains that this sub-continent experiences comparatively milder winters as compared to Central Asia.
7. Pressure and Winds
(i) The pressure and wind system of any area depend on the latitude and altitude of the place.
(ii) These influence the temperature and rainfall pattern.
(iii) The climate and associated weather conditions in India are governed by the following atmosphere conditions
(iv) Pressure and surface winds.
(v) Upper air circulation.
(vi) Western cyclonic disturbances and tropical cyclones.
(vii) The pressure and wind conditions over India are unique.During winter, there is a high pressure area north of the Himalayas.
(viii) Cold dry winds blow from this region to the low pressure areas over the oceans to the south. In summer, a lowpressure area develops over interior Asia as well as overnorth-western India. This causes a complete reversal of the direction of winds during summer.
(ix) Air moves from high pressure area over the southern Indian ocean, in a south easterly direction, crosses theequator and turns right toward, the low-pressure areasover the Indian subcontinent. These are known as theSouthwest Monsoon winds.
(x) These winds blow over the warm oceans, gather moistureand bring widespread rainfall over the mainland of Indialasting for 100-120 days.
8. Coriolis Force This is an apparent force caused by the Earth’s rotation. The Coriolis force is responsible for deflecting winds towards the right in the northern hemisphere and toward the left in the southern hemisphere. This is known as Ferrel’s law.
9. Jet Stream
(i) There are a narrow belt of high altitude (above 12,000 m)westerly winds in the troposphere.
(ii) Their speed varies from about 110 km/h in summer to about 184 km/h in winter.
(iii) A number of separate jet streams have been identified.
(iv) The most constant are the mid latitude and the sub-tropical jet stream.
(v) The jet streams are located approximately over 27° – 30°north latitude and therefore, they are known as subtropical westerly jet streams.
(vi) In summer, the sub-tropical westerly jet stream moves north of the Himalayas with the apparent movement of the Sun.
(vii) An easterly jet stream called the sub-tropical easterly jetstream blows over peninsular India approximately over 14°N during the summer months.
10. Western Cyclonic Disturbances
(i) The western cyclonic disturbances are weather phenomena of the winter brought in by the westerly flow from the Mediterranean region.
(ii) They usually influence the weather of the north andnorth-western regions of India.
(iii) Tropical cyclones occur during the monsoon as well as in October-November and are part of the easterly flow. They affect the coastal regions of the country.
(iv) The factors influencing the mechanism of monsoons are
(a) The differential heating and cooling of land and water creates low pressure on the landmass of India while these as around experience comparatively high pressure.
(b) The shift of the position of Inter Tropical convergence zone (ITCZ) in summer over the Ganga plains. This is the equatorial through normally positioned about 5°Nof the equator. It is also known as the monsoon trough during the monsoon season.
(c) The presence of the high pressure area east of Madagascar approximately at 20°S over the Indian ocean. The intensity and position of the high pressure area affects the Indian monsoon.
(d) The Tibetan plateau gets intensely heated during summer which results in strong vertical air currents and the formation of low pressure over the plateau at about 9 km above sea level.
(e) The movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during the summer.
11. Southern Oscillation (SO)
(i) Normally, when the tropical eastern south Pacific Ocean experiences high pressure, the tropical eastern Indian ocean experiences low pressure. But in certain years, there is a reversal in the pressure in comparison to the eastern Indian ocean. This periodic change in pressure conditions is known the Southern Oscillation (SO).
12. El Nino
(i) A feature connected with the Southern Oscillation (SO) is the El Nino event during which a warm ocean current that flows past the Peruvian coast in place of the cold Peruvian current every 2 to 5 years. The change in pressure conditions is connected to the El Nino. Hence, the phenomenon is referred to as ENSO (El Nino Southern Oscillations).
(ii) El Nino is a Spanish word meaning ‘the child’ and refers to the baby Christ, as this current starts flowing during Christmas.
(iii) The presence of El Nino leads to an increase in sea surface temperatures and weakening of the trade winds in the region.
13. The Onset of the Monsoon
(i) The monsoon winds, unlike the trades are not steady winds but are pulsating in nature and are affected by different atmospheric conditions encountered by them on their way over the warm tropical seas.
(ii) The duration of the monsoon is between 100-120 days from early June to mid September.
(iii) Around the time of its arrival, the normal rainfall increases suddenly and continues constantly for several days. This is known as ‘burst’ of monsoon and can bedistinguished from the premonsoon showers.
(iv) The monsoon arrives at the southern tip of the Indian peninsula generally by the first week of June.Subsequently, it divides into two, the Arabian Sea branchand the Bay of Bengal branch.
(v) The Arabian Sea branch reaches Mumbai about ten days later on approximately 10th June.(vi) This is a fairly rapid advance.
(vii) The Bay of Bengal branch also advances rapidly and arrives in Assam in the first week of June.
(viii) The lofty Himalaya mountain cause the monsoon winds to deflect towards the west over the Ganga plain.
(ix) By mid June the Arabian Sea branch of the monsoon arrives over Saurashtra-Kuchchh and the central part of the country.
(x) The Arabian Sea and the Bay of Bengal branches of the monsoon merge over the north-western part of the Ganga plains.
(xi) Delhi receives the monsoon showers by the end of June(around 29th June). By the first weak of July, western Uttar Pradesh, Punjab, Haryana and eastern Rajasthan experience the monsoon. By mid July, the monsoon reaches Himachal Pradesh and the rest of the country.
14. Withdrawal or Retreat of Monsoon
(i) Withdrawal or the retreat of the monsoon is a more gradual process.(ii) The withdrawal of the monsoon begins in the north-western states of India by early September.
(iii) By mid October, it withdraws completely from the northern half of the peninsula.
(iv) The withdrawal from the southern half of the peninsula is fairly rapid. By early December, the monsoon has withdrawn from the rest of the country.
(v) The islands receive the very first monsoon showers, progressively from south to north from the last week of April to the first week of May.
(vi) The withdrawal takes place progressively from north to south from the 1st week of December to the first week of January. By this time, the rest of the country is already under the influence of winter monsoon.
15. The Seasons Four main seasons can be identified in India: the cold weather season, the hot weather season, the advancing monsoon and the retreating monsoon, with some regional variations.
(i) The Cold Weather Season (Winter)
(a) The cold weather season begins from mid November in northern India and stays till February.
(b) December and January are the coldest months in the northern part of India.
(c) The temperatures decreases from south to the north.
(d) The average temperature of Chennai on the East coast is between 24°-25° Celsius, while in the Northern plains it ranges between 10° – 15° Celsius.
(e) Days are warm and nights are cold
(f) Frost is common in the North and the higher slopes of the Himalayas experience snowfall.
(g) In the northern part of the country, the weather is normally marked by clear sky, low temperature, low humidity and feeble variable winds.
(h) The winter rainfall locally known as ‘Mahawat’ is small, but it is of immense importance for the cultivation of Rabi crops.
(i) The peninsular region does not have a well defined cold season. There is hardly any noticeable seasonal change in temperature pattern, during winter due to moderating influence of the sea.
(ii) The Hot Weather Season (Summer)
(a) The summer season occurs from March to May.
(b) In May, temperatures between 42° to 45° are common.
(c) In peninsular India, temperatures remain lower due to the moderating influence of the ocean.
(d) ‘Loo’ these are strong gusty, hot dry winds blowing during the day over the north and north-western India. Direct exposure to these winds may even proveto be fatal.
(e) Dust storms are very common during the month of May in northern India.
(f) This is also the season for localised thunderstorms associated with violent winds and torrential downpours often accompanied by hail.
(g) In West Bengal, these storms are known as ‘KaalBaisakhi’ or calamity of the month of Baisakhi.
(h) In Kerala and Karnataka, premonsoon showers are common. They help in the early ripening of mangoes and are often referred to as ‘mango showers’.
(iii) Advancing Monsoon (The Rainy Season)
(a) By early June, the trade winds are attracted by the low pressure condition over the northern plains.
(b) They cross the equator and blow in a south westerly direction. They enter the Indian peninsula as the south west monsoon.
(c) As these winds blow over the warm oceans, they bring abundant moisture to the subcontinent.
(d) Early in the monsoon season the windward side of the Western Ghats receives very heavy rainfall, more than 250 cm.
(e) The Deccan plateau and parts of Madhya Pradesh also receive some amount of rain inspite of lying in the rainshadow area.
(f) The maximum rainfall of this season is received in the north-eastern part of the country.
(g) Mawsynram in the Southern ranges of the Khasi Hills receives the highest average rainfall in the world.
(h) Rajasthan and parts of Gujarat get scanty rainfall.
(i) Another phenomenon associated with the monsoon is its tendency to have ‘breaks’, i.e., the monsoon rain stake place only for a few days at a time.
(j) These breaks in monsoon are related to the movement of the monsoon trough.
(k) When the axis of the monsoon trough lies over the plains, rainfall is good in these parts. Whenever the axis shifts closer to the Himalayas, there are longer dry spells in the plains.
(l) Widespread rain occurs in the mountainous catchment areas of the Himalayan rivers. These heavy rains bring in their wake devastating floods causing damage to life and property in the plains.
(m) The frequency and intensity of tropical depressions too determine the amount and duration of monsoon rains.
(iv) Retreating Monsoon (The Transition Season)
(a) During October and November the south west monsoon winds weaken and start withdrawing gradually. By the beginning of October, the monsoon withdraws from the northern plains.
(b) The months of October – November form a period of transition from hot rainy season to dry winter conditions. The retreat of the monsoon is marked by clear skies and rise in temperature.
(c) Day temperatures are high, nights are cool and pleasant. The land is moist.
(d) Owing to the conditions of high temperature and humidity, the weather becomes rather oppressive during the day. This is commonly known as ‘OctoberHeat.’ In the second half of October, the mercury begins to fall rapidly in northern India.
(e) The low-pressure conditions over north western India, get transferred to the Bay of Bengal by early November. This shift is associated with the occurrence of cyclonic depressions which originates over the Bay of Bengal. Deltas of Krishna and Kaveri are struck by cyclones, which are often very destructive and cause damage to life and property.
(f) Sometimes these cyclones arrive on the coasts of Orrisa, Paschim Banga and Bangladesh.
(g) The maximum rainfall on the coromandal coast is derived from depressions and cyclones.
20. Distribution of Rainfall
(i) Parts of the western coast and north-eastern India receive over about 400 cm of rainfall annually.
(ii) It is less than 60 cm in Western Rajasthan adjoining parts of Gujarat, Haryana and Punjab.
(iii) Rainfall is equally low in the interior Deccan plateau and east of Sahyadris.
(iv) A third area of low precipitation is around Leh in Jammuand Kashmir.
(v) The rest of the country receives moderate rainfall.
(vi) Snowfall is restricted to the Himalayan region.
(vii) Owing to the nature of monsoons, the annual rainfall ishighly variable from year to year.
21. Monsoon As a Unifying Bond
i) India is a land of diversities. It has diverse food habits, languages, customs and festivals; also it has diverse climatic conditions.
(ii) However, the monsoons have a unifying influence on the Indian subcontinent.
(iii) The Indian landscape, its animals and plant life, its entire agricultural calender and the life of the people including their festivals revolve around this phenomenon (monsoon).
(iv) Year after year the people of India from north to south and from east to west eagerly await the arrival of the monsoon.
(v) These monsoon winds bind the whole country by providing water to set the agricultural activities in motion.
(vi) The river valleys which carry this water also unite as a single river valley unit.
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