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India has ‘Monsoon’ type of climate. The word, monsoon has been derived from the Arabic, word ‘mausil m’ which means seasonal reversal, of the winds during the course of the year., This implies a rhythmic change in the, direction of winds and in the distribution of, temperature and rainfall with the change of, season., , Unity and Diversity in the Indian, Monsoon Climate, , pica ea, Notwithstanding its broad climatic unity, the, dimate of India has many regional variations,, expressed in the pattern of winds,, temperature and rainfall, rhythm of seasons, and the degree of wetness or dryness. These, climatic differences are due to location,, altitude, distance from sea and relief. The, main examples of diversities in the monsoonal, unity of Indian climate are briefed as under:, , 1. The day temperature in the month of, June may soar to 48°-50°C at Barmer, in Rajasthan while it may hardly reach, 22°C at Gulmarg or Pahalgam in, Kashmir at the same time., , 2. On a December night, the temperature, may dip to — 45°C at Dras or Kargil in, J & K while it may be as high as 20°C22°C at Thiruvananthapuram or, Chennai at the same time., , 3. Mawsynram in Meghalaya receives as, high as 1221 cm of annual rainfall while, at Jaisalmer (Rajasthan) the annual, rainfall rarely exceeds 12 cm. Tura, in, the Garo Hills, may receive as much, rainfall in a single day as is received by, Jaisalmer in 10 years. F, , 4. The Ganga delta and the coastal plains, , of Orissa are hit by strong rain storms, , , , Climate, , almost every third or fifth day in July, and August, while the Coromandel, Coast, a thousand kilometres to the, south, goes dry during these months., , 5. There are severe floods in Brahmaputra, in July whereas there may be very little, rainfall in Rajasthan., , 6. The people of Mumbai and the Konkan, Coast do not have to suffer the extremes, of climate but these extremes affect the, life of people of interior parts of the, country such as Delhi and Agra., , 7. Places like Goa, Hyderabad, Bhubaneswar, and Patna get rains by the first quarter, of June while the rains are awaited till, the end of June or early July at places, like Agra, Delhi and Chandigarh., , 8. In Kerala and in the Andaman Islands,, the difference between day and night, temperatures may be hardly seven or, eight degree Celsius. But in the Thar, desert, if the day temperature is around, 50°C, at night, it may drop down, considerably upto 15°-20°C., , 9. Most parts of the country get rainfall, during June-September, but on the, coastal areas of Tamil Nadu, it rains in, the beginning of the winter season., , In spite of these differences and, variations, the climate of India is, monsoonal in rhythm and character., , Factors Determining the Climate, of India, , ————————————, India’s climate is controlled by a number of, factors which can be broadly divided into two, groups—factors related to location and relief,, and factors related to air pressure and winds., , — B-53, , _ GUIMATE
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Factors related to Location and Relief, , 1. Latitude. The mainland of India, extends roughly from 8°N to 37°N latitude, and the Tropic of Cancer passes through the, middle of the country. Areas lying in the, north of Tropic of Cancer are in sub-tropical, and temperate zone while those lying in the, south of this line fall in the tropical zone., There is a gradual fall in temperature as we, move from south to north. The tropical zone, is nearer to the equator and experiences, high temperature throughout the year. This, zone experiences small daily and annual, range of temperature. The part of country, in the north of the Tropic of Cancer is away, from the equator and as such these areas, experience extremes of climate with high, diurnal and annual range of temperature., , 2.The Himalayan Mountains. The, Himalayan mountains along with its extensions in the north-west and north-east act, as a very effective climate divide. It saves, the Indian subcontinent from the cold and, chilly winds originating near the Arctic Circle, and blowing across central and eastern Asia., The Himalayan wall also traps the monsoon, winds from the Bay of Bengal and Arabian, Sea and cause widespread rainfall in the subcontinent., , 8. Distribution of Land and Water., India is flanked by the Indian Ocean on three, sides in the south and girdled by a high and, continuous mountain wall in the north. As, compared to the landmass, water heats up or, cools down slowly. This differential heating, of land and sea creates different air pressure, zones in different seasons in and around the, Indian subcontinent. Difference in air, pressure causes reversal in the direction of, monsoon winds., , 4. Distance from Sea. As mentioned, above, landmasses heat up and cool down, more quickly than water. As such coastal, areas enjoy equable climate while areas located in the interior i.e., far away from the, , B-54, , , , , , , , , , , , , , , sea experience extremes of climate. This ia, the season that people living in coastal areas, have no idea of extremes of climate while, seasonal contrasts are quite evident in the, interior of the country., , 5. Altitude. There is a gradual fal] of, temperature with increasing altitude at an, average rate of 1°C for 165 metres (or 0.65°C, per 100 metres). Consequently places located, in mountains are cooler than places located, in plains. For example, Agra and Darjiling, are located on the same latitude, but temperature of January in Agra is 16°C whereas, it is only 4°C in Darjiling., , 6. Relief. India has great physiographic, variations including lofty mountains, vast, plains and extensive plateaus. These aspects, of relief have a great bearing on temperature, air pressure, direction and speed of wind, and amount and distribution of rainfall. The, windward sides of Western Ghats and Assam, receive high rainfall during June-September, whereas the southern plateau remains dry, due to its leeward situation along the Western Ghats., , Factors Related to Air Pressure and, Wind, , To understand the differences in local, climates of India, we need to understand the, mechanism of the following three factors:, , (a) Surface distribution of pressure and, winds., , (6) Upper air circulation caused by factors, controlling global weather and the i, flow of different air masses and jet, streams., , (c) Weather conditions leading to rainfall, due to inflow of western disturbance®, during winter and tropical depressions, during the rainy season., , These mechanisms can be described with, reference to the two main seasons of the ye", i.e, winter and summer, when strikiné, changes in weather occur.
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Indian weather in the winter season is, generally influenced by the presence of high, atmospheric pressure in Central and West, jsia. This causes the flow of surface winds, fom the north to the Indian sub-continent., These winds form a dry continental air-mass., This air mass comes in contact with the, Indian trade winds. Sometimes, this contact, zone between the dry continental air-mass, and the Indian trade winds reaches as far as, middle Ganga Valley. As a result of this, the, , entire north-western India upto middle Ganga, Valley comes under the influence of dry northwestern winds., , The above pattern of pressure and winds, is found only in the lower parts of the, atmosphere near the surface of the earth only., The upper air circulation is explained by the, jet stream described below., , The Jet Streams. These are upper level, winds which blow above 6 to 7 km from the, surface and attain greater velocity at higher, levels. These winds blow upto a height of 12, to 16 km within the troposphere. The areas, over which they blow vary latitudinally with, , , , MEE Main branch of jet-stream in winter, , oO cS Jet-stream in summer, , (After Pedelaborde), , , , FIG. 4.1. JET STREAM, , , , , , , , , IN SUMMER AND, , x, , cS |, , , , , , WINTER
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seasons and height, At 12 to 13 km the upper, level winds attain speeds exceeding 180 km, per hour and are known as the jet streams., They are of two types : westerly jet streams, which blow from west to east at a height of, 12 km and easterly jet streams which blow, from east tOwest at 18 km from the earth, surface. The position of these jet streams, changes seasonally. The axis of the westerly, jet stream lies along the southern slopes of, , _ the Himalayas in winter, but with the advent, of the southwest monsoon it shifts to the, north and remains there during that season, (Fig. 4.1). The easterly jet stream blows to, the south of 25° N parallel during the, southwest monsoon. In June it blows over, the southern part of the peninsula and has a, maximum speed of 90 km per hour. In August, it is confined to 15° N parallel and in, September upto 22° N parallel. The easterlies, normally do not extend to the north of 30° N, parallel in upper troposphere., , The axis of the jet streams shifts northwards and southwards at different times of, the year and their study helps in forecast of, the on set and withdrawal of the southwest, monsoon, and also breaks of the rainy season., , Western Disturbances, ai opical, Cyclones. The upper ain Feeetee a., , tropical westerlies that move eastwards over, north India during winter months are known, as western disturbances in the Indian, meteorology. These moving troughs often give, rise to closed circulation of winds and are, responsible for winter precipitation. The, westerly troughs and the associated low level, cyclonic storms originate in the Mediterranean, or the east Atlantic region with secondaries, developing over west Asia. They have a life, history similar to that of the temperate cyclones, of the Pacific or the Atlantic and arrive over, India more or less in an occluded state., , Tropical cyclones originate over the Bay, of Bengal and the Arabian Sea. These tropical cyclones have very high wind velocity, and heavy rainfall and hit the Tamil Nadu,, Andhra Pradesh and Orissa coast. Most of, these cyclones are very destructive due to, high wind velocity and torrential rain that, accompanies it., , B-56, , , , , , , , , , INDIA : PHYSICAL ENVIRONMENT, , sisi Mth e Slat, , , , , , the last over three centuries, not DU, , Mechanism of weather in the, , Summer Season, eee, Surface Pressure and Winds. With th., onset of summer season, the sun shift,, towards the north and there is a complet,, reversal of pressure conditions and wing, circulation both in the lower and the upper, layers of the atmosphere. Near the earth’;, surface, the Inter-Tropical Convergence Zone, (ITCZ) establishes itself at about 25°y, latitude over India and Pakistan. Inter, Tropical Convergence Zone is a low pressure, zone located at equator where trade winds, converge and the air tends to ascend. With, the northwards shift of the ITCZ, the westerly, jet stream withdraws from the Indian plain., This low pressure ITCZ attracts winds trom, different directions. The maritime tropical air, (MT) from southern hemisphere rushes to, this low pressure zone after crossing the, equator between 40° and 60° east longitudes., This is known as the south-west monsoon., According to another view, the south-west, monsoon is just a continuation of the, equatorial westerlies which are attracted by, ITCZ., , Jet Streams and Upper Air, Circulation. In the upper layers of the, troposphere, the conditions are entirely, different. An easterly jet stream flows over, the southern part of the Peninsular in June, It has a maximum speed of 90 km per hour, In August, it is confined to 15°N latitude,, aad in September up to 20°N latitudes. The, easterlies normally do not extend to the nort, of 30°N latitude in the upper atmosphere:, , Easterly Jet Stream and Tropical, Cyclones. The easterly jet stream steers the, tropical depressions into India. anes, depressions affect the distribution of rainfal, in India. The highest rainfall occurs alone, the track of these depressions. The fre enc!, at which these depressions visit India, the, direction and intensity, all go a long wav, determining the rainfall pattern during, southwest monsoon period., , The Nature of Indian Mons00?”, , «4 for, Although monsoons have been studied fo
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ynown about the nature of the Indian, onsoon and it still continues to be a puzzle, forthe scientists. Some breakthrough has been, achieved during the last few decades when it, yas studied at the global rather than at, regional level., , A brief discussion of the main, characteristics and genesis of the monsoon, will be useful to understand the monsoonal, regime of the Indian climate. Following, aspects of monsoonal rainfall will help in, understanding the monsoons in a better way:, , 1. The onset and advance of the monsoon., , 2. Rain-bearing system and rainfall distribution., , 3. Break in the monsoon., 4. The retreat of the monsoon., , 1.The Onset and Advance of the, Monsoon. Many scholars still believe that, the main mechanism of monsoons is, differential heating of land and sea in the, Summer season. The landmass, particularly, in the north-west India, gets intensely heated, luring summer season causing a very low, Pressure area there. The Indian Ocean, femains comparatively cooler leading to high, Pressure condition there. This causes a, Northward shift in the position of the ITCZ, and the south-east trade winds cross the, ‘tuator and, after deflection, these winds, *nter India as South-West Monsoon winds., oe shift in the position of the ITCZ is also, shut %0 the withdrawal of the westerly jet, “am from the south of the Himalayan range., e easterly jet stream is caused by the, v heating of the Himalayan and the, A an highlands, These highlands occupy a, St area of 4.5 million square kilometres with, ‘Verage altitude of more than 4,000 metres., to , “Se highlands are heated in summer due, irpttward shift of the sun, a clockwise, a ‘ation of the wind takes place in the middle, Stre. © troposphere. This results in two main, oe of air from this landmass. One of oe, oe Ses towards the equator while rie, eauat, 18 deflected towards the pole. ee, Stre, torward flow enters India as easterly ie, Centra) le the poleward stream goes to Bast, 1a as westerly jet stream., , Summe;, , © South-west monsoon sets in over Me, , Kerala coast by 1 June and reaches Mumbai, and Kolkata by 10-13 June. It spreads all over, the sub-continent by 15 July (Fig. 4.2), , 2. Rain-bearing System and Rainfall, Distribution. The monsoon rainfall is never, continuous but comes as wet spells followed, by dry spells. The tropical depressions from, the Bay of Bengal cause rainfall in the, northern plain of India while the west coast, of India receives rainfall from the Arabian, Sea current of the monsoon. The intensity of, rainfall on the west coast of India depends, upon the off-shore meteorological conditions, and the position of the equatorial jet stream, along the eastern coast of Africa., , The frequency and the tracks of Bay of, Bengal depressions depend upon the position, of ITCZ which is generally known as the, monsoon trough. A change in the position of, ITCZ causes a change in the axis of the tropical, depression which brings about a change in, the intensity and distribution of rainfall. The, amount of rainfall decreases from west to eastnorth-east over the west coast and from eastsouth-east towards north-west over the Indian, plain and the northern peninsula., , 3. Break in the Monsoon. When the, south-west monsoon fails to bring rainfall for, two or more weeks and there is a dry spell in, the rainy season, it is called the break in, the monsoon. This break may be caused, either by failure of the tropical depressions, or by dislocation of the monsoon trough (ITCZ), over north India. Over the west coast, we, experience dry spell when the winds blow, parallel to the coast. In the western part of, Rajasthan, the inversion of temperature, prevents rain-bearing winds from rising up, and a dry spell is experienced., , 4, The Retreat of the Monsoon. The, south-west monsoon starts retreating by 1st, September from the western strip of, Rajasthan and, by 15th September, it retreats, from most parts of Punjab, Haryana,, Rajasthan and Gujarat. It retreats from most, parts of India by mid-October, except the, southern peninsula. The retreating monsoon, , icks up moisture from the Bay of Bengal, ee establishes itself over Tamilnadu coast, me the month of December and causes rain, fall there (Fig. 4.3)., , , , CLIMATE
