Chapter 15 — 5.4 Climate of India

Climate is the average condition of the atmosphere over a large area for a long period, usually 30 years or more. Weather is the condition of the atmosphere at a place for a short time. India's climate is described as tropical monsoon, though the country stretches from about 8°N to 37°N and so crosses the Tropic of Cancer, which passes through the middle of the country at about 23°30'N. The northern half is therefore in the sub-tropical or warm temperate zone by latitude, but the Himalayas shut out the cold winds of central Asia and the whole country behaves as a tropical land.

The word monsoon comes from the Arabic mausim, meaning season. It refers to winds that reverse their direction completely with the seasons. In summer the winds blow from the sea to the land (the south-west monsoon) and bring rain; in winter they blow from the land to the sea (the north-east monsoon) and are dry. Because rainfall depends on these winds, the whole year of India is arranged around them, and the four seasons are named by their relationship with the monsoon.

The main characteristics of India's climate are these. First, it is dominated by the seasonal reversal of winds. Second, rainfall is concentrated in a few months: about three-quarters of the annual rain falls between June and September. Third, rainfall is unevenly distributed: Mawsynram in Meghalaya receives more than 1,100 cm a year, while Jaisalmer in Rajasthan receives less than 15 cm. Fourth, the monsoon is uncertain: it may come early or late, break for weeks in the middle, and fail altogether in some years, producing floods in one region and drought in another. Fifth, there are great regional contrasts of temperature: in December a night at Dras in Ladakh may touch −45°C while Chennai stays at 25°C, and in June the day at Barmer may reach 50°C while Shillong is at 20°C. Sixth, the temperature range is continental in the north-west interior and equable along the coasts.

Despite these contrasts the climate is one unit, because every part of India waits for the same monsoon, and the failure of the monsoon anywhere is felt everywhere through the price of food. This unity in diversity is why geographers speak of one Indian climate with many regional variations rather than many separate climates.

Several factors act together to give India its climate. The first is latitude. The Tropic of Cancer divides the country into two nearly equal halves. South of it the sun is overhead twice a year and temperatures are high throughout the year, with a small annual range; this is the tropical part. North of it the sun is never overhead and there is a marked winter; this is the sub-tropical part. The difference is felt in the winter: Thiruvananthapuram in January averages 27°C while Amritsar averages 12°C.

The second factor is the Himalayas. This great wall, 2,400 km long and rising above 6,000 m, has two effects. In winter it blocks the bitterly cold winds that blow out of the high-pressure centre over Siberia and central Asia, so that northern India is much warmer than places at the same latitude in China or Iran. In summer it stops the moist south-west monsoon winds from crossing into Tibet, forces them to rise, and squeezes rain from them over the whole northern plain and the hills of the north-east. Without the Himalayas, India would be a dry land like the deserts of west Asia at the same latitude.

The third factor is distance from the sea. India is a peninsula with a coastline of more than 7,500 km, and the sea moderates the temperature of every coastal region. Water warms and cools slowly, so the coast has cooler summers and milder winters, and a small annual range: Kochi varies only between 26°C and 30°C on average. The interior of the north-west is 1,000 km or more from the sea and has a continental climate of hot summers and cold winters: Bikaner may reach 48°C in June and fall to 2°C in January.

The fourth factor is the surrounding seas themselves. The Arabian Sea and the Bay of Bengal are the sources of the moisture that the monsoon carries. The Bay of Bengal, being warmer and enclosed, is also the breeding ground of the cyclones that strike Odisha, Andhra Pradesh, West Bengal and Bangladesh in the pre-monsoon and post-monsoon months. The Indian Ocean to the south sends the equatorial current of air that becomes the monsoon.

Altitude is the fifth factor. Temperature falls with height at the normal lapse rate of about 6.5°C for every 1,000 m. This is why the hill stations exist: Darjeeling, at 2,045 m, averages about 17°C in June while Siliguri at its foot averages 28°C. Shimla, Ooty, Mussoorie and Kodaikanal are cool for the same reason. In Ladakh, above 3,500 m, the climate is cold desert, and the highest Himalayan peaks are under permanent snow.

Relief is the sixth. The direction in which a mountain range lies decides where the rain falls. The Western Ghats run north-south, right across the path of the Arabian Sea branch of the monsoon; the windward western slope receives 250-400 cm of rain and the leeward Deccan plateau just behind it, 60-80 cm. This is the rain-shadow effect. The Aravalli range runs south-west to north-east, parallel to the Arabian Sea winds, so the winds pass along it without rising and Rajasthan stays dry. The Meghalaya plateau faces the funnel of the Bay of Bengal branch, so Mawsynram and Cherrapunji on its southern edge are the wettest places on earth.

The jet streams are the seventh factor and belong to the upper air. They are narrow bands of very fast westerly wind, 150-300 km/h, at 9-12 km height. In winter the sub-tropical westerly jet lies over northern India, south of the Himalayas; it brings the western disturbances from the Mediterranean and keeps the north-west dry and cool. In late May or early June, as the Tibetan plateau heats up, the jet shifts north of the Himalayas; its withdrawal allows the low pressure of the plains to deepen and the monsoon to burst. At the same time the tropical easterly jet establishes itself over the peninsula at about 15°N and helps the monsoon to spread. The retreat of the easterly jet and the return of the westerly jet in October end the rainy season.

El Nino is an occasional warming of the surface of the eastern Pacific Ocean off the coast of Peru. In an El Nino year the pressure over the Indian Ocean rises and the monsoon tends to be weak or late; the years 1987, 2002, 2009 and 2015 were drought years linked to El Nino. The opposite condition, La Nina, is linked to strong monsoons. The relationship between the Pacific and the Indian Ocean pressure is called the Southern Oscillation, and together the two are called ENSO.

The classical explanation of the monsoon treats it as a giant land and sea breeze. Land heats faster than water. From March the sun moves north of the equator and the plains of northern India, Rajasthan and Punjab heat intensely; by May a strong low-pressure centre forms over the north-west, sometimes called the monsoon trough. The Indian Ocean to the south is cooler and has higher pressure. Air flows from high to low pressure, so moist winds are drawn from the ocean towards the land: this is the south-west monsoon. In winter the situation reverses: the land cools, high pressure forms over the northern plains, and dry winds blow outward from the land to the sea as the north-east monsoon.

The modern explanation adds three elements to this. First, the Inter-Tropical Convergence Zone (ITCZ), the belt of low pressure where the trade winds of the two hemispheres meet, shifts north with the sun in summer and settles over the Ganga plain at about 20-25°N. The south-east trade winds of the southern hemisphere cross the equator, are deflected to the right by the Coriolis force (Ferrel's law) and become the south-west monsoon. This is why the monsoon comes from the south-west and not simply from the south. Second, the Tibetan plateau, 4,000 m high and 1,000 km wide, is a heat source in summer that strengthens the low pressure aloft and drives the tropical easterly jet, which pulls the monsoon in. Third, the jet streams: the monsoon cannot burst until the sub-tropical westerly jet has moved north of the Himalayas, which usually happens in the first week of June.

The arrival of the monsoon is sudden and is called the burst of the monsoon. It reaches the Kerala coast around 1 June, Mumbai by 10 June, Kolkata by about 7-10 June through the Bay of Bengal, Delhi by the end of June and the whole country by mid-July. The rain comes in spells separated by dry periods called breaks in the monsoon, which occur when the monsoon trough shifts to the foot of the Himalayas. The monsoon retreats from the north-west from early September and leaves the peninsula by early December.

The rain of the monsoon is mainly orographic, produced when the moist winds are forced to rise over the Western Ghats, the Himalayas and the north-eastern hills, though the depressions that move up from the Bay of Bengal add cyclonic rain to the northern plains.

The Indian Meteorological Department divides the year into four seasons: the cold weather season (December to February), the hot weather season (March to May), the south-west monsoon or rainy season (June to September) and the retreating monsoon or post-monsoon season (October and November). Traditional Indian calendars recognise six ritus: grishma, varsha, sharad, hemanta, shishir and vasanta; the four-season scheme is the geographer's grouping of these.

The cold weather season begins in December when the sun is overhead in the southern hemisphere. Temperatures fall from south to north. The mean January temperature is about 25°C at Chennai, 20°C at Kolkata, 14°C at Delhi and 12°C at Amritsar; nights in the north-west plain may fall to 0°C and frost is common in Punjab and Haryana. The Himalayan valleys are snow-bound. The peninsula, being near the sea and the equator, has no real winter: Thiruvananthapuram in January is as warm as in July.

Pressure is high over the northern plains because of the low temperature, and light winds blow outward from this high: north-westerly in the Ganga plain and north-easterly over the Bay of Bengal. These are the north-east monsoon winds. Blowing from the land, they are dry, and most of India has clear skies, low humidity, cool sunny days and cold nights. Rainfall is negligible except in two areas. The Coromandel coast of Tamil Nadu receives its main rain now, because the north-east winds pick up moisture over the Bay of Bengal before reaching the coast; Chennai gets about 60 per cent of its rain in October-December. The north-western plains receive light rain from the western disturbances, shallow low-pressure systems that travel from the Mediterranean Sea across Iran and Afghanistan along the sub-tropical jet. They give Punjab, Haryana and western Uttar Pradesh 2-5 cm of rain in December-February and snow to Kashmir and Himachal. This winter rain, called mahawat, is small in amount but of great value to the rabi crop of wheat, and a local saying values one shower in Magh above a whole crop of manure.

Cold waves sometimes sweep the north when the western disturbance passes, and fog is a feature of the Ganga plain in January, when calm air and moisture from the fields cool at night.

The hot weather season lasts from March to May. As the sun moves north the temperature rises rapidly, and the belt of highest temperature moves north with it: in March the Deccan reaches 38°C, in April Gujarat and Madhya Pradesh 42°C, and in May the north-western plain and Rajasthan 45-48°C. The peninsula stays a little cooler because of the sea on three sides. The daily range is very large in the north-west: 45°C by day and 20°C at night at Jaisalmer.

Pressure falls over the heated land and by May a strong low develops over the north-west, centred on the Thar desert and the Punjab. Winds begin to blow inward. In the north-west a hot, dry, dusty wind called the loo blows during the afternoon from the west and south-west across Punjab, Haryana, Delhi and Uttar Pradesh; exposure to it may cause sunstroke, and it is followed by dust storms called andhi in Uttar Pradesh. These storms bring a temporary fall of temperature and sometimes a little rain.

The pre-monsoon showers of this season have local names. In West Bengal, Assam and Odisha the violent thunderstorms of April-May that come in the late afternoon from the north-west are the Kalbaishakhi ('the calamity of Baisakh'), called Nor'westers by the British. In Kerala and Karnataka light showers in April-May are the mango showers, so named because they help the mango to ripen; in Karnataka the same showers are the cherry blossoms because they help the coffee to flower. The hot season is also the first season of cyclones in the Bay of Bengal; the cyclone of May 2020 (Amphan) struck West Bengal in this season.

The Himalayan hill stations are at their most pleasant now, and the season is the time of the harvest of the rabi wheat in the north and the sowing of the early kharif rice in Bengal. The dryness of the season also makes it the season of forest fires and of the drying of tanks and wells, so that the arrival of the monsoon is awaited with anxiety everywhere.

The rainy season runs from June to September and gives India about 75 per cent of its annual rain. When the monsoon bursts over Kerala the south-west winds are split by the shape of the peninsula into two branches, the Arabian Sea branch and the Bay of Bengal branch. The Arabian Sea branch is the stronger, since it is the more direct continuation of the flow from the Indian Ocean.

The Arabian Sea branch itself divides into three currents. The first current strikes the Western Ghats at right angles. The winds rise sharply up the windward slope and give 250-400 cm of rain to the Konkan and Malabar coasts and the crest of the Ghats: Mumbai receives about 200 cm, Mangaluru 330 cm and Agumbe in Karnataka more than 750 cm. Having crossed the crest, the winds descend to the Deccan plateau, warm up and hold their moisture; the plateau just behind the Ghats is in a rain shadow and receives only 50-70 cm: Pune 70 cm, Bengaluru 90 cm, Solapur 60 cm. Rainfall increases again as the winds approach the Bay of Bengal and pick up more moisture, so Nagpur receives 120 cm.

The second current enters through the Narmada and Tapti valleys, where there is no mountain barrier, and gives rain to the plateau of central India, Madhya Pradesh and eastern Gujarat; it reaches the Chhotanagpur plateau and joins the Bay of Bengal branch. The third current moves north over Kutch and Rajasthan parallel to the Aravalli range. The Aravallis run in the same direction as the wind and do not force it to rise; the winds pass over the hot desert, which heats them and prevents condensation, so western Rajasthan receives less than 25 cm. This is why the Thar is a desert though the monsoon winds pass over it. This current continues to the Punjab and Himachal, where the Himalayas finally make it rise and rain falls at Dharamsala (300 cm) and Shimla.

The distribution of rain on the western side of India is therefore governed by the direction of the hills, and the classic examination contrast is between Mumbai and Pune, or between Mangaluru and Bengaluru, on the two sides of the Western Ghats.

The Bay of Bengal branch moves north-east across the Bay, gathering moisture from its warm surface. It is met by the Arakan Yoma mountains of Myanmar, which deflect the greater part of it westward, so that it enters India from the south-east over the Bengal delta and Bangladesh. This branch too divides into two currents.

The first current moves north into Assam and the north-eastern hills. It is caught in the funnel-shaped valley between the Meghalaya plateau and the Himalayas, is forced to rise up the southern face of the Khasi hills, and yields the heaviest rain in the world: Mawsynram receives about 1,187 cm a year and Cherrapunji (Sohra), 16 km away, about 1,080 cm. The Brahmaputra valley gets 200-300 cm; but Shillong, on the leeward northern side of the same plateau only 50 km from Cherrapunji, gets about 220 cm, a rain-shadow effect in miniature.

The second current turns west and moves up the Ganga plain, guided by the Himalayas on its right. Because it is moving away from its source it becomes drier as it goes, so that rainfall decreases from east to west along the plain: Kolkata 160 cm, Patna 120 cm, Allahabad 100 cm, Lucknow 90 cm, Delhi 65 cm, Amritsar 60 cm, Bikaner 30 cm. The Himalayan foothills get more than the plain at every longitude, since the winds are forced to rise there. Near the Punjab the Bay of Bengal current meets the third current of the Arabian Sea branch and the two together give the western Himalaya its rain.

The rain of the plain comes largely from monsoon depressions, low-pressure systems that form at the head of the Bay of Bengal three or four times a month and travel west-north-west along the monsoon trough, each giving two or three days of heavy rain along its track. When the trough shifts north to the foot of the Himalayas the plain has a break in the monsoon while the hills and the north-east get excessive rain and the Brahmaputra floods. The whole country is under the monsoon by about 15 July. Tamil Nadu is the exception: it lies in the rain shadow of the Arabian Sea branch and parallel to the Bay of Bengal branch, so it receives little rain in June-September and waits for the retreating monsoon.

By September the sun has moved back towards the equator and the low pressure over the north-western plain weakens. The monsoon trough moves south and the south-west monsoon begins to withdraw, first from Rajasthan and Punjab in the first week of September, from the Ganga plain by the end of the month, from central India and Bengal by the middle of October and from the southern peninsula by early December. This period of withdrawal, October and November, is the retreating monsoon or the post-monsoon season.

The weather now is transitional. The sky clears, humidity remains high and the temperature rises again to give the hot, sticky, oppressive weather called October heat: Kolkata averages 32°C by day with 80 per cent humidity. In the second half of October the nights begin to cool, first in the north, and by November the cold season is setting in over the plains, while the peninsula is still warm.

The winds over the Bay of Bengal are now reversing from south-west to north-east. This is the season of tropical cyclones. Deep depressions form over the Andaman Sea and the south Bay of Bengal, intensify into cyclones with winds above 120 km/h, and strike the coasts of Tamil Nadu, Andhra Pradesh, Odisha and West Bengal, and less often the Arabian Sea coast of Gujarat. The Odisha super cyclone of October 1999, Phailin in October 2013 and Hudhud in October 2014 all belong to this season. The cyclones bring torrential rain, storm surges that flood the deltas, and great loss of life and crops.

The retreating monsoon is the main rainy season of the Coromandel coast. The winds blowing from the north-east across the Bay of Bengal pick up moisture and give rain to Tamil Nadu, southern Andhra Pradesh and the interior of the southern peninsula: Chennai receives about 80 cm in October-December out of an annual 140 cm, and Tamil Nadu as a whole gets about half its rain now. This is why Tamil Nadu is called the region of the north-east monsoon. For the rest of the country the retreating monsoon is dry, and this is the season of the harvest of kharif rice in Bengal, the festival of Durga Puja and the sowing of rabi wheat in the north.

India's mean annual rainfall is about 118 cm, but the average hides extremes from below 10 cm to above 1,100 cm. On a rainfall map the country falls into four belts. The regions of heavy rainfall, above 200 cm, are the western slopes of the Western Ghats and the Konkan-Malabar coast, the hills of the north-east including Meghalaya, Assam, Arunachal Pradesh, Nagaland, Manipur, Mizoram and Tripura, the Himalayan foothills from Sikkim to northern West Bengal (Jalpaiguri, Darjeeling foothills), and the Andaman and Nicobar Islands. The regions of moderate rainfall, 100-200 cm, are the Ganga plain from West Bengal to eastern Uttar Pradesh, Bihar, Jharkhand, Odisha, Chhattisgarh, eastern Madhya Pradesh, the northern part of Andhra Pradesh and the eastern Tamil Nadu coast. The regions of low rainfall, 50-100 cm, are western Uttar Pradesh, Punjab, Haryana, eastern Rajasthan, Gujarat, the Deccan plateau of Maharashtra, Karnataka and Telangana, and the interior of Tamil Nadu. The regions of scanty rainfall, below 50 cm, are western Rajasthan, Kutch, the rain shadow of the Ghats around Solapur and Bellary, Ladakh and the cold desert of the Himalaya.

Three rules explain this map. Rainfall decreases from east to west along the northern plain because the Bay of Bengal winds dry out as they travel. Rainfall decreases from west to east across the peninsula because of the rain shadow of the Western Ghats, and then increases again near the Bay of Bengal coast. And rainfall increases with altitude on the windward side of every range and falls sharply on the leeward side.

The variability of rain is as important as its amount. The coefficient of variation is lowest, under 15 per cent, in the wet regions of the north-east and the west coast, and highest, over 50 per cent, in the dry regions of Rajasthan, Gujarat and the rain shadow. The regions that get the least rain are also the ones in which it varies most, and they are the regions of drought: about a third of the country is drought-prone, and the same map shows that the flood-prone regions are the plains of the Ganga and Brahmaputra where the wettest months bring the rivers over their banks.

The monsoon has a set of characteristics that examiners ask for repeatedly. It is seasonal: three-quarters of the rain falls in the four months from June to September, and the remaining eight months are mostly dry. It is uncertain in time: the onset over Kerala has varied from 19 May to 8 June, and the withdrawal from the north-west from 1 to 25 September; a late monsoon delays the sowing of kharif rice and an early withdrawal ruins its ripening. It is uncertain in amount: all-India rainfall in a bad year may be 20 per cent below normal and in a good year 20 per cent above, and the swing is larger in the dry regions. It is unevenly distributed across the country, as the previous topic showed. And it is irregular within the season, coming in active spells and breaks.

A break occurs when the monsoon trough shifts from its normal position over the Ganga plain to the foot of the Himalayas. Then the plains have a week or two of dry weather while the Himalayan rivers rise; the Kosi and the Brahmaputra floods of Bihar and Assam often coincide with breaks over the rest of the country. A break of three weeks in July, when the rice seedlings have just been transplanted, can be as damaging as a failed monsoon.

The consequence of these characteristics is that floods and droughts occur in the same year in different parts of India, and sometimes in the same place in different months. About 40 million hectares are flood-prone, chiefly in the Ganga-Brahmaputra plain, and about 68 per cent of the cropped area is drought-prone to some degree, chiefly in Rajasthan, Gujarat, the Deccan and Bundelkhand. Severe all-India droughts occurred in 1965-66, 1972, 1979, 1987, 2002, 2009 and 2015, and many are linked to El Nino years.

The monsoon rain is also heavy and torrential, falling in bursts of several centimetres an hour; this erodes soil, floods the fields and runs off quickly to the sea, so that much of the water is lost before it can soak in. This is why the storage of monsoon water in tanks, reservoirs and the ground is the central problem of Indian water management, and why the phrase 'the gamble in the monsoon' is used for Indian agriculture.

Beside the great monsoon there are smaller weather systems and local winds that every student of India's climate must know, because they have local names and appear in short-answer questions.

Western disturbances are extra-tropical cyclones that originate over the Mediterranean Sea and the Caspian region and are carried east by the sub-tropical westerly jet stream across Iran, Afghanistan and Pakistan into north-western India between November and March. They bring cloud, 1-5 cm of rain to Punjab, Haryana, Delhi and western Uttar Pradesh and heavy snow to Jammu and Kashmir, Himachal and Uttarakhand. Their passage is followed by a sharp fall of temperature and a cold wave. The rain is of great value to the wheat crop and is the source of the phrase that winter rain is worth its weight in gold. In April and May the disturbances still occasionally reach the north-west and cause the hail that damages the ripening wheat.

The Kalbaishakhi is the pre-monsoon thunderstorm of West Bengal, Assam, Bangladesh and eastern Bihar. In April and May the plain of Bengal heats strongly, moist air from the Bay of Bengal flows in at the surface while dry, cool air from the Chhotanagpur plateau and the north-west lies above it; the unstable column overturns in the afternoon into a towering cumulonimbus, which comes down the Ganga valley from the north-west with a squall of 60-100 km/h, hail, lightning and 2-5 cm of rain in an hour. The storm cools the air, brings the rain that makes the jute and aus rice sowing possible, and is so much a part of the Bengali year that Rabindranath Tagore wrote of it. It is dangerous for boats on the rivers and for the tin roofs of villages.

The loo is the hot, dry, scorching afternoon wind of the north Indian plain in May and June, blowing from the west and south-west from the Thar and Baluchistan; it raises the temperature to 45-48°C and causes heat stroke. The andhi is the dust storm of Uttar Pradesh and Rajasthan that follows it. The mango showers of Kerala and Karnataka and the cherry blossom showers of the Karnataka coffee country are the gentle pre-monsoon rains of the south. The Blossom shower and Kalbaishakhi are together called the pre-monsoon rains, and their total, about 10 per cent of India's annual rain, is not part of the south-west monsoon proper.

West Bengal stretches from the Himalaya at Darjeeling in the north to the Sundarbans at the Bay of Bengal in the south, so it contains within one state most of the variety of India's climate. The Tropic of Cancer passes through the middle of the state (through Nadia, Bardhaman, Bankura and Purulia), and the state has a tropical monsoon climate with a hot, humid summer, a heavy monsoon and a short, mild winter.

The seasons in Bengal are traditionally six, but climatically four. Summer (March-May) is hot: Kolkata averages 35°C by day, and the western districts of Purulia, Bankura and Birbhum, farther from the sea and on the edge of the plateau, reach 42-45°C with a dry westerly wind, while the coast stays around 34°C but very humid. This is the season of the Kalbaishakhi. The monsoon (June-September) bursts over the state in the second week of June; the Bay of Bengal branch enters directly over the delta and rainfall is heavy: Kolkata about 160 cm annually, of which 120 cm falls in these four months, the coastal Sundarbans 180 cm, and the western plateau margin 120-140 cm. Autumn (October-November) is the retreating monsoon, with October heat and the danger of cyclones. Winter (December-February) is pleasant: Kolkata averages 20°C in January, with nights near 12°C, and the western districts near 8°C; the north-west winds are dry and there is almost no rain except a little from western disturbances in the far north.

North Bengal is different. The Darjeeling hills have a temperate climate: Darjeeling at 2,045 m has a June mean of 17°C and a January mean of 6°C, with occasional snow; the Terai and Duars at the foot of the hills (Jalpaiguri, Alipurduar, Siliguri) are the wettest part of the state, receiving 300-400 cm because the Bay of Bengal winds are forced to rise against the Himalaya. Buxa Duar records over 400 cm.

The climate governs the state's agriculture: aman rice is transplanted with the monsoon in July and harvested in November-December; jute is sown after the Kalbaishakhi rain in April; boro rice and potatoes are the winter crops under irrigation; and tea in the Darjeeling hills and Duars depends on the heavy, well-distributed rain and the mist of the hills.

The monsoon is not merely a weather event in India; it is the frame of the whole economy and of the calendar of life. Its influence can be listed under several heads.

Agriculture. About 55 per cent of India's net sown area has no irrigation and depends directly on the rain. The kharif crops, rice, maize, jowar, bajra, cotton, jute, groundnut and soyabean, are sown with the burst of the monsoon in June-July and harvested at its retreat in September-October. The rabi crops, wheat, barley, gram, mustard and linseed, are sown in October-November in the moisture the monsoon has left in the soil and are helped by the winter rain of the western disturbances. The zaid crops of summer, melons, cucumbers and vegetables, are grown on irrigated land between March and June. A good monsoon means a good kharif, full reservoirs for the rabi and a fall in food prices; a failed monsoon means shortage, higher prices, distress among farmers and the import of foodgrains. Even today the monsoon forecast of the Meteorological Department in April is national news.

Water and power. The rivers, tanks and groundwater are recharged by the monsoon; the great multipurpose reservoirs fill in July-September and are drawn down through the year. Hydro-electricity and the irrigation of the rabi crop both depend on a good monsoon. The failure of the monsoon lowers the water table and dries the wells, and in Chennai and other cities the water supply is rationed after a poor north-east monsoon.

Industry, transport and prices. Agro-industries such as sugar, cotton textiles, jute and edible oil depend on the harvest; a poor monsoon reduces their raw material and the purchasing power of the villages that buy manufactured goods. Floods cut roads and railways, and heavy rain slows construction. The price of food, and so the rate of inflation, follows the monsoon.

Society and culture. The festivals of India follow the agricultural calendar the monsoon sets: Onam and Durga Puja after the kharif, Baisakhi and Pongal at the rabi harvest, Holi at the end of winter. Houses in the wet regions have sloping roofs of tile, and in the dry regions flat roofs; the diet is rice in the wet east and south and wheat in the drier north-west. Poetry, music (the raga Megh Malhar) and painting celebrate the rain. The monsoon is thus rightly called the unifying bond of the Indian people.

The Madhyamik examination tests this chapter in three ways: short factual answers, explanatory answers on the mechanism and distribution of the monsoon, and map-pointing. The student should be able to identify a station from a table of monthly temperature and rainfall and to locate rainfall features on an outline map.

To identify a station from its climatic data, look at three things. First, the temperature range: if the difference between the hottest and coldest month is small (under 8°C) and the temperatures are high all year, the station is coastal and southern (Chennai, Mumbai, Kochi); if the range is large (over 20°C), the station is in the interior north (Delhi, Amritsar, Jodhpur). Second, the rainy months: if the rain is concentrated in June-September, it is a south-west monsoon station, which is most of India; if the maximum is in October-December, it is on the Coromandel coast (Chennai); if there is winter rain in December-February with a summer maximum, it is in the north-west (Amritsar, Delhi). Third, the amount: above 200 cm means the west coast or the north-east; below 40 cm means Rajasthan or Ladakh. Mumbai, for instance, shows a range of about 6°C, no rain from November to May, and 200 cm between June and September; Chennai shows a range of about 8°C and its wettest months in October and November.

For map-pointing, the examination regularly asks the student to mark: the wettest place in India (Mawsynram, Meghalaya); the driest region (western Rajasthan, Jaisalmer); the area of the Kalbaishakhi (the Bengal plain); a region of rain shadow (the Deccan plateau east of the Western Ghats); the direction of the south-west monsoon (arrows from the south-west over the Arabian Sea and from the south over the Bay); a region receiving winter rain (Tamil Nadu coast, or Punjab); and the Tropic of Cancer.

Common examination questions ask: why Tamil Nadu gets its rain in winter; why Rajasthan is dry though the Arabian Sea branch passes over it; why Shillong gets less rain than Cherrapunji; why rainfall decreases from Kolkata to Delhi; why Mumbai gets more rain than Pune; what the western disturbances are; and why the monsoon is called uncertain. Each has a two- or three-sentence answer built from the topics above, and the student should practise writing them with the figures.