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Chapter 6: Climate

In the middle of May, Nagpur, a city in the heart of central India, can record a maximum temperature of 45 °C. The ground is hot enough to shimmer. Work stops through the worst of the afternoon, and people stay indoors until the sun drops. Nothing about that heat suggests that relief is anywhere close.

By July, the same city records a maximum of 31 °C. The city has not moved. The sun has not gone away. Fourteen degrees have simply disappeared from the hottest part of the day, and they will come back next April.

Something similar happens on the coast. In Mumbai, a single week of rain can pull the temperature down by about 10 °C.

What changed in those weeks was the direction of the wind. For part of the year the air over India moves out from the land towards the sea, dry and unable to give rain. Then the whole system turns around, and the air arrives from the ocean instead, carrying moisture with it. That reversal is the monsoon, and almost everything in this chapter follows from it — when the rain comes, where it falls heavily, and where it hardly falls at all.

Diagram

Fig. 6.A1 Caption: The same country, two opposite wind systems — the reversal that gives India its monsoon climate. Brief: Two side-by-side outline panels of India showing the full extent of India as officially defined; left panel labelled summer with a low-pressure centre over north-west India, high pressure over the ocean, and arrows pointing from sea to land; right panel labelled winter with high pressure over the northern landmass, low pressure over the Indian Ocean, and arrows pointing from land to sea.

NoteCuriosity Corner

Q1. Why do the winds over India blow from the sea in July but from the land in January? Q2. Mumbai and Pune lie on either side of the same hills, so why does Pune get far less rain? Q3. Why does the Tamil Nadu coast stay dry in July and August, when most of India is at its wettest? Q4. Why is October, a month of clear skies, more uncomfortable than rainy July? By the end of this chapter you will be able to answer each of these from the way India’s winds, pressure and relief work together.

6.1 Regional Variations

India has a tropical monsoon type of climate. It is called tropical because India lies in the tropical belt, the warm zone lying on either side of the equator, where the sun’s rays strike steeply for much of the year. It is called monsoon because its weather is governed by the monsoon winds, which are largely confined to the Tropics, that is, between 20°N and 20°S.

Two features mark this type of climate. Temperatures stay relatively high through most of the year, and the winters are dry.

A Barrier In The North, An Ocean In The South

India does not, however, behave exactly like other tropical countries. The Himalayas in the north and the Indian Ocean in the south give it climatic conditions of its own.

The Himalayan ranges, which run along India’s northern edge, stand between the country and the cold winds of Central Asia and Siberia, the vast landmass lying north of the mountains. Sheltered in this way, northern India takes on a continental climate — the climate of a large landmass, far from the moderating touch of the sea. Its characteristics are the following. (a) Land winds prevail, so the air reaching a place has usually travelled over dry ground rather than over water. (b) The air is dry, because winds that have crossed a landmass carry little moisture with them. (c) The diurnal range of temperature is large, which means the gap between the highest temperature by day and the lowest by night at the same place is wide.

Why Northern India Is Not Colder Himalayas stand across the north → cold Central Asian and Siberian winds are kept out → northern India is shielded but is also left under land winds → air stays dry and the day-to-night gap widens → a continental climate in the north.

The Indian Ocean in the south works the other way. It gives the southern part of the country a hot monsoon climate, one that is far more typical of the tropical zone than of the temperate zone.

Variations Within One Climate

Even though the monsoon type of climate covers the whole country, conditions are not the same everywhere in it. The climatic conditions of Bihar and Uttar Pradesh in the north differ from those of Kerala and Tamil Nadu in the south, yet all of these States still have a monsoon type of climate.

These variations show themselves in several ways. (a) The pattern of winds differs, because a coastal State meets the moisture-bearing winds first while an inland State receives them weakened. (b) Temperature differs, since distance from the sea and height above it change how much a place heats and cools. (c) Rainfall differs, both in how much falls and in where it falls heavily. (d) The rhythm of the seasons differs, so the same season does not begin or end at the same time everywhere. (e) The degree of wetness or dryness differs, which is why one part of the country can be waterlogged while another is parched.

These regional diversities within one broad climate are known as the sub-types of monsoon climate.

Q. If Kerala and Bihar share one type of climate, why does the weather in the two feel so different?

A. Because a climate type is a broad family. Within it, distance from the sea, relief and latitude give each region its own sub-type.

6.2 Distribution Of Temperature

Temperature in India is never one figure. It changes with the seasons, and it changes from place to place and from region to region as well. It also changes at a single place, and even within a single day at that place.

That gives three different scales to keep in mind at once. There is the difference between one part of the country and another, the difference between one season and the next at the same station, and the difference between afternoon and dawn on the same date.

Five Measured Contrasts

The chapter gives five examples of how far these variations can go. (a) Barmer in Rajasthan, in the dry west of the country, may record a temperature of 48°C or 50°C on a June day, while it hardly reaches 22°C at Pahalgam or Gulmarg in Kashmir on the same day. In Dras, near Kargil in the high mountains of the north, the temperature may go down to –40°C during winter. (b) Kerala has a tropical climate with warm and moist air, whereas Punjab, lying inland in the north-west, has a continental climate in which severe heat alternates with severe cold. (c) The temperature touches –40°C in Kargil in the month of December, whereas Kerala records 20°C or 22°C in the same month. (d) The annual range of temperature, which is the gap between the highest and the lowest temperatures a place records through the year, is 3°C along the Malabar Coast, the coastal strip of the south-west, and more than 20°C in the interiors. (e) The difference between day and night temperatures in the Andaman Islands, in the Bay of Bengal, and in Kerala is hardly seven or eight degree celsius, whereas in the Thar desert of western Rajasthan it is between 25°C to 30°C.

Q. Why is the annual range only 3°C on the coast but more than 20°C inland?

A. Water heats and cools slowly, so the sea holds a coastal station steady through the year. Land heats and cools fast, so the interior swings.

Read those five together and a pattern appears. The extremes belong to places far from the sea or high in the mountains, and the steady figures belong to places the sea can reach. The same country therefore holds a station that touches 50°C and a station that falls to –40°C.

Each of these figures, though, is a reading taken at one station. An average worked out for a whole State would smooth all of them away, and the difference between Barmer and the Malabar Coast would disappear into a single middling number.

6.3 Precipitation

Precipitation is water that falls to the ground from the atmosphere, whether as rain, snow or hail. In India it varies in three separate ways at once — in its type, in its amount, and in how it is spread through the year.

The type varies with height. Snowfall occurs in the Himalayas, where the air is cold enough for water to reach the ground frozen, whereas it only rains over the rest of the country.

Four Measured Contrasts

Amount and timing vary even more sharply, and the chapter gives four examples. (a) Cherrapunji and Mawsynram in Meghalaya, in the hills of the north-east, get about 1100cm rain in a year, whereas Jaisalmer in Rajasthan hardly receives 9cm of rainfall in a year. Rainfall in centimetres is the depth of water that would stand on flat ground if none of it drained away, so 1100cm is a column of water eleven metres deep. (b) Tura in Meghalaya gets an amount of rainfall in a single day which is equal to 10 years of rainfall at Jaisalmer in Rajasthan. (c) The Coromandel Coast, the eastern coastal strip of Tamil Nadu, remains dry in the months of July and August, whereas the Ganga delta and the coastal plains of Odisha are hit by strong storms almost every third or fifth day during these months. (d) Most parts of India receive rainfall during June-September, but the coastal areas of Tamil Nadu get rains in the beginning of the winter season.

WC 6.A1 — One Day At Tura Against Ten Years At Jaisalmer The situation: Points (a) and (b) of this section give both figures — Jaisalmer receives about 9cm of rainfall in a year, and Tura receives in one day what Jaisalmer receives in ten years. The working: ten years of Jaisalmer’s rainfall is \(9 \text{ cm} \times 10 = 90 \text{ cm}\), where cm is the depth of rainwater standing on level ground; so a single day at Tura delivers about 90 cm of rain, which is close to a metre of water in twenty-four hours.

The arithmetic turns a striking sentence into a usable idea. A town in the north-east may have to move nearly a metre of water off its slopes in one day, so its problem is drainage, landslides and flooding. A town in the desert receives less in a decade, so its problem is storage, and every centimetre has to be saved. The same rainfall figure that reads as a disaster in one place would be a gift in the other.

Rain Comes In A Short Window

Points (c) and (d) are about timing rather than amount. Most of India is wet only during June to September, and dry for the rest of the year, so a place’s yearly total says nothing about whether water is available in April.

The timing is not the same everywhere either. While the eastern deltas are being struck by storms every few days in July and August, the Coromandel Coast is waiting, and its rain arrives only as winter begins.

6.4 Factors Affecting The Climate Of India

Climate refers to a generalised and composite picture of the average weather conditions spread over a long period, for a given large area. Weather is what a place is doing today; climate is what it usually does, worked out over many years.

That word ‘usually’ is important. A single hot April or a single failed monsoon does not change a region’s climate, because climate is built from a long record and covers a large area.

Ten factors decide what that long record looks like in India. They work together, and at any one place two or three of them are usually doing most of the work.

Latitude And The Two Zones

The Indian subcontinent is divided into two parts by the Tropic of Cancer. The northern portion lies in the temperate zone and the southern portion lies in the tropical zone.

The warm temperate or sub-tropical climate of the northern zone gives it a cold winter season and a hot summer season. The southern tropical climatic zone of India is warmer than the north and does not have a clear-cut winter season.

The position of the sun follows the same divide. The northern zone does not have the mid-day sun almost vertically overhead during any part of the year, while the southern zone has the mid-day sun almost vertically overhead at least twice every year. Rays that arrive steeply deliver more heat to a given patch of ground than rays that arrive at a slant, which is why the south stays warm.

Altitude And Cooling With Height

Temperature decreases as you move to higher altitudes. There is a decrease of 1°C for every 166m rise in height, so the mountains are cooler than the plains standing in the same latitude.

Ooty, a hill station in the Nilgiris in the south, has a much lower temperature than Kochi on the Kerala coast, because of the altitude. Both lie deep in the tropical south, so latitude cannot explain the difference; height can.

Varied Relief

Relief — the shape of the land, its ranges, plateaus and plains — plays an important role in the climatic conditions of India. A range lying across the path of a moist wind forces that wind to rise, and rising air gives up its moisture as rain.

The side of a range that faces the wind is called the windward side and receives the rain, while the sheltered side beyond it is the leeward side and is left dry. The dry belt lying beyond a range in this way is its rain-shadow area. Four examples from India show what relief can do. (a) The Western Ghats get heavy rainfall on the western side because they stand in the way of South West Monsoon winds which come from the Arabian Sea. (b) The Deccan Plateau gets less rainfall, as it lies in the rain-shadow area of the Western Ghats. (c) In Rajasthan, the Aravalli Range is parallel to the direction of the South West Monsoon winds, so it is unable to stop the moisture laden winds, and this makes Rajasthan a dry area. (d) The southern hills of Assam get heavy rainfall but the northern ones do not, because the southern hills force the moisture laden winds to shed their moisture before they proceed northwards.

The Rajasthan case is worth looking at on a map, because a range that fails to catch rain looks no different from one that succeeds until you see which way it runs.

Map

Map 6.1 Caption: No rainfall in Rajasthan. Brief: North-western India with the Thar Desert shaded and the Aravalli Range drawn as a ridge; blue arrows for South West Monsoon winds sweeping in from the Arabian Sea and running alongside the range rather than across it; two numbered labels beside the range reading that the Aravalli Range is parallel to the S.W. monsoon winds and that it is lower in height; where India’s boundary is drawn it must show the full extent of India as officially defined.

The Western Ghats do the opposite job, and two towns on either side of them show it plainly.

Map

Map 6.2 Caption: Mumbai gets more rainfall than Pune. Brief: The western coastal belt with the Western Ghats drawn as a ridge running north to south, Mumbai marked on the coastal side and Pune marked inland, the rain-shadow area shaded east of the ridge; a key distinguishing winds full of moisture from winds without moisture, with arrows running from the Arabian Sea eastwards across the ridge; where India’s boundary is drawn it must show the full extent of India as officially defined.

The same rule works on the other coast as well, but with a different wind and in a different season.

Map

Map 6.3 Caption: Relief and rainfall: the windward side of the Western Ghats receives rainfall from the S-W Monsoon and the windward side of the Eastern Ghats receives rainfall from the N-E Monsoon. Brief: Peninsular India showing the full extent of India as officially defined, with the Western Ghats and Eastern Ghats drawn as ridges, the rain shadow area labelled between them, Mangalore and Chennai marked; solid arrows for S.W. Monsoon winds arriving from the Arabian Sea onto the western side and dashed arrows for N.E. Monsoon winds arriving from the Bay of Bengal onto the eastern side, with windward sides labelled on both ranges.

Map 6.3 Two ranges, two wind directions. Arrows from the Arabian Sea strike the Western Ghats; arrows from the Bay of Bengal strike the Eastern Ghats, so each range has its own windward side in its own season. The shaded belt between them is the rain shadow. The map does not tell you how much rain falls, or in which months — only which side gets it.

Distance From The Sea

Areas in the interiors of India have an extreme type of climate, or continental climate, whereas coastal areas have an equable or maritime climate, in which the difference between summer and winter stays small.

The reason lies in the land breeze and the sea breeze, caused by the differential rate of heating and cooling of land and sea. Land heats up quickly and cools quickly, while water does both slowly, so a daily exchange of air between the two keeps the coastal air from reaching either extreme. These breezes are responsible for the moderate climate in coastal areas.

That benefit does not travel far inland everywhere. In the Peninsula, the Western Ghats prevent the winds from carrying such benefits far inwards, and therefore the moderating influence of the sea is limited to the coastal areas.

The Surrounding Seas

India is flanked by three water bodies, namely, the Indian Ocean, the Bay of Bengal and the Arabian Sea. They have a profound impact on the climate, especially in areas lying close to the sea.

These water bodies act as the major source of moisture to the summer monsoons and bring heavy rainfall to the whole area. It is because of the nearness to the sea that these areas have a moderate climate.

The Monsoon Winds

The monsoon winds have almost a universal effect in bringing summer rains over the whole of South Asia. During summers, winds move quickly into Peninsular India towards the low-pressure system of north west India from the Arabian Sea and Bay of Bengal.

The moisture carried by these south-westerly winds from the Arabian Sea and Bay of Bengal brings rain to the entire subcontinent, leaving Tibet, the high plateau lying north of the Himalayas, dry in the ‘rain-shadow’ of the mountains.

The retreat of these winds is staggered across the country. (a) The south-west monsoon starts retreating by the end of September from Punjab and Uttar Pradesh. (b) It retreats by October from Central India. (c) It retreats by the end of November from South India.

The retreating monsoon brings rain in the eastern coastal region. During winter, dry offshore North-East Monsoon winds blow from a high pressure to a low pressure region which is centred at the head of Bay of Bengal.

These winds originate in Mongolia and north west China, far to the north-east of India. As they blow over a vast landmass, they are cold and dry. These cold dry winds join the Trade Winds to pass over the Bay of Bengal, and there they pick moisture from the Bay of Bengal to bring rain to Tamil Nadu in winters.

Western Disturbances

The weather conditions during winter are generally influenced by the distribution pattern of pressure in Central and West Asia. In the winter season, due to the shifting of the pressure belts, the westerlies blow from 20°N to 50°N latitude.

North India then comes under the influence of the winds and cyclones rising from the Mediterranean Sea. These cyclones bring rain to the Northern Plains and snow in Jammu and Kashmir in India.

The Himalayan Barrier

The Himalayas form a climatic barrier separating the Indian subcontinent from the rest of Asia. The range does two jobs at the same time. (a) It prevents the cold Siberian winds from entering the Indian region, and so stops India from becoming a cold desert. (b) It forces the moisture laden South West Monsoon winds to shed rainfall in India, or else India would be a dry region.

An India Without The Himalayas Remove: the Himalayan wall along the northern edge. Then: cold Siberian air pours south with nothing to stop it, and winters in the north turn bitter; the monsoon winds, no longer forced to rise over anything, carry their moisture onward instead of dropping it. Which tells us: one range is holding the cold out and the rain in at the same time.

Jet Streams

Jet streams are cold fast blowing winds that develop in the upper layers of the atmosphere, far above the winds we feel at the surface. They influence the climate of India.

The westerly jet stream prevails over the North Indian Plains during the winter months, while the easterly jet stream steers the tropical depression over India. These depressions play a significant role in the distribution pattern of the monsoon rainfall in the subcontinent, and the highest rainfall occurs along the track of these depressions.

The El Nino Effect

El-Nino refers to an abnormal warming of surface waters in the equatorial Pacific region, an ocean far to the east of India. It increases the surface temperature of the sea and affects the movement of monsoon winds in the Indian Ocean.

The result reaches India as drier conditions over the subcontinent and a delay in the monsoon. A change in one ocean therefore shows up in the rain gauges of another, which is one of the clearest signs that climate is a system rather than a set of separate local weathers.

A Warm Pacific, A Late Monsoon Surface waters of the equatorial Pacific warm abnormally → sea surface temperature rises → the movement of monsoon winds over the Indian Ocean is disturbed → the monsoon reaches India late → drier conditions over the subcontinent.

6.5 Monsoon Phenomena And Its Mechanism

Monsoon winds are periodic or seasonal winds, which means they blow one way for part of the year and the opposite way for the rest of it. They develop because of the differential heating as well as cooling of the land and the sea — land takes in heat quickly and loses it quickly, while water does both slowly.

Because the two surfaces never warm at the same rate, the air above them is never at the same pressure. Air presses down on the earth with a weight of its own, and warm air, being lighter, rises and leaves lower pressure behind, while cooler and heavier air settles and builds higher pressure. Wind is simply air moving from the high pressure area towards the low.

The monsoon winds are divided into two wind systems — the Summer Monsoon and the Winter Monsoon.

6.5.1 Summer Monsoons

In summer, the land gets heated more than the sea. Hence a centre of low pressure develops on the land. Over the adjoining sea the air is comparatively cool, and a high pressure develops there.

This difference causes the winds to blow from the sea to the land. It is the ‘Summer Monsoon’.

Q. Why does the wind blow from the sea in summer and not from the land?

A. Because the heated land has low pressure and the cooler sea has high pressure, and air always moves from high pressure towards low.

In May, June and July, the plains of the Indian subcontinent are heated by the vertical rays of the sun. The intense heat develops a low pressure. During these months, a high pressure area develops over the Indian Ocean.

So the winds blow from the Indian Ocean northward and north-westward into Asia. As they blow from the sea to the land, they bring heavy rainfall in some parts of the Indian subcontinent, because air that has crossed a warm ocean arrives loaded with moisture.

These summer monsoon winds blow from the south-west, and so they are known as the ‘South-West Summer Monsoon’.

6.5.2 Winter Monsoons

During the winter season, the conditions are the reverse of those in summer. A high pressure develops over a big landmass stretching from Central Asia up to the north-west Indian plains, while at the same time a low pressure zone develops in the Indian Ocean.

As the winds now blow from the land to the sea, they bring cold dry weather. They are incapable of producing rain, since they have crossed dry ground rather than water and carry almost no moisture to give.

There is one exception, and it comes from the sea again. When these winds blow over seas and pass over the adjoining land, they bring some rainfall. The Southern Coromandel Coast, which is Tamil Nadu and the southern tip of Andhra Pradesh, gets rain from the winter monsoons in this way.

The winter monsoon winds blow from the north-east, and so this monsoon is known as the ‘North-East Winter Monsoon’. The mechanisms of the monsoon are taken further later in this chapter, season by season.

Flowchart

Fig. 6.A2 Caption: The monsoon is one turning system, not two separate winds. Nodes: summer sun heats the land faster than the sea → low pressure over the landmass, high pressure over the ocean → winds blow from sea to land as the South-West Summer Monsoon → moist air brings heavy rainfall → the sun’s vertical rays move south and the land cools faster than the sea → high pressure over the landmass, low pressure over the Indian Ocean → winds blow from land to sea as the North-East Winter Monsoon → cold dry weather, with rain only where the winds cross the Bay of Bengal → back to the land heating in summer

Q1. Why do the winds over India blow from the sea in July but from the land in January?

A. Because pressure changes place with the season. In July the heated land holds low pressure and the ocean high, so air moves inland. In January the land is cold and high, the ocean low, so the flow reverses.

6.6 Four Seasons

The monsoon type of climate has a distinct seasonal pattern, marked by a significant change from one season to the other. These changes are clearly visible in the interior parts of the country, where nothing moderates the swing.

The coastal areas behave differently. Because of the influence of the seas they do not experience much variation in temperature, though there does occur variation in the pattern of rainfall. On the coast, therefore, the seasons are told apart by when it rains rather than by how hot it is.

Thus, according to the India Meteorological Department (IMD), the country’s official weather agency, the year may be divided into four principal seasons on the basis of monsoon variations. (a) The Hot and Dry Summer, from March to May. (b) The Hot and Wet or Rainy Season, also called the South-West Monsoon, from June to September. (c) The Retreating South-West Monsoon, also called the Post-monsoon, in October and November. (d) The Cold and Dry Winter Season, also called the North-East Monsoon, from December to February.

Notice that three of the four are named after the monsoon itself — its arrival, its retreat and its opposite. The seasons of India are really four stages of one wind system, and each of the sections that follow takes up one of them in turn.

Flowchart

Fig. 6.A3 Caption: Four seasons, one turning wind system that returns to where it began. Nodes: Hot and Dry Summer, March to May → Hot and Wet Rainy Season or South-West Monsoon, June to September → Retreating South-West Monsoon or Post-monsoon, October to November → Cold and Dry Winter or North-East Monsoon, December to February → back to the Hot and Dry Summer in March

Where One Season Ends Evidence: the IMD keeps long records of temperature, pressure, wind direction and rainfall from stations across India. Reasoning: the year is cut where those records change together — when winds reverse and rain begins or stops. Limit: the dates are averages built from many years, so in any single year the monsoon may arrive early or late.

6.7 Summer

In India, the hot season begins in March and lasts until June. The vertical rays of the sun fall directly over the Tropic of Cancer during this period, which is why the heat builds over the middle of the country rather than at its southern tip.

6.7.1 Temperature

From March to May, due to the apparent northward movement of the path of the sun’s vertical rays, the length of the day increases. A longer day means more hours of heating, and it results in the increase in solar radiation which moves northward over the tropical areas of South Asia. The temperature starts rising.

In most parts of India, temperature ranges between 30°C and 32°C at this time. The highest day temperatures increase as the heat belt moves further north, and in the north-western part of India temperatures around 48°C are not uncommon.

In south India, the hot weather is not as intense as in north India. The moderating influence of the oceans, together with the Peninsular shape of south India, which puts sea on three sides of the land, keeps the temperatures lower than that in north India. The temperatures, therefore, remain between 26°C and 32°C in south India.

Height and nearness to the sea both offer some escape. There is some respite from the heat in the coastal regions due to the influence of the sea, and plateaus and hills are also relatively cool because of the elevation.

The nights tell a different story from the days. The diurnal variation is large, especially in the interior part lying on the west of the subcontinent, where the ground gives up its heat quickly once the sun has set.

6.7.2 Pressure Conditions

As the heat belt shifts, so does the pressure pattern. The warmest area slowly shifts from the Deccan to north west India, and the high temperature in the subcontinent causes a low pressure between the Thar Desert and the Chota Nagpur Plateau, which lies in eastern India across Jharkhand and parts of Odisha and Chhattisgarh.

The seas do not warm at the same pace. The surrounding seas are cooler and develop high pressure conditions in the Indian Ocean, and this difference between land and sea is what will later set the monsoon winds moving.

By the end of May a comparatively high pressure area develops near Cape Comorin, at the southern tip of the peninsula, extending towards the Arabian Sea. The April map below shows this pattern midway through its development.

Map

Map 6.4 Caption: India — temperature, pressure, prevailing winds and cyclones in April. Brief: Political outline of India showing the full extent of India as officially defined, shaded in bands of mean temperature in degrees Celsius with a key from below 22.5 upwards; isobars in millibars at 08.30 IST drawn as thin lines and labelled, with LOW marked over the interior and HIGH marked over the seas; pink arrows for prevailing winds; major cities named; scale bar included.

Map 6.4 The colour bands give mean temperature in degrees Celsius; the thin labelled lines are isobars, joining places of equal air pressure in millibars, the unit in which pressure is measured. Closely drawn isobars mean pressure changes fast. The arrows run from HIGH towards LOW. What it will not tell you is the day’s actual maximum, because a monthly mean smooths every heatwave away.

By June, the inland low pressure system becomes more intense and moves northward, with its centre over the Indus lowlands and Baluchistan in the north-west of the subcontinent. The influx of these winds by mid-June brings about a change in the weather towards the rainy season.

The July map that follows shows how far the pattern has travelled by then, and it is worth comparing the two side by side.

Map

Map 6.5 Caption: India mean temperatures in July. Brief: Political outline of India showing the full extent of India as officially defined, shaded in bands of mean temperature in degrees Celsius with a key from below 25.0 upwards; isobars in millibars at 08.30 IST labelled, with LOW marked over the north-west and HIGH marked to the south; pink arrows for prevailing winds; major cities named; scale bar included.

6.7.3 Storms And Rainfall

The hot-dry weather is marked by weak winds and dryness over the area. Air that is barely moving and carries little moisture cannot give rain.

The low pressure over the Northern Plains, however, draws winds from the surrounding areas and gives rise to thunderstorms with strong dusty winds. These winds cause a shower of rain mostly in Punjab, Haryana and Uttar Pradesh.

6.7.4 Local Winds

Local winds are winds that blow over one region only, and in the hot season India has several. Four kinds are worth knowing. (a) A striking feature of the hot weather season is the strong and dusty winds, called loo, which blow during the day over northern and north-western India. These winds have a temperature range between 45°C and 50°C, which is hot enough to cause heatstrokes. (b) There are occasional tornado-like dust storms in Punjab and Haryana and Uttar Pradesh. (c) Thunderstorms accompanied with strong winds and heavy rainfall occur in Assam and West Bengal. These local winds are known as Kalbaisakhi, which means ‘the calamity of the month of Baisakh’, Baisakh being a spring month of the Indian calendar. In Assam, these storms are known as Bardoli Chheerha. They originate over the Chota Nagpur Plateau and are carried eastward by westerly winds, bringing rainfall in Assam, West Bengal and Odisha. (d) Thunderstorms cause rainfall along the Kerala and Karnataka coasts. The little rainfall even that they bring is important for mango, tea and coffee plants. Such rains are called mango showers, as they help in the early ripening of the mango crop, and they are also called cherry blossoms in Karnataka.

The rainfall from the Kalbaisakhi is quite beneficial for growing jute and rice in West Bengal and tea in Assam. A storm that is dangerous while it lasts is therefore also the reason a crop gets planted on time, and this double character runs through much of Indian climate.

A Storm That Feeds A Crop Intense summer heating over the Chota Nagpur Plateau → thunderstorms build → westerly winds carry them eastward → heavy rain falls over Assam, West Bengal and Odisha → the soil is wet early enough for jute and rice in West Bengal and tea in Assam.

6.8 Monsoon

The South-West Monsoon season is the season of general rainfall, and it lasts from June to September. The differential heating of land and sea during the summer months causes the monsoon winds to drift towards the subcontinent.

How The Monsoon Is Set Off

The large landmass to the north of the Indian Ocean gets intensely heated during April and May. This causes the formation of an intense low pressure in the north-western part of the Indian subcontinent.

Since the pressure in the ocean to the south of the landmass is high, it attracts the South East (S.E.) Trade Winds which prevail in the southern hemisphere. These S.E. Trade winds, after crossing the Equator, are deflected towards the right because of the Coriolis force, which is the turning effect the earth’s rotation has on any wind moving over its surface, and they reach the west coast as the South-West Monsoon.

These winds bring heavy rainfall accompanied by violent thunder and lightning. This sudden violent onset of rainfall in the first week of June is termed the Burst of the Monsoon.

The rain does not always continue once it has started. When the South-West Monsoon fails to bring rainfall for two or more weeks and there is a dry period in the rainy season, it is called the Break of Monsoon. For a farmer who has sown on the strength of the burst, a break is far more serious than a late start.

The first State to receive the monsoon showers is Kerala, and it is also the last to see its withdrawal.

Two Branches, And What The Rain Feels Like

As a result of the tapering topography of peninsular India, which narrows to a point in the south, the South-West Monsoon winds divide into two branches. (a) The Arabian Sea Branch. (b) The Bay of Bengal Branch.

The rains bring down the temperature and hence give some respite from the heat. However, there is relief only as long as it rains, because the relative humidity — the amount of moisture in the air compared with the most it could hold at that temperature — is quite high during July-August. Air already close to saturation cannot take up sweat from the skin, so a wet day at 31°C can feel heavier than a dry day much hotter.

The figures show how much the rain takes off. In Mumbai the temperature drops by 10°C after a week’s rain, and Nagpur sees a maximum of 45°C in May which drops to 31°C in July.

The two monsoons of the year are opposites in almost every respect, as the table sets out.

Table 6.1 Caption: The two monsoons compared, season by season. | South-West Monsoon | North-East Monsoon | |—|—| | These winds blow from June to September. | These winds blow from December to February. | | The direction of these winds is from South-West. | The direction of these winds is from North-East. | | They blow from sea to land and are moisture laden winds. | These winds blow from land to sea. | | They bring heavy rainfall. This is a high temperature and high humidity season. The pressure is low on the land area and high over surrounding water bodies. | Clear skies, pleasant weather with low temperature and low humidity prevails over the Indian region. | | During these months there is rainfall in the whole subcontinent. | When they move over the Bay of Bengal they pick up moisture to bring rainfall only to the Coromandel coast. |

6.8.1 The Arabian Sea Branch

The South-West Monsoon winds originating over the Arabian Sea further split into three branches. (a) One branch is obstructed by the Western Ghats. These winds go up the slopes of the Western Ghats, become cool and bring heavy rains in the windward side of the Sahyadris, which is the other name for this range, and of the Western Coastal Plains. After crossing the Ghats these winds descend, get heated up and thereby the humidity in the winds is reduced, so consequently these winds cause little rainfall east of the Western Ghats, that is, in the rain-shadow area. (b) The second branch strikes the coast north of Mumbai. These winds move further along the Narmada and Tapi river valleys, which run westward across central India, and cause a fair amount of rainfall in Central India. The Chota Nagpur Plateau receives 15cm of rainfall from this branch, after which it enters the Ganga plains and mingles with the Bay of Bengal Branch. (c) The third branch strikes the Saurashtra Peninsula and the Kutch, on the western coast of Gujarat. After that it passes over west Rajasthan and along the Aravallis causing scanty rainfall. The Arabian Sea Branch joins the Bay of Bengal Branch in Punjab and Haryana, and together these two branches cause rainfall in the western Himalayas.

Point (a) is worth reading twice, because it contains the whole mechanism of the rain shadow in two sentences. Air forced up a slope cools and rains; the same air, coming down the far side, warms again and holds on to what little moisture it still has.

Q2. Mumbai and Pune lie on either side of the same hills, so why does Pune get far less rain?

A. Mumbai sits on the windward side of the Western Ghats, where the moist Arabian Sea winds are forced to rise, cool and rain. Pune lies beyond the crest, in the rain shadow, where the same air has descended and warmed.

6.8.2 The Bay Of Bengal Branch

The Bay of Bengal branch is directed towards the coast of Myanmar and part of south-east Bangladesh. However, owing to the presence of the Arakan Hills along the coast of Myanmar, a large part of this branch of the monsoon is deflected towards the Indian subcontinent.

The monsoons therefore enter West Bengal and Bangladesh from the south and south-east instead of the south-westerly direction. Now this branch bifurcates into two branches, one along the Ganga plains and the other along the Brahmaputra valley.

These branches cause widespread rain in the Ganga plains, the Brahmaputra valley and the Garo and Khasi Hills of Meghalaya. Cherrapunji and Mawsynram, located on the crest of the southern range of Khasi Hills, receive the highest average annual rainfall in the world, and the map below shows why their position on the hills matters so much.

Map

Map 6.6 Caption: Cherrapunji is located on the windward side of hills. Brief: North-eastern India and its neighbours, showing the full extent of India as officially defined, with the Garo, Khasi and Jaintia Hills drawn as ridges and Shillong and Cherrapunji marked; blue arrows for South-west Monsoon winds arriving from the Bay of Bengal, the wind-ward side labelled on the southern face of the hills and the rain shadow area labelled beyond the crest.

Map 6.6 The arrows show the wind arriving from the Bay of Bengal; the ridges show what it must climb. Cherrapunji sits on the face that meets the wind, and the rain shadow lies past the crest. The map explains the position, not the amount — it cannot tell you how many centimetres fall there, or that almost all of it comes in a few months.

Relief Decides Where The Rain Falls

The distribution of rainfall received from the South-West Monsoons is largely governed by the relief or topography. Mahabaleshwar, a hill town on the windward side of the Western Ghats, registers a rainfall of over 250cm, whereas Pune on the leeward side of these Ghats receives less than 70cm of rainfall.

The same rule works in the north-east, where the heavy precipitation in the north-eastern states is due to the occurrence of hilly ranges in these states.

Distance from the source of moisture matters as well as relief. There is a decrease in rainfall as one goes from east to west, and during this particular season Kolkata receives about 120cm, Patna 102cm, Allahabad 91cm and Delhi 56cm.

WC 6.A2 — The Rain Thins Out From East To West The situation: four stations on and near the Ganga plains, with their South-West Monsoon rainfall as given in this section — Kolkata about 120cm, Patna 102cm, Allahabad 91cm and Delhi 56cm, all measured as depth of rainwater in centimetres. The working: from Kolkata to Patna the fall is \(120 - 102 = 18 \text{ cm}\); from Patna to Allahabad it is \(102 - 91 = 11 \text{ cm}\); from Allahabad to Delhi it is \(91 - 56 = 35 \text{ cm}\); so across the whole traverse the total fall is \(120 - 56 = 64 \text{ cm}\), and Delhi receives less than half of what Kolkata receives.

The figures show a wind steadily running out of what it carries. The Bay of Bengal branch arrives at Kolkata loaded, and every stretch of land it crosses takes some of that moisture out of it, so by the time it reaches Delhi there is little left to give. The drop is not even, though: the largest single fall comes in the last stretch, which is a reminder that the wind is weakening and moving away from its source at the same time.

Flowchart

Fig. 6.A4 Caption: One monsoon, splitting again and again as the land divides it. Nodes: South-West Monsoon reaches the tapering peninsula → splits into Arabian Sea Branch and Bay of Bengal Branch; Arabian Sea Branch → obstructed by the Western Ghats, heavy rain on the windward side, rain shadow to the east; Arabian Sea Branch → strikes the coast north of Mumbai, moves along the Narmada and Tapi valleys, on to the Chota Nagpur Plateau, then into the Ganga plains; Arabian Sea Branch → strikes Saurashtra and Kutch, crosses west Rajasthan along the Aravallis with scanty rain, joins the Bay of Bengal Branch in Punjab and Haryana → rainfall in the western Himalayas; Bay of Bengal Branch → deflected by the Arakan Hills into West Bengal and Bangladesh → bifurcates into one arm along the Ganga plains and one along the Brahmaputra valley → widespread rain in the Ganga plains, Brahmaputra valley and the Garo and Khasi Hills

The two branches do not arrive on equal terms, and the table sets out why.

Table 6.2 Caption: The two branches of the South-West Monsoon winds compared. | Arabian Sea Branch | Bay of Bengal Branch | |—|—| | It enters Indian landmass after blowing over a vast open sea. During this period it collects lot of moisture. | It travels a shorter distance (a bay) and carries less moisture. | | The Arabian Sea Branch blows over India. | The Bay of Bengal Branch partly enters India; its major part blows over Myanmar and Thailand. | | It hits against the Western Ghats as soon as it blows over the land. | It has to travel a long distance over land before it hits against the mountains. |

The Coast That Stays Dry

One stretch of coast misses this season altogether. The Tamil Nadu coast remains dry during this season, and two reasons work together.

The first is direction. The Tamil Nadu coast is located parallel to the Bay of Bengal branch of the south-west monsoon, so the wind runs alongside it rather than into it, exactly as the Aravallis fail to catch the winds that run parallel to them. The second is relief, because it also lies in the rain-shadow area of the Arabian Sea branch of the south-west monsoon.

Q3. Why does the Tamil Nadu coast stay dry in July and August, when most of India is at its wettest?

A. The Bay of Bengal branch runs parallel to that coast instead of striking it, and the Arabian Sea branch has already dropped its rain on the Western Ghats, leaving Tamil Nadu in the rain shadow.

6.9 Retreating Monsoon

The South-West Monsoon starts retreating from northern India in early October. Hence the months of October and November are known for the retreating monsoon, the season in which the rain-bearing wind system withdraws from the country.

Why The Wind System Withdraws

The reason for this retreat is that the monsoon trough of low pressure over the Ganga plains becomes weaker due to the apparent southward movement of the sun. As the sun’s vertical rays move away, the land is no longer heated hard enough to hold that low, and the low pressure trough is gradually replaced by high pressure.

The retreat of the monsoon is marked by clear skies and a drop in night temperature. The land remains moist, however, after four months of rain.

That combination is uncomfortable. High temperature together with high humidity gives rise to an oppressive weather, commonly known as ‘October heat’, and the day temperature rises further because clear skies let the sun through without interruption.

Map

Map 6.7 Caption: Weather conditions in October. Brief: Political outline of India showing the full extent of India as officially defined, shaded in bands of mean temperature in degrees Celsius with a key from below 22.5 upwards; isobars in millibars at 08.30 IST labelled, with LOW marked over the north-west and again towards the Bay of Bengal, and HIGH marked over the interior and the seas; pink arrows for prevailing winds; major cities named; scale bar included.

The Low Pressure Moves Out To Sea

The low pressure conditions are transferred to the centre of the Bay of Bengal by early November. This shift of the low pressure area is marked by cyclonic depressions which originate over the Andaman Sea, in the eastern part of the Bay of Bengal.

Some of these cyclonic depressions manage to cross the eastern coasts of the southern peninsula, resulting in heavy and widespread rains on the coast of Tamil Nadu and parts of Odisha. These cyclonic storms move from the north-east to the south-west.

Elsewhere the season is a dry one. The retreating monsoons are generally dry except on the coastal areas of Tamil Nadu, Odisha and parts of Karnataka, and the bulk of the rainfall of the Coromandel Coast is derived from depressions and cyclones.

Where they do give rain, they give a great deal of it. Retreating Monsoon winds cause rainfall in some places that is as heavy as the summer Monsoons. The interior parts of the Deccan, though, remain dry, because they lie in the rain-shadow of the Eastern Ghats — the same relief rule as before, working now on a wind that arrives from the opposite side.

Map

Map 6.8 Caption: Retreating monsoons — the low pressure trough shifts, causing the retreat of the monsoon winds. Brief: Two outline maps of India side by side, each showing the full extent of India as officially defined; the left panel titled monsoon winds in September with arrows running inland from the seas across the peninsula, the right panel titled monsoon winds in November with arrows confined to the south-west and running offshore; Tropic of Cancer marked on both, scale 1:36,000,000 stated on both.

Map 6.8 Two panels of the same country, two months apart. Compare only the arrows: in September they cover the whole peninsula, in November they survive in the south-west alone. That shrinking is the retreat. The pair shows direction and extent, not rainfall — a place can still be under an arrow in November and receive nothing.

6.9.1 Tropical Cyclones

There are tropical depressions, which are low-pressure systems in which air spirals inward and rises, originating in the Bay of Bengal, caused by local variations of heat and moisture. They lead to tropical cyclones in November and December.

Such cyclones generally originate in the neighbourhood of the Andaman Islands between 12°N and 17°N, and travel west or north west over the Bay of Bengal.

Whenever they occur, they cause great loss of life and property due to heavy rains on the eastern coastal regions of India. The rain that ends a dry October on the Coromandel Coast and the storm that destroys homes on the same coast are the same weather system, and a warning issued days ahead is what separates the two outcomes.

6.9.2 October Heat

October is marked by a clear cloudless sky, high temperature and high humidity. As explained earlier, this sultry and oppressive weather is referred to as ‘October Heat’.

It is a transition period between the hot rainy season and the cold dry season. By the end of October, or by the beginning of November, fine weather conditions prevail over the rest of the subcontinent.

Q4. Why is October, a month of clear skies, more uncomfortable than rainy July?

A. The rain has stopped but the land is still moist, so humidity stays high. Clear skies then let the day heat up. High temperature with high humidity is what makes October heat oppressive.

6.10 Winter

The cold weather season commences at the end of November and continues till March. The skies are relatively clear with dry weather.

Night temperatures are low, specially in northern India, but the days are pleasantly warm. The mean temperature in the northern plains is below 20°C, and it gets even lower in Himachal Pradesh, Jammu and Kashmir, Punjab and Haryana.

Air Quality The retreating monsoon is characterised by clear skies, high temperature, low wind velocity and negligible rainfall. Wind is what carries smoke and dust away, so when it drops, the pollutants released each day stay near the ground instead of dispersing. This leads to an accumulation of pollutants close to the ground and a worsening of air quality in major cities in the country. The period also coincides with agricultural practices such as stubble burning in north India, which further increases air pollution and can turn large cities like Delhi into toxic gas chambers for weeks at a time.

Map

Map 6.9 Caption: Weather conditions in January. Brief: Political outline of India showing the full extent of India as officially defined, shaded in bands of mean temperature in degrees Celsius with a key running from below 12.5 upwards; isobars in millibars at 08.30 IST labelled, with HIGH marked over the north and LOW marked to the far south; pink arrows for prevailing winds running outward from the landmass; major cities named; scale bar included.

Map 6.9 Read the key first: the cold bands sit in the north, the warm ones in the south, so temperature increases as you move down the map. The arrows leave the land for the sea, which is the winter reversal. What the map cannot show is the night, and January nights in the plains are far colder than these monthly means suggest.

6.10.1 Temperature

In the winter season, January is usually the coldest month, and the temperature remains quite low during the winter months over the Indian subcontinent. Unlike summer, the temperatures decrease from south to north.

The January figures make the gradient plain. The mean maximum temperature for the month of January at Thiruvananthapuram is as high as 31°C, at Calicut 26°C, at Chennai 24°C, at Varanasi about 16°C and at Jammu only 10°C.

The night temperature in the Gangetic Plains varies from 2.5°C to 17.5°C. Temperatures fall below freezing point in the higher reaches of the Himalayas, and the Dras Valley in Kashmir near Kargil records a minimum temperature of –45°C, which makes it the coldest place in India.

The Coldest Place In India Factors: great altitude in the higher Himalayas, snowfall in the surrounding ranges, and a position far inland with no sea to moderate it. The decisive one: altitude, since temperature falls by 1°C for every 166m of rise, and Dras sits high in the mountains. Move it: put the same valley at plain level in Jammu and its winter minimum would climb towards Jammu’s own figures.

The excessive cold in north India during this season is due to the following reasons. (a) In the month of February, the cold winds from the Caspian Sea and Turkmenistan, both lying to the north-west of the subcontinent, bring a cold wave over the northwestern parts of India. (b) Punjab, Haryana and Rajasthan experience continental type of climates, as they are located far away from the sea to experience its moderating influence. (c) The snowfall in the nearby Himalayan ranges creates a cold wave situation, because air passing over fresh snow arrives chilled.

The south is a different country in winter. In Peninsular India, the average temperature lies between 20°C to 25°C, and it does not have any well defined cold weather season.

The coastal areas hardly experience any seasonal change in the distribution pattern of temperature, due to the moderating influence of the sea and proximity to the Equator. For example, the mean maximum temperature for June at Thiruvananthapuram is 29.5°C and it is 31°C for January — the two ends of the year barely a degree and a half apart, and the January figure the higher of the two.

6.10.2 Pressure

The weather in this season is characterised by feeble high pressure conditions over the north-western part of the plain. This is because of the oblique rays of the sun which reach the subcontinent as the sun moves to the Southern Hemisphere, and slanting rays deliver less heat to the ground than vertical ones.

During this season, the North-East Trade Winds prevail over the country. These winds blow from land to sea and hence, for most parts of the country, it is a dry season.

There is one exception, and it is the same one as before. When these winds pass over the Bay of Bengal they pick up some moisture and cause some amount of rainfall on the Coromandel Coast. As these winds blow from north-east to south-west, they are called the North-East Monsoon winds.

6.10.3 Temperate Cyclones (Western Disturbances)

A characteristic feature of the cold weather season is the inflow of depressions from the west and the north-west. These low pressure systems, called the western disturbances, originate in West Asia and the regions near the Mediterranean Sea.

They travel eastwards across Iran and Pakistan and reach India during the winter season. They bring the much-needed winter rains over the plains and snowfall in the mountains.

They are not rare events. On an average, four or five such depressions affect India in each of the winter months, and they are generally active between December and February.

Though the amount of winter rainfall is small, it is of considerable importance for the cultivation of rabi crops, which are sown in winter and harvested in spring. A few centimetres arriving at the right week can matter more than a great deal arriving at the wrong one.

A Mediterranean Low, A North Indian Harvest Low pressure systems form near the Mediterranean Sea → they travel east across Iran and Pakistan → they reach the northern plains between December and February → light winter rain and mountain snowfall follow → the rabi crop gets moisture at the stage it needs it.

The winter storms of the north and the storms of the eastern coast are not the same kind of system at all, and the table sets out how they differ.

Table 6.3 Caption: Tropical and temperate cyclones compared. | Tropical Cyclones | Temperate Cyclones | |—|—| | Tropical cyclones are largely a summer phenomenon. | Temperate cyclones are most intense in the winter season. | | They are generally smaller in size and their shape is more or less circular. | They are bigger in size than the tropical cyclones and are oval in shape. | | Tropical cyclones develop over the seas. | Temperate cyclones develop over continents. | | They develop in the tropical region of Bay of Bengal and influence Indian coastal climate. | They develop in the temperate region of Mediterranean sea. | | They bring heavy rainfall with strong winds over the Coromandel Coast of India. | They bring light to moderate rainfall in the North western part of India under the influence of westerly jet streams. |

6.10.4 Rainfall

Most parts of India do not receive rainfall in the winter season. This is because the winter monsoons have little humidity, and due to anticyclonic circulation on land — air spiralling outward and sinking from a high pressure centre, which prevents the rising motion that rain needs — the possibility of rain from them decreases.

However, there are some areas which do receive rainfall in the winter season. They are as follows. (a) Central parts of India and northern parts of the Peninsula get occasional rainfall in winter. (b) Some weak temperate cyclones from the Mediterranean Sea cause rainfall in Delhi, Haryana, Punjab and western Uttar Pradesh. Though the amount of this rainfall is meagre, it is quite beneficial for the rabi crops. (c) The north-eastern part of India also gets rainfall during the winter months, and Arunachal Pradesh and Assam get rains between 25mm and 50mm during winters. (d) In the months of October and November, the North-East Monsoon, while crossing over the Bay of Bengal, picks up moisture and brings torrential rainfall over the Tamil Nadu coast and the southern tip of Andhra Pradesh.

Map

Map 6.10 Caption: India — winter rainfall. Brief: Outline map of India showing the full extent of India as officially defined, with an index distinguishing areas of heavy precipitation, whether rainfall or snowfall, from wind arrows; hatching over the western Himalayas and the north-east to mark heavy precipitation; red arrows showing winds moving outward from the high pressure area in the north-west across the country and towards the seas; the Himalayas, Khasi Patkai, Naga hills, Arakan Yoma, Aravalli, Western Ghats and Eastern Ghats labelled; high pressure and low pressure marked; scale bar included.

Where A Region’s Rain Comes From

Because different wind systems reach different parts of the country, no single source explains a region’s water. Two regions make the point.

For Punjab and Haryana, the three sources of rainfall are the following. (a) Western disturbances. (b) South-West monsoon winds, Arabian Sea branch. (c) South-West monsoon winds, Bay of Bengal branch.

For the Tamil Nadu Coast, the three sources of rainfall are the following. (a) South-West monsoon winds. (b) North-East monsoon winds. (c) Retreating monsoon winds.

Notice that both lists have three entries but share almost nothing. Punjab and Haryana draw on a summer wind and a winter depression; the Tamil Nadu coast draws on the same monsoon at three different stages of its year.

6.11 Distribution Of Rainfall

Rainfall is not scattered across India at random. Its distribution is determined by the following. (a) The pressure conditions and the direction of the relief features, since a range lying across a moist wind gives rain and a range lying along it does not. (b) The direction of the winds bearing moisture, because only a wind that has crossed water has anything to give. (c) Cyclonic depression, determined by the pressure gradient, which is how sharply pressure changes over distance — the steeper the gradient, the stronger the inflowing winds.

The annual rainfall map draws all three of these influences into one picture.

Map

Map 6.11 Caption: India — annual rainfall. Brief: Outline map of India showing the full extent of India as officially defined, shaded by mean annual rainfall in centimetres with a graded key running from 0–30, 30–60, 60–120, 120–180, 180–240, 240–300, 300–420 and above 420; the Tropic of Cancer marked, the Arabian Sea, Bay of Bengal, Andaman Sea and Indian Ocean labelled; scale bar included.

Map 6.11 The key gives mean annual rainfall in centimetres, each band a range rather than a single value. Trace the darkest shading and you find the west coast strip and the north-east; the palest lies in the north-west. The map gives a yearly total only. It cannot tell you that almost all of that total arrives inside four months.

The Four Rainfall Regions

India can be divided into four rainfall regions on the basis of how much falls in a year. (a) The Heavy Rainfall Region experiences more than 200cm of rainfall annually, and includes the following areas. (i) The windward side of the Western Ghats, like the Western Coastal Plains and the slopes of the Western Ghats. (ii) The Meghalaya Hills, that is the Garo, Khasi and Jaintia hills, the southern slopes of the Eastern Himalayas, Assam, Arunachal Pradesh and West Bengal. (b) The Moderate Rainfall Region receives rainfall ranging from 100cm to 200cm a year, and includes the following areas. (i) The northern parts of Andhra Pradesh and the southern parts of Tamil Nadu. (ii) The Middle Ganga Valley, some portions of the Western Ghats, Eastern Maharashtra, Madhya Pradesh and Odisha. (c) The Low Rainfall Region has rainfall varying from 50 to 100cm, and the areas of low rainfall are the following. (i) Parts of the Deccan Plateau comprising the regions of Karnataka, Andhra Pradesh and Tamil Nadu. (ii) Eastern Rajasthan, Punjab, Haryana and Kashmir. (d) The Scanty Rainfall Region gets the least rainfall, that is, less than 50cm annually, and desert and semi-desert areas are included in this. The following areas come in this region. (i) Southern Punjab, Western Rajasthan and parts of Ladakh. (ii) The rain-shadow regions of the Western Ghats lying in the Deccan Plateau.

The Handover Between Two Monsoons

The retreating monsoon is the withdrawal of the south west monsoon, and it happens in October till the mid-November from the northern and central part of India. This withdrawal occurs due to the change in temperature and pressure conditions.

The North East Monsoon starts after this, when there is a change in the pressure conditions with the apparent movement of the sun. The reversal of the monsoon winds from the south west monsoon to the north east monsoon takes place with this change in temperature and pressure.

During this transition period the coastal areas experience cyclonic conditions, which result in moderate to high rainfall, and tropical cyclones over the Bay of Bengal are more prominent at this time.

The pattern is not fixed for ever. Over the past few years, change in the world climatic pattern has influenced the precipitation pattern in the Indian sub-continent too, along with a rise in global temperature.

6.11.1 Features Of The Rainfall

The main features of the rainfall in India are the following. (a) There is rainfall over three months and the rest of the year is mostly dry, and seventy per cent of the annual rainfall occurs in the rainy season. (b) The rains are mainly of the relief type, so the windward slopes of the mountains get more rainfall than the leeward side. (c) Only a small portion of the rainfall is received from sources other than the monsoon, like cyclonic rainfall and convectional rainfall, the second of which comes from air heated at the surface rising straight up. (d) The quantity and the time of occurrence of rainfall cannot be predicted, as the rainfall is erratic. It is because of this uncertainty that sometimes there are floods and sometimes droughts. (e) India has an agrarian economy dependent on rainfall, and as such rainfall affects the economy of the country.

Points (d) and (e) belong together, and they are the reason this whole chapter matters. A year’s sowing is planned on an expectation, and the expectation is built on an average that any single year is free to ignore.

When The Average Does Not Arrive Rainfall is erratic in quantity and timing → a season delivers far more or far less than expected → floods in one year, drought in another → an economy that depends on rainfall for its farming carries the loss → the effect reaches far beyond the fields that were flooded or left dry.

6.11.2 Interpretation Of Climate Data

A climate graph drawn from climate data shows a location’s average monthly temperature and precipitation over a year. Temperature is typically represented by a line graph and precipitation by a bar graph.

The horizontal axis represents the 12 months, while the two vertical axes display the scales for temperature and precipitation. Two different quantities are therefore plotted on one drawing, which is why each has its own scale on its own side.

Temperature, shown as a line graph, indicates the average temperature for each month, revealing seasonal patterns and the overall temperature range.

Precipitation, displayed as a bar graph, shows the average amount of precipitation, whether rain or snow, for each month. Combined with temperature, this gives us information about the climatic conditions as well as the location of the place in India.

Four cities are given below, and read together they cover much of what this chapter has described.

Graph

Fig. 6.1 Caption: Climate graph for Delhi, showing average monthly temperature and rainfall. Brief: Combined climograph as printed in this chapter for Delhi; x-axis the twelve months from January to December; left y-axis average temperature in °C running to 40; right y-axis rainfall in mm running to 260; rainfall plotted as blue bars, average temperature as a smooth line; index box naming the two series; data exactly as printed in the chapter’s Delhi climate graph.

Fig. 6.1 Two scales, one drawing. Bars are rainfall in mm, read on the right; the line is average temperature in °C, read on the left. The tall bars stand in July and August, and the line peaks earlier, in the hot months before the rain. It shows averages only — an unusually dry July would not appear anywhere on it.

Graph

Fig. 6.2 Caption: Climate graph for Chennai, showing average monthly temperature and rainfall. Brief: Combined climograph as printed in this chapter for Chennai; x-axis the twelve months; left y-axis average temperature in °C running to 40; right y-axis rainfall in mm running to 340; rainfall as blue bars peaking towards the end of the year, average temperature as a smooth line; index box naming the two series; data exactly as printed in the chapter’s Chennai climate graph.

Graph

Fig. 6.3 Caption: Climate graph for Kozhikode, showing average monthly temperature and rainfall. Brief: Combined climograph as printed in this chapter for Kozhikode; x-axis the twelve months; left y-axis average temperature in °C running to 40; right y-axis rainfall in mm running to 800; rainfall as tall blue bars in the middle of the year, average temperature as a nearly flat line; index box naming the two series; data exactly as printed in the chapter’s Kozhikode climate graph.

Graph

Fig. 6.4 Caption: Climate graph for Cherrapunji, showing average monthly temperature and rainfall. Brief: Combined climograph as printed in this chapter for Cherrapunji; x-axis the twelve months; left y-axis average temperature in °C running to 25; right y-axis rainfall in mm running to 3500; rainfall as very tall blue bars from May to August, average temperature as a low arched line; index box naming the two series; data exactly as printed in the chapter’s Cherrapunji climate graph.

Compare the four and the geography of the chapter appears in the shape of the bars alone. Delhi and Cherrapunji are wet in the middle of the year, when the south-west monsoon is over the country. Chennai’s bars stand late in the year instead, because its rain arrives with the retreating monsoon and the north-east monsoon. Kozhikode’s flat temperature line is the signature of a coastal station, while Delhi’s steep one belongs to the interior.

6.12 Summary

  1. India has a tropical monsoon type of climate, because it lies in the tropical belt and its weather is governed by the monsoon winds, which are largely confined to the Tropics, between 20°N and 20°S. Its main characteristics are relatively high temperatures and dry winters.
  2. The Himalayas in the north and the Indian Ocean in the south give India climatic conditions of its own.
    1. The Himalayan ranges keep out the cold winds of Central Asia and Siberia and leave northern India with a continental climate, marked by land winds, dry air and a large diurnal range of temperature.
    2. The Indian Ocean gives the south a hot monsoon climate, more typical of the tropical zone than of the temperate zone.
  3. Within this one climate there are regional variations, seen in the pattern of winds, temperature and rainfall, the rhythm of the seasons and the degree of wetness or dryness. These are the sub-types of monsoon climate.
  4. Temperature varies enormously across the country and within a single day at one place. Barmer may record 48°C or 50°C on a June day while Dras near Kargil falls to –40°C in winter, and the annual range is 3°C along the Malabar Coast but more than 20°C in the interiors.
  5. Precipitation varies in type, amount and seasonal distribution. Cherrapunji and Mawsynram get about 1100cm of rain a year while Jaisalmer hardly receives 9cm, and snowfall occurs only in the Himalayas.
  6. Ten factors affect the climate of India, and at any one place two or three of them do most of the work.
    1. Latitude, since the Tropic of Cancer divides the subcontinent into a temperate northern zone and a tropical southern zone.
    2. Altitude, since temperature decreases by 1°C for every 166m rise in height.
    3. Varied relief, which decides which slope is watered and which is left in the rain shadow.
    4. Distance from the sea, which separates continental interiors from equable maritime coasts.
    5. The influence of the surrounding seas, which supply moisture to the summer monsoons.
    6. The monsoon winds themselves, which bring the summer rains and retreat in stages between September and November.
    7. Western disturbances, which bring winter rain to the Northern Plains and snow in Jammu and Kashmir.
    8. The Himalayas, which keep the cold out and force the monsoon winds to shed their rain over India.
    9. Jet streams, high fast winds that steer the depressions along whose tracks the highest rainfall occurs.
    10. The El-Nino effect, an abnormal warming of the equatorial Pacific that brings drier conditions and a delayed monsoon.
  7. Monsoon winds are periodic or seasonal winds that develop from the differential heating and cooling of land and sea, and they form two systems.
    1. In summer the heated land holds low pressure and the cooler sea high pressure, so winds blow from sea to land as the South-West Summer Monsoon.
    2. In winter the pattern reverses, and cold dry winds blow from land to sea as the North-East Winter Monsoon, giving rain only where they cross the Bay of Bengal.
  8. The IMD divides the year into four principal seasons: the hot and dry summer from March to May, the rainy south-west monsoon from June to September, the retreating monsoon in October and November, and the cold dry winter from December to February.
  9. In summer the heat belt moves north, temperatures around 48°C are not uncommon in the north-west, and low pressure builds between the Thar Desert and the Chota Nagpur Plateau while the seas stay cool and high.
  10. Local winds mark the hot season, including the loo of northern and north-western India at 45°C to 50°C, the Kalbaisakhi of Assam and West Bengal, and the mango showers of the Kerala and Karnataka coasts.
  11. The south-west monsoon bursts in the first week of June and divides into two branches because of the tapering shape of the peninsula.
    1. The Arabian Sea branch splits three ways, watering the windward Western Ghats, the Narmada and Tapi valleys and central India, and Saurashtra and Kutch.
    2. The Bay of Bengal branch is deflected by the Arakan Hills into West Bengal, then bifurcates along the Ganga plains and the Brahmaputra valley.
  12. Relief and distance govern how much rain falls where. Mahabaleshwar receives over 250cm while Pune receives less than 70cm, and rainfall falls from about 120cm at Kolkata to 56cm at Delhi as one moves west.
  13. The retreating monsoon of October and November brings clear skies, October heat, and cyclonic depressions that give the Coromandel Coast the bulk of its rainfall.
  14. In winter, January is the coldest month, temperatures decrease from south to north, and western disturbances bring the small but valuable rainfall on which rabi crops depend.
  15. India has four rainfall regions — heavy above 200cm, moderate between 100cm and 200cm, low between 50 and 100cm, and scanty below 50cm.
  16. Seventy per cent of the annual rainfall falls in the rainy season, the rains are mainly of the relief type, and their quantity and timing are erratic, which is why floods and droughts both occur in a country whose economy depends on rainfall.
  17. A climate graph plots average monthly temperature as a line and average monthly precipitation as bars, on twelve months across the horizontal axis and two separate vertical scales, and its shape alone reveals where in India a station lies.

Flowchart

Fig. 6.A5 Caption: The whole chapter in one chain — heating drives pressure, pressure drives wind, and wind decides the rain. Nodes: the sun’s vertical rays move north → land heats faster than the sea → low pressure over the north-west, high pressure over the oceans → South-West Monsoon blows from sea to land, bursting in June → the peninsula splits it into the Arabian Sea branch and the Bay of Bengal branch → relief decides where it rains: windward slopes heavy, rain-shadow areas scanty → the sun’s rays move south, the trough weakens and is replaced by high pressure → the monsoon retreats through October and November, with cyclonic depressions watering the Coromandel Coast → land cools faster than the sea, high pressure over the landmass → North-East Monsoon blows from land to sea, dry except where it crosses the Bay of Bengal, with western disturbances bringing winter rain to the north → back to the land heating in March

What This Chapter Cannot Tell You

Almost every figure here is a mean. A monthly mean temperature hides the afternoon that reached 48°C, and an annual rainfall total hides the fact that most of it arrived inside four months, or in a single week of that season.

The maps carry the same limit at a different scale. A national rainfall map paints a whole district in one colour, so the wet windward valley and the dry village a few kilometres beyond the crest disappear into the same band.

The four-season calendar is an average too. The chapter itself notes that El Nino can delay the monsoon and that the world’s changing climatic pattern is already affecting precipitation in the subcontinent — how far those dates and totals will shift is not something the data in this chapter can answer. ## 6.13 Check Your Concepts

  1. Look at the climate graphs for Chennai (Fig. 6.2) and Kozhikode (Fig. 6.3).
    1. In which months do the tallest rainfall bars stand on each graph?
    2. Using the difference between the two, say which station gets its rain from the south-west monsoon and which from the retreating and north-east monsoon, and give your reason.
  2. The chapter gives the mean maximum temperature at Thiruvananthapuram as 29.5°C for June and 31°C for January. Work out the difference between the two figures. Compare your answer with the annual range of 3°C given for the Malabar Coast and with the range of more than 20°C given for the interiors, and explain what this tells you about the influence of the sea.
  3. Map 6.1 shows the Aravalli Range and Map 6.3 shows the Western Ghats, and both ranges stand in a region reached by the South-West Monsoon. Explain why one of them causes heavy rainfall while the other leaves its region dry.
  4. The south-west monsoon does not withdraw from the whole country at once.
    1. Arrange the three stages of its retreat in the order in which they happen, naming the regions and the months.
    2. Explain what change in pressure over the Ganga plains causes this retreat to begin.
  5. Give a geographical reason for each of the following.
    1. Pune receives less rainfall than Mumbai.
    2. Tropical cyclones bring heavy rainfall but temperate cyclones bring light rainfall.
    3. Kerala has a tropical climate while Bihar has a continental climate.
  6. Distinguish between ‘Burst of Monsoon’ and ‘Break of Monsoon’.
  7. Most parts of India do not receive rainfall in the winter season. Why? Name two regions that receive winter rainfall and identify each region’s source.
  8. On an outline map of India:
    1. Colour the areas which receive less than 30 cm rainfall in a year.
    2. Write L over the region which is the last to receive the South-West Monsoon.
    3. Show by arrows the direction of the rain bearing winds during winter and summer.
    4. Mark the wind direction in the Ganga basin and on the Tamil Nadu coast in the month of January.
    5. Shade an area which receives rain in January and February.
    6. Shade and name the regions of India which experience cyclonic rain and convectional rain.
    7. Mark dots on the areas which are affected by the loo.
    8. Write HAR in an area having a high annual range of temperature and LAR in an area having a low annual range of temperature.
    9. Shade, in different colours or patterns, the three sources of rain in Punjab.
    10. Mark and name the mountain range and the direction of rain-bearing winds, as a result of both of which Rajasthan gets scanty rainfall.

6.14 Case-Based Questions

  1. Study the climatic data given below and answer the questions that follow.
Month Jan Feb Mar Apr May June July Aug Sep Oct Nov Dec
Temperature (°C) 23.1 24.8 26.5 29.3 32 32.8 33.1 32.1 30.5 29.3 28.7 26.1
Rainfall (cm) 15.3 10.1 0.3 0.1 1.3 4.5 6.1 10.2 10.5 20.1 16.8 19.0
  1. Calculate the annual rainfall experienced by the station.
  2. What can you conclude about the climate of this station — is it maritime or continental? How did you come to this conclusion?
  3. Name the season during which rainfall is the heaviest. Name a probable station satisfying this criteria.
  1. At Lucknow, during the month of May, the hot dry season is characterised by a local wind which increases the temperature for the city and keeps most people indoors during the afternoon.
    1. What type of local wind is discussed?
    2. How is it responsible for the climatic condition prevailing in the city?