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Chapter 5: Climate

Every year, from about the middle of May, forecasters at the India Meteorological Department watch the southern seas closely. They are waiting for a single event that will shape the year for hundreds of millions of people: the onset of the monsoon.

For most of the year, the surface winds over India blow outward — from the dry interior toward the sea. Then, over just a few days at the start of June, they flip. Moist air from the Arabian Sea and the Bay of Bengal comes rushing onto the land, and the first heavy rains break over the coast of Kerala. The arrival is so sudden that it has a name of its own — the ‘burst’ of the monsoon.

The rain does not stop at Kerala. A ragged rain-line sweeps northward: Mumbai around the tenth of June, then the Gangetic plain, until by the middle of July the whole subcontinent lies under monsoon skies. Temperatures fall, rivers rise, and fields that were cracked and brown turn green within weeks.

The word monsoon comes from the Arabic mausim, meaning season. That is the heart of the matter. India’s climate is not a single kind of weather but a rhythm — a yearly reversal of the winds that arrives, more or less, on schedule, and then, months later, retreats the way it came.

Diagram

Fig. 5.0-1 Caption: The monsoon is a reversal: in January the winds carry dry air off the land, in July they carry moist air onto it. Brief: Two side-by-side outline maps of India, showing the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh; standard international boundaries. Left panel labelled ‘January’: mark a high-pressure cell over north-west India and the interior; draw bold arrows blowing from the land outward to the sea (north-east to south-west over the peninsula, offshore along both coasts) — the retreating north-east winds. Right panel labelled ‘July’: mark a low-pressure trough over north-west India; draw bold arrows blowing from the sea inward onto the land (south-west to north-east) — the advancing south-west monsoon. Show the Tropic of Cancer in both. Emphasise that the arrow directions are opposite between the two panels.

NoteCuriosity Corner

Q1. Why do the winds over India reverse direction almost completely between winter and summer? Q2. The monsoon reaches Kerala first and Punjab last — so why does it also leave Punjab first and Kerala’s coast last? Q3. Kerala and Tamil Nadu are neighbours, yet one gets its heaviest rain in June and the other in November. Why? Q4. Why do the same monsoon rains that flood the Ganga plain leave western Rajasthan a near-desert? Q5. If the monsoon waters almost the whole country, why does India still face droughts and floods in the very same year?

By the end of this chapter you will be able to answer each of these — not by memory, but by following the winds.

5.1 Factors Affecting India’s Climate

Weather is the state of the atmosphere at one place at one moment — today’s temperature, wind and rain. Climate is the average of that weather, taken over many years, together with its seasonal rhythm. Weather is what you dress for in the morning; climate is what a place is like.

India’s climate is described in one word — monsoon. Yet within that single word lies enormous variety. Leh sits in a cold high-altitude desert; the coast of Kerala is warm and drenched; western Rajasthan is hot and dry. The same country holds all of these. Our task is to understand both the unity and the variety, and both come from a small set of controlling factors.

These factors fall into two families. One family fixes temperature — how hot or cold a place is: its latitude, its distance from the sea, and its relief. The other family fixes the winds and the rain: the great mountain wall, the shape of the land, and above all the monsoon winds, which are themselves driven by upper-air currents and ocean conditions. This section deals with the geographical controls. The atmospheric machinery that runs the monsoon is taken up in the next section.

We can see most of these controls on a single map.

Map

Map 5.1 Caption: The geographical controls on India’s climate — its latitude, its three seas, and its northern wall of mountains. Brief: Outline map showing the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh; standard international boundaries. Mark and label: the Tropic of Cancer across the middle; the latitude limits of the mainland (about 8°N in the south, about 37°N in the north); the Arabian Sea, the Bay of Bengal and the Indian Ocean around the peninsula; the arc of the Himalayan and adjoining ranges from the north-west to the north-east, shown as a bold mountain barrier; the Western Ghats and the Aravali range. Do not show rainfall or temperature values — this map shows the controls, not the outcome.

Map 5.1 The Tropic of Cancer marks where the noon sun is overhead in June; the mountain shading marks the barrier wall; the seas mark the moisture sources. Together they explain the broad pattern. But this is a country-scale map: it cannot show a single hill range or a local rain shadow, and it shows the causes of climate, not the rainfall or temperature they produce.

5.1.1 Location and Latitudinal Extent

Latitude is the angular distance of a place north or south of the equator; it decides how steeply the sun’s rays strike the ground. The mainland of India stretches roughly from 8°N in the far south to 37°N in the north, with the Tropic of Cancer, at about 23½°N, running across its middle.

This spread matters. The southern parts lie close to the equator, so the sun’s rays fall almost vertically all year and temperatures stay high. Insolation — the incoming solar radiation a place receives — is strong and steady there. The northern parts lie in the warm temperate belt, where the rays are more slanting, especially in winter, and real cold sets in.

So a single control, latitude, already splits the country in two: a tropical south that is warm the year round, and a subtropical north that has a genuine winter. This is the first source of India’s climatic variety.

5.1.2 Distance from the Sea

The sea is a giant store of heat that warms and cools far more slowly than the land. A place near the coast therefore has its temperatures smoothed by the water; a place deep inland does not.

Peninsular India is surrounded on three sides by the Arabian Sea, the Bay of Bengal and the Indian Ocean. Its coastal belts have a maritime climate — equable, with only a small difference between the hottest and coldest months. The interior of the country is cut off from this moderating touch and has a continental climate — extreme, with scorching summers and cold winters.

The gap can be striking. A coastal station may vary by only a few degrees across the whole year, while an interior station can swing through twenty degrees or more. We will read exactly this contrast off the temperature graphs later in the chapter.

5.1.3 The Northern Mountain Ranges

Along India’s northern edge runs a wall of mountains — the Himalayas and their neighbouring ranges — sweeping from Kashmir in the north-west to Arunachal Pradesh in the north-east. This wall acts as a climatic divide: a barrier that separates the climate of the subcontinent from that of Central Asia.

It works in two ways. In winter it blocks the bitterly cold, dry winds that pour off the interior of Asia, so northern India stays milder than its latitude alone would allow. Places far away on the same latitude, deep inside continents, are far colder.

In summer it works the other way. It stands across the path of the rain-bearing south-west monsoon and stops the moist air from escaping northward. The winds are forced to rise, cool and shed their rain over India instead of over Tibet. The wall keeps the warmth in during winter and the rain in during summer.

An Earth Without The Himalayan Wall Remove: the northern mountain barrier. Then: cold, dry Central Asian air would sweep unchecked into the plains, and north-Indian winters would turn far colder; and the moist monsoon air, no longer trapped, would carry much of its rain away over the north. Which tells us: the wall does two jobs at once — it warms India’s winters and it holds back the summer rain.

5.1.4 Physiography

Physiography — the arrangement of highlands and lowlands — steers the winds and decides where their moisture is released. Where moist air is forced to climb a slope, it cools, the water vapour in it condenses, and rain falls; this is orographic rainfall. The side of the hill facing the wind, the windward side, gets soaked, while the sheltered leeward side lies in a rain shadow — a belt of dry air that has already lost its moisture.

Diagram

Fig. 5.1-1 Caption: Air forced up a mountain rains on the windward slope and descends dry on the leeward side, leaving a rain shadow. Brief: Cross-section of a mountain range with the sea on the left. Draw a bold arrow of moist air moving from the sea, rising up the windward (left) slope; show clouds and heavy rain on that slope. Over the crest and down the leeward (right) slope, show the arrow descending, the clouds thinning, and a dry rain-shadow zone. Label ‘windward slope’, ‘leeward slope’ and ‘rain shadow’, and show the direction of air movement with arrows pointing up the windward side and down the leeward side.

India offers textbook examples. The Western Ghats stand square across the Arabian Sea winds, so their western slopes and the coastal plain are drenched, while the Deccan interior just behind them stays comparatively dry. In the north-east, the funnel-shaped hills of Meghalaya force the Bay of Bengal winds sharply upward and produce some of the heaviest rainfall found anywhere on Earth.

Relief also splits and directs the winds. The Bay of Bengal branch of the monsoon is divided by the lie of the land into one stream that runs up the Ganga valley and another that runs up the Brahmaputra valley. Physiography does not merely receive the winds; it organises them.

Q4. Why do the same monsoon rains that flood the Ganga plain leave western Rajasthan a near-desert?

A. The Aravali range runs parallel to the Arabian Sea current, so it gives the winds no slope to climb and forces out no rain. And by the time the Bay of Bengal branch reaches the far west, its moisture is spent. Rajasthan lies off the path of both.

5.1.5 The Monsoon Winds

The master control over India’s climate is the monsoon wind system — the winds that reverse their direction with the season, which we first met in the opener (Fig. 5.0-1). This reversal is so complete, and its effect on rainfall so decisive, that India’s climate is called the monsoon type.

In summer the south-west monsoon blows from the sea onto the land. Having crossed warm water, it is heavy with moisture, and it delivers about four-fifths of India’s total annual rainfall between June and September. This is the rainy season for most of the country.

In winter the flow reverses. The north-east monsoon blows from the land toward the sea. Because it starts over dry land it brings little rain — except where it crosses the Bay of Bengal and picks up moisture, wetting the Coromandel coast of Tamil Nadu while the rest of India stays dry.

These winds are the immediate cause of India’s rhythm of wet and dry seasons. But what makes them reverse so faithfully, year after year? That question takes us from the geography of the land to the machinery of the atmosphere, which is the subject of the next section.

5.2 The Monsoon: Concept and Mechanism

5.2.1 The Meaning of Monsoon

A monsoon is a wind system in the tropics whose direction reverses completely between the summer and the winter. The word comes from the Arabic mausim, meaning season, and that is exactly what it names — a wind that changes with the season.

Under this system the winds blow from the land to the sea in winter and from the sea to the land in summer. Because the sea-bound summer winds are moist and the land-bound winter winds are dry, most of the rain in a monsoon land falls in the summer, while the winter stays largely rainless.

That single fact — rain concentrated into one season and withheld in another — shapes farming, water supply and daily life across India. To understand why the winds reverse at all, we have to look at what drives them.

5.2.2 Two Explanations of the Monsoon

The oldest explanation, put forward by the British scientist Edmund Halley, rests on the different way land and sea heat up. In summer the land grows hotter than the sea; a low-pressure area forms over the land and draws in moist winds from the neighbouring ocean. In winter the land cools faster than the sea; high pressure builds over the land and pushes dry winds outward toward the ocean. The monsoon, in this view, is simply a giant land-and-sea breeze.

This thermal idea is true as far as it goes, but the German meteorologist Hermann Flohn argued that the difference in heating is too small, by itself, to reverse winds across a whole continent. He explained the monsoon instead as part of the yearly migration of the planet’s pressure and wind belts, which shift with the overhead sun.

Central to this is the Inter-Tropical Convergence Zone (ITCZ) — a belt of low pressure near the equator where the trade winds of the two hemispheres meet and rise. The trade winds are the steady tropical surface winds that blow toward this belt. As the vertical rays of the sun move north over the Tropic of Cancer in summer, the ITCZ is dragged north with them, and settles as a trough over the northern plains of India.

This northward pull is decisive. The south-east trade winds of the southern hemisphere are drawn across the equator toward the low-pressure trough. As they cross into the northern hemisphere they are bent to the right by the Coriolis force — the deflection that the Earth’s rotation gives to any moving air, to the right in the northern hemisphere and to the left in the southern. Turned right, the south-east winds arrive over India as the moist south-west monsoon.

In winter the whole arrangement slides south. The sun’s rays retreat toward the Tropic of Capricorn, the ITCZ moves south of India, high pressure settles over the cooled northern plains, and the winds blow outward from the land as the dry north-east monsoon.

Flowchart

Fig. 5.2-1 Caption: The monsoon is the seasonal migration of pressure and winds that follows the overhead sun. Nodes: Sun overhead near the Tropic of Cancer (June) → north-west India and the interior heat intensely → a strong thermal low forms over north-west India → the ITCZ shifts north over India → south-east trade winds of the southern hemisphere are drawn across the equator → Coriolis force deflects them to the right → they arrive as the moist south-west monsoon → crossing warm seas they gather moisture → forced up over the land and hills they shed rain over most of India → (as the sun retreats south in January) land cools, high pressure builds over north-west India, the ITCZ moves south → winds blow outward as the dry north-east monsoon → back to the summer flow when the sun returns north.

Q1. Why do the winds over India reverse direction almost completely between winter and summer?

A. Because the overhead sun migrates. In summer it stands over the Tropic of Cancer; the land heats, a low forms over the north-west, the ITCZ shifts north, and the southern trades cross the equator to blow onshore. In winter the sun retreats, the land cools, high pressure builds, and the flow reverses outward.

5.2.3 Upper Air Circulation and the Jet Streams

The surface winds are only half the story. High in the atmosphere run narrow ribbons of very fast wind called jet streams, and their behaviour helps explain both the timing and the sudden onset of the monsoon.

In winter a subtropical westerly jet stream blows from west to east about eight to twelve kilometres above the ground. The high wall of the Himalayas and the Tibetan plateau splits it into two branches. The northern branch flows north of Tibet; the southern branch flows south of the Himalayas, along about 25°N. This southern branch steers winter storms into north-west India and helps keep the high pressure that drives the dry north-east monsoon.

In summer the pattern turns over. As the sun moves north, the westerly jet withdraws to the north of the Himalayas, and its retreat clears the way for the monsoon. Meanwhile the intense summer heating of the Tibetan plateau sets up a tropical easterly jet stream, blowing from east to west high over peninsular India at about 15°N. Its arrival is closely tied to the sudden burst of the south-west monsoon.

Diagram

Fig. 5.2-2 Caption: The high-level jet streams reverse with the season — a split westerly jet in winter, an easterly jet in summer. Brief: Two outline maps of India and its northern surroundings, showing the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh. Winter panel: show the subtropical westerly jet flowing west to east, splitting at the Himalayan–Tibetan barrier into a northern branch (north of Tibet) and a southern branch (south of the Himalayas, near 25°N); arrows point west to east. Summer panel: show the westerly jet withdrawn to the north of the Himalayas, and a tropical easterly jet flowing east to west over peninsular India near 15°N; arrows point east to west. Label the branches and the Himalayan barrier; show arrow directions clearly.

Fig. 5.2-2 The arrows show the direction of the wind about ten kilometres up, not at the ground; a westerly jet blows west-to-east, an easterly jet east-to-west. The pattern is the split winter westerly and the summer easterly. But this maps the average position only: the real jet wanders north and south from week to week, and it says nothing about the surface weather below it.

5.2.4 Western Disturbances and Tropical Cyclones

Two kinds of travelling storm bring rain to India outside the main monsoon, one in winter and one at the edges of the monsoon season.

Western disturbances are low-pressure systems that form over the Mediterranean Sea and West Asia and travel eastward on the subtropical westerly jet. On average, four to five of them reach north-west India in each winter month. They bring light but valuable rain and snow to the northern plains and the Western Himalaya, and are usually followed by cold waves. This winter rain, though small in amount, is crucial for the rabi (winter-sown) crops, above all wheat, which are ripening at that time.

Tropical cyclones are intense low-pressure storms that form over warm tropical seas, chiefly the Bay of Bengal. They strike India mainly in the pre-monsoon months of April and May and in the retreating-monsoon months of October and November. In the retreating season they cross the eastern coast and bring heavy, sometimes destructive, rain to Tamil Nadu, Andhra Pradesh and Odisha.

5.2.5 El Niño and the Southern Oscillation

The strength of the monsoon does not depend on India alone. It is tied to conditions far out in the Pacific Ocean, through a linked pair of events known together as the El Niño–Southern Oscillation.

The Southern Oscillation is a seesaw of air pressure between the Indian Ocean and the Pacific Ocean: when pressure is high over one, it is low over the other. When pressure is high over the Pacific and low over the Indian Ocean, the south-west monsoon tends to be strong. When the seesaw tips the other way, the monsoon tends to be weak.

El Niño is the warm partner of this seesaw. It is a periodic warming of the surface waters of the central and eastern equatorial Pacific, which appears off the coast of Peru around December — its name means ‘the child’, after the Christmas season. In an El Niño the eastern Pacific warms by a few degrees above normal, and this shift in the ocean tends to weaken the Indian monsoon. Several of India’s severe drought years, including the deep drought of 1987, occurred in El Niño years. Its cool opposite, La Niña, tends to bring a stronger monsoon.

Linking El Niño To A Weak Monsoon Evidence: decades of sea-surface temperature readings across the Pacific, set beside long records of India’s monsoon rainfall. Reasoning: in most years when the eastern Pacific runs unusually warm, India’s rainfall falls below normal, so the two are statistically linked through the shifting ocean circulation. Limit: the link is a tendency, not a rule — some El Niño years still bring normal rain, because the monsoon answers to many drivers at once.

5.3 The Indian Monsoonal Regime

The monsoon is not a single event but a whole life cycle: it arrives, it delivers its rain in a particular pattern, it pauses, and it withdraws. Following that cycle — the monsoonal regime — is the best way to understand India’s rainy months.

5.3.1 The Onset and Advance of the Monsoon

The south-west monsoon does not creep in gently; it arrives with a sudden downpour called the ‘burst’ of the monsoon. It sets in over the coast of Kerala around the first of June, reaches Mumbai and Kolkata by the middle of June, and spreads across the whole subcontinent by about the middle of July.

Its advance is a rain-line sweeping from the south-east toward the north-west. The north-west of the country is reached last of all — a fact worth remembering, because it explains a good deal about the retreat as well.

The onset brings quick relief from the summer heat. As the moist winds and their clouds arrive, temperatures fall by about five to eight degrees between mid-June and mid-July. The mechanism behind all this — the northward shift of the ITCZ and the switch to the tropical easterly jet — was set out in the previous section.

Map

Map 5.2 Caption: The advance of the south-west monsoon, showing the normal dates on which it reaches different parts of India. Brief: Outline map of India showing the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh; standard international boundaries. Draw isochrones (lines joining places reached on the same date) for the advancing south-west monsoon: about 1 June over Kerala and the far south, mid-June across the peninsula and up to Mumbai and Kolkata, early July over the central and northern plains, and about 15 July over the north-west. Label a few key dates and the Arabian Sea and Bay of Bengal branches with arrows pointing inland (Arabian Sea branch from the south-west, Bay of Bengal branch curving up from the south-east).

Map 5.2 Each line joins places that the monsoon normally reaches on the same date, so the closer the lines, the faster the advance. The pattern is a sweep from the south-east to the north-west. But these are average dates: in any single year the monsoon may arrive a week early or late, and the map cannot show how much rain actually fell once it came.

5.3.2 The Rain-bearing System and Rainfall Distribution

The moisture arrives in two great streams. The monsoon splits into an Arabian Sea branch and a Bay of Bengal branch, and each of these divides again as it meets the land.

Flowchart

Fig. 5.3-1 Caption: The south-west monsoon splits into two branches, each of which divides again as it strikes the coast. Nodes: South-west monsoon → Arabian Sea branch and Bay of Bengal branch. Arabian Sea branch → (a) strikes the Western Ghats: heavy rain on the windward coast, dry rain shadow behind; (b) enters the Narmada–Tapi gap: rain to central India; (c) runs north-east parallel to the Aravalis: little rain over the Thar. Bay of Bengal branch → (a) up the Ganga delta into the funnel-shaped Meghalaya hills: extreme rain; (b) deflected west along the Himalayas up the Ganga plain: rain declining westward.

The eastern stream of the Bay of Bengal branch runs up the Ganga delta and strikes the funnel-shaped hills of Meghalaya. Squeezed and forced upward, the air pours out its moisture. Mawsynram, in these hills, is one of the wettest inhabited places on Earth, receiving over 11,000 mm — more than 1,100 cm — of rain a year, with nearby Cherrapunji almost as wet.

The other stream of the Bay branch is turned westward and travels up the Ganga plain, roughly parallel to the Himalayas. As it moves, it keeps shedding rain, so it grows steadily drier toward the west.

Worked Case 5.1 — Rain Fading Across The Ganga Plain The situation: the Ganga plain from the delta in the east to the neighbourhood of Delhi in the far west. The working: the Bay of Bengal branch enters from the east already loaded with moisture. As it is deflected west along the Himalayas, it releases rain at every step, so each place downstream receives what the places before it left behind. The eastern plain of Bengal and Bihar collects over 100 cm during the monsoon season; the central plain less; and by the time the winds reach the western plain near Delhi they are much weakened, giving well under 60 cm.

This westward fade is not a small detail. It is one reason the wet eastern plain is classic rice country, while the drier western plain leans on wheat and depends heavily on irrigation. The same monsoon feeds both, but hands them very different amounts.

The Tamil Nadu coast is the great exception in these months. It lies in the rain shadow of the Arabian Sea branch, and it runs parallel to the Bay of Bengal branch rather than across it, so the winds slide past instead of climbing. As a result the Coromandel coast stays dry through the very season when the rest of India is soaked — a puzzle whose other half we solve when the monsoon retreats.

Across the country, then, rainfall decreases inland from both coasts and thins out toward the west and the Thar. The heaviest falls come on the windward Western Ghats, the Meghalaya hills and the north-east; the least in western Rajasthan.

5.3.3 Break in the Monsoon

The monsoon rain is not continuous. It comes in wet spells broken by dry ones, and when a dry spell in the rainy season lasts two weeks or more, it is called a break in the monsoon.

A break has two main causes. Either the tropical depressions — the moderate low-pressure systems, weaker than cyclones, that steer the rain up from the Bay of Bengal — fail to form and arrive; or the monsoon trough, which is simply the ITCZ lying as a low-pressure trough over the northern plains, shifts away from its usual position. When the trough swings north toward the Himalayan foothills, the rain follows it, and the plains go dry while the hills get soaked.

Break In The Monsoon Monsoon trough shifts north to the Himalayan foothills → the rain-line leaves the plains → transplanted rice in the plains goes thirsty in mid-season → a long break can cut the harvest even in a year of normal total rainfall.

5.3.4 The Retreat of the Monsoon

By the first week of September the south-west monsoon begins to withdraw from the north-west. It leaves Punjab, Haryana, Rajasthan and Gujarat by about the middle of September, most of the rest of India by the middle of October, and the southern peninsula last of all. Unlike the sudden burst of the advance, the retreat is gradual — the clouds thin, the skies clear, and the rains fade away rather than stop.

The geography of the retreat mirrors the advance in reverse. Because the north-west was reached last and held the monsoon most weakly, it is the first to be given up. Punjab, reached only around the first of July, loses the monsoon by mid-September and so has barely two and a half months of rain. The Coromandel coast, by contrast, keeps monsoon-season conditions for around six and a half months.

Q2. The monsoon reaches Kerala first and Punjab last — so why does it also leave Punjab first and Kerala’s coast last?

A. As the sun retreats south from September, the north-west — reached last and held only weakly — is the first to lose the low pressure that drew the winds in, so the monsoon withdraws there first. The south, reached earliest and held longest by the warm trough, keeps it into December.

Map

Map 5.3 Caption: The withdrawal of the south-west monsoon, showing the normal dates on which it retreats from different parts of India. Brief: Outline map of India showing the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh; standard international boundaries. Draw isochrones for the retreating monsoon: about 1 September from the western strip of Rajasthan, about 15 September from Punjab, Haryana, Rajasthan and Gujarat, mid-October from most of the central and eastern plains, and progressively later dates southward, reaching the Tamil Nadu coast by December. Label a few key dates. Emphasise that withdrawal begins in the north-west and ends in the far south.

5.4 The Rhythm of Seasons

The monsoon divides the Indian year into a clear sequence of seasons. Meteorologists usually recognise four: the winter or cold-weather season, the summer or hot-weather season, the rainy season of the advancing monsoon, and the season of the retreating monsoon. Each has its own temperature, pressure and wind pattern, and its own kind of rain — or lack of it.

5.4.1 The Winter Season

The cold-weather season runs from about mid-November to February, with January the coldest month. In the north the mean daily temperature stays below about 21°C, and at many places the night temperature falls below freezing. Spells of unusually cold weather, called cold waves, sweep the northern plains. The peninsula has no marked winter: temperatures there rarely fall below the low twenties, and the far south stays warm, with January afternoons still near 30°C at places like Thiruvananthapuram.

Map

Map 5.4 Caption: Mean January temperature across India, shown by isotherms in degrees Celsius. Brief: Outline map of India showing the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh. Draw isotherms (lines of equal mean January temperature) running roughly east–west across the north — about 10°C in the far north rising toward 25°C in the south — and bending to run parallel to both coasts in the peninsula. Label a few isotherm values. Show the Tropic of Cancer.

Map 5.4 An isotherm joins places with the same temperature — here, the mean for January. The pattern is a fall from the warm south to the cold north, and in the peninsula the lines bend to run parallel to the coast, showing the sea’s evening-out effect. But a monthly mean hides the daily swing between a warm afternoon and a freezing night, and at country scale it cannot show a single cold hill valley.

Because the north is cold, pressure there is high. Air pressure is measured in millibars (mb); at sea level it is normally about 1013 mb, and in winter the north-west rises to around 1019 mb. The winds therefore blow gently outward from this high toward the low pressure of the south — the dry north-east monsoon.

Little rain falls in winter, but there are three exceptions. The north-east winds pick up moisture over the Bay of Bengal and wet the Tamil Nadu coast, southern Andhra Pradesh and south-east Karnataka. Western disturbances bring light rain and hill snow to the north-west, precious for the ripening wheat. And the far north-east receives some winter rain of its own.

5.4.2 The Summer Season

The hot-weather season lasts from about April to June. With the sun now vertical over the Tropic of Cancer, the north and north-west heat fiercely; by mid-May the interior reaches 41–42°C, and parts of the north-west touch 45–47°C. The south, cooled by the sea, is milder, with mean maximums of 26–30°C, and its isotherms run parallel to the coast.

As the land heats, the winter high collapses into low pressure. The lowest pressure, about 997 mb, sits over west Rajasthan near the border, while higher pressure of around 1009 mb still lies over the far south and the surrounding seas. The winds begin their reversal, blowing from the south-west across the Arabian Sea and the Bay of Bengal.

This is the season of fierce local winds and sudden storms. The loo is a hot, dry, oppressive wind that blows by day across the northern plains from Punjab to Bihar, fiercest between Delhi and Patna. Towards the end of the summer, pre-monsoon mango showers fall in Kerala and coastal Karnataka, helping the mangoes ripen, while similar blossom showers help the coffee flower. In the east, dreaded evening thunderstorms called Nor’westers, known locally as kalbaisakhi — the calamity of the month of Baisakh — strike Bengal and Assam; harsh as they are, they help the tea, jute and rice, and in Assam they are called Bardoli Chheerha.

The Loo sun overhead near the Tropic of Cancer → the north-western plains heat fiercely → very low pressure and steep heating → hot, dry air is dragged across the plains as the loo → afternoons from Punjab to Bihar turn dangerous, until a local dust storm brings brief relief.

5.4.3 The Rainy Season

With the burst of the south-west monsoon in June, the rainy season begins and lasts until about the end of September. This is the true wet season, when the monsoon winds are active over the whole country and India receives roughly four-fifths of its annual rain. The temperature drops by 5–8°C as the rains set in, and the air turns humid and cloudy.

The rain does not fall evenly. It comes in the wet-and-dry spells and along the two great currents we followed in the monsoonal regime (section 5.3), and it is heaviest where the land forces the moist air upward. July temperatures show the sea’s steadying hand at work.

Map

Map 5.5 Caption: Mean July temperature across India, shown by isotherms in degrees Celsius. Brief: Outline map of India showing the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh. Draw isotherms of mean July temperature, showing the highest values over the north-west (near 30°C) and lower values elsewhere, with the lines running parallel to both coasts in the peninsula to show the equalising effect of the sea and the cooling by monsoon cloud. Label a few values. Show the Tropic of Cancer.

Compared with the cloudless heat of May, July is cooler and more even. Cloud cover and rain hold the daytime peaks down, and the sea keeps the coasts mild, so the hottest belt is now the still-dry north-west rather than the whole plain.

5.4.4 The Season of Retreating Monsoon

Through October and November the monsoon withdraws. The retreat is gradual: the clouds break up and the sky clears, the day temperature rises again while the nights turn pleasant, and the diurnal range — the gap between the day’s and the night’s temperature — widens. October is often uncomfortable, a spell of clear-sky warmth and high humidity known as the ‘October heat’, when strong radiation and lingering moisture make the air feel heavy.

This is also the season of the north-east monsoon’s rain. The retreating winds cross the Bay of Bengal, gather moisture, and deliver it to the Tamil Nadu coast. Cyclonic storms forming over the Bay in this season move from south-east to north-west and can bring heavy, damaging rain to the eastern coast.

Q3. Kerala and Tamil Nadu are neighbours, yet one gets its heaviest rain in June and the other in November. Why?

A. Kerala faces the Arabian Sea, so the June monsoon strikes the Western Ghats and drenches it. Tamil Nadu lies in that current’s rain shadow and stays dry then. Its rain comes later, from the retreating north-east monsoon, which crosses the Bay of Bengal and reaches the Coromandel coast in October and November.

5.5 The Distribution and Variability of Rainfall

Rainfall is the element of India’s climate that varies most — from place to place across the country, and from year to year at the same place. The first kind of variation shapes where crops can grow without irrigation; the second decides how safe that growing is.

5.5.1 The Pattern of Annual Rainfall

The amount of rain a place receives depends on how close it lies to the sea and whether the land lifts the moist winds. The wettest parts of India are the Western Ghats, the west coastal plain, the sub-Himalayan north-east and the hills of Meghalaya, all of which receive more than 200 cm a year, with parts of Meghalaya well over 1,000 cm. A broad belt across eastern and central India — Bengal, Odisha, Chhattisgarh, Jharkhand, Bihar and their neighbours — gets 100 to 200 cm.

A useful marker is the isohyet, a line joining places of equal rainfall. The 100 cm isohyet runs down from the Gujarat coast, roughly parallel to the crest of the Western Ghats, toward the far south. West and south-west of this line, rainfall is generally deficient, and successful farming there leans heavily on irrigation. In the north-west — Punjab, Haryana, western Rajasthan and the Kachchh region — rainfall drops below 60 cm, and in the driest heart of western Rajasthan below 20 cm.

Two broad trends run through the map. Rainfall decreases from the Bengal and Odisha coasts westward and north-westward, and it declines from both the eastern and western coasts inward toward the interior of the plateau.

Map

Map 5.6 Caption: The average annual rainfall of India, showing the wet coasts and north-east against the dry north-west. Brief: Outline map of India showing the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh. Shade rainfall bands: under 20 cm, 20–60 cm, 60–100 cm, 100–200 cm, and over 200 cm. Mark the heaviest zones (Western Ghats windward slopes, west coast, sub-Himalayan north-east, Meghalaya hills) and the driest zone (western Rajasthan and Kachchh). Draw the 100 cm isohyet clearly. Include a key.

Map 5.6 The shading bands each stand for a range of annual rainfall in centimetres; the darkest zones are the wettest. The pattern is wet coasts and north-east, dry north-west and interior. But a band shows only the yearly total — it hides whether that rain arrives in a few violent bursts or spreads evenly, and it hides how much the figure swings from one year to the next.

5.5.2 The Variability of Rainfall

The annual total is only an average. In practice the rain in any single year may run far above or below it, and this year-to-year swing is measured by the coefficient of variation. It is found from the formula:

\[\text{Coefficient of variation (C.V.)} = \frac{\text{Standard deviation}}{\text{Mean}} \times 100 \quad (\text{per cent})\]

Here the mean is the long-run average rainfall, and the standard deviation is a measure of how far individual years typically stray from that average. The larger the coefficient, the more unreliable the rainfall.

Worked Case 5.2 — Working Out a Coefficient of Variation The situation: suppose the annual rainfall at a station over five years was 80, 120, 100, 60 and 140 cm. How variable is it? The working: \[\text{Mean} = \frac{80 + 120 + 100 + 60 + 140}{5} = \frac{500}{5} = 100 \text{ cm}\] Deviations from the mean: \(-20, +20, 0, -40, +40\) cm. Squared deviations: \(400, 400, 0, 1600, 1600\); their sum is \(4000\). \[\text{Standard deviation} = \sqrt{\frac{4000}{5}} = \sqrt{800} \approx 28.3 \text{ cm}\] \[\text{C.V.} = \frac{28.3}{100} \times 100 \approx 28 \text{ per cent}\]

A coefficient of about 28 per cent means the rainfall typically departs from its average by roughly that fraction — so a “100 cm” station could easily see 72 cm one year and 128 cm the next. The higher the coefficient, the less a farmer can trust the average when deciding what, and when, to sow.

Across India the coefficient generally lies between 15 and 30 per cent. It is lowest, under 15 per cent, in the heavy-rainfall areas — the west coast around Mangaluru, and the sub-Himalayan and north-eastern hills — where the rain is both plentiful and dependable. It rises over the interior plateau and is highest of all, over 40 per cent and reaching 50 to 80 per cent, in the deserts of Rajasthan and Kachchh.

The pattern is an inverse one: areas of heavy rain have low variability, while areas of scanty rain suffer high variability. The very regions that can least afford a bad year are the ones whose rainfall is least reliable.

Low Rainfall, High Variability a region already receives little rain → what falls is also erratic, so its coefficient of variation is high → farming depends on an average it cannot count on → the same dry regions become the most drought-prone.

Take the rainfall figures of any station from a temperature-and-rainfall table in this chapter, add the twelve monthly values to get the year’s total, and compare two stations. Which has the higher annual total, and which do you expect to have the steadier year-to-year rainfall?

5.6 Droughts and Floods

Rainfall that is unreliable brings two opposite disasters. Too little rain, for too long, is a drought; too much, too fast, is a flood. They sit at the two ends of the same scale, and India, because of its variable monsoon, meets both — often in the same year.

5.6.1 Droughts: Meaning, Causes and Affected Areas

A drought is a distress situation caused by the failure of rainfall, when surface water runs short and groundwater is drawn down. It is a relative thing: what matters is not the bare amount of moisture but whether it is enough, and arrives in time, for the crops and people that depend on it.

Droughts in India follow a weak south-west monsoon. Rain that is deficient, that arrives late or withdraws early, or that fails during a long break in the season, all leave the land parched. Although a drought can strike anywhere, the regularly affected areas are those with marginal rainfall — generally below 75 cm a year — and high variability, and with little irrigation to fall back on. Roughly one-third of the country is counted as drought-prone.

Three main tracts stand out. One is the desert and semi-desert belt, forming a rough rectangle from Ahmedabad to Kanpur and Kanpur to Jalandhar. A second is the dry rain-shadow strip on the leeside of the Western Ghats, reaching down to the southern tip of the peninsula. Beyond these lie scattered pockets — Coimbatore in Tamil Nadu, Saurashtra and Kachchh, Bundelkhand in Uttar Pradesh, the Palamau plateau in Jharkhand, Purulia in West Bengal and the Kalahandi region of Odisha.

The heaviest human toll, though, has sometimes fallen in normally wet states such as West Bengal, Bihar and Odisha. There, plentiful rain supports a dense population, so when the rain fails, the failure reaches millions of people at once.

Table 5.1 — How Often Severe Drought Recurs, By Region (India Meteorological Department) | Region | Recurrence of highly deficient rainfall | |—|—| | Assam | Very rare — about once in 15 years | | West Bengal, Chhattisgarh, Bihar, Madhya Pradesh, coastal Andhra Pradesh, Maharashtra, Konkan, Kerala, Odisha | Once in 5 years | | South interior Karnataka, eastern Uttar Pradesh, Vidarbha | Once in 4 years | | Gujarat, eastern Rajasthan, western Uttar Pradesh, Tamil Nadu, Kashmir, Rayalaseema, Telangana | Once in 3 years | | Western Rajasthan | Once in about 2.5 years |

Table 5.1 The left column names a region, the right column tells how often severe drought returns there. The pattern matches the rainfall map: the drier and more variable the region, the shorter the interval. But “once in five years” is a long-run average, not a timetable — it cannot say which year — and it lumps a whole region together, hiding the wetter and drier districts inside it.

Map

Map 5.7 Caption: The drought-prone areas of India, graded from extreme to moderate. Brief: Outline map of India showing the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh. Shade three zones — extreme, severe and moderate drought risk — concentrated in western Rajasthan and Kachchh (extreme), the interior peninsula in the rain shadow of the Western Ghats (severe), and adjoining tracts (moderate); leave the wet coasts and north-east as drought-free. Include a key.

5.6.2 Coping with Drought

India’s response to drought has shifted over time from relief to management — from stepping in after a famine to acting on early warnings before one takes hold. The old approach waited until crops had failed and then provided food and work; the newer approach watches rainfall, reservoir levels and the progress of sowing, so that action can begin in the monsoon season itself.

Several programmes carry this out. The Drought Prone Areas Programme, begun in 1973, treats a whole area at once, working to restore its ecological balance by conserving land, water, livestock and human resources together. A Crop-weather Watch Group, set up during the drought of 1979, issues weekly reports on rainfall and farming and aims to secure what its planners called Trikal — Akal, Jalkal and Tinkal, meaning care of food, water and fodder. Integrated watershed management, a preventive measure, plans on the basis of all the water a watershed holds, using surface water and groundwater together.

For all these programmes, the surest defence against drought on the farm remains irrigation. Once-precarious tracts of Punjab, Haryana and western Uttar Pradesh, long haunted by famine, were transformed once irrigation reached them.

5.6.3 Floods: Meaning, Causes and Affected Areas

A flood is a state of high water along a river or coast that submerges land normally kept dry. A rising river is a natural part of the water cycle; it becomes a hazard only when it reaches the lives and property of people.

Floods have many causes. The excessive rain of the south-west monsoon is the chief one, joined by tropical cyclones before and after the monsoon, the silting of riverbeds that shrinks their capacity, changes in a river’s course, and deforestation that speeds runoff. Now and then a flood follows the bursting of a dam, as when the Machhu dam in Gujarat failed in August 1979.

Most flood damage falls on the plains of northern India — Punjab, Haryana, Uttar Pradesh, Bihar, West Bengal and Assam. In the Ganga plain, tributaries such as the Kosi, the Gandak and the Ghaghara bring huge volumes of water from the Himalaya; the Kosi is so prone to shifting its course and drowning new land that it has earned a grim reputation. The Brahmaputra basin floods almost every year, as heavy rain, a heavily silted channel, and earthquakes and landslides all conspire together. The peninsular deltas of the Mahanadi, Godavari, Krishna and Cauvery flood occasionally too. In all, national assessments have put India’s flood-prone area at around 40 million hectares, later revised upward to nearly 50 million, of which about 8 million hectares are flooded in an average year.

Q5. If the monsoon waters almost the whole country, why does India still face droughts and floods in the very same year?

A. The one monsoon fails in two opposite ways. Where it is weak, late or breaks for weeks, the dry, high-variability regions get too little and drought sets in. Where it is heavy, or the rivers are silted and swollen, the well-watered plains get too much and flood — the same season, two failures, different places.

Map

Map 5.8 Caption: The flood-hazard zones of India, concentrated in the great river plains of the north and north-east. Brief: Outline map of India showing the full extent of India as officially defined by the Government of India, including Jammu and Kashmir and Ladakh. Shade the areas liable to flood: the Punjab–Haryana plain, the Ganga plain across Uttar Pradesh, Bihar and West Bengal, the Brahmaputra valley of Assam, and patches along the peninsular deltas (Mahanadi, Godavari, Krishna, Cauvery). Include a key marking ‘area liable to flood’.

5.6.4 Managing Floods

After the severe floods of 1954, the Government of India launched a National Flood Management Programme in three phases — an immediate phase of data collection and emergency protection, a short-term phase of building embankments (raised earthen banks along a river to keep it in its channel) and spurs, and a long-term phase of reservoirs and further embankments.

Large dams and reservoirs help by holding back and releasing floodwater gradually, softening the flood peak downstream. Yet they are not a complete cure: even after major dams were built, the Mahanadi and the Damodar have flooded again. Structural works can protect only about 32 of the nearly 50 million flood-prone hectares; the rest must be managed in other ways.

The most important of these is flood forecasting — giving warning before the water arrives, so that people, livestock and movable property can be moved in time. The Central Water Commission began forecasting in 1958 and now runs a wide network of stations. India has also signed agreements with its upstream neighbours, with Nepal in 1988 and with China in 2002, to share the river data on which good forecasts depend.

Who Gains From The Embankment? The farmer behind the embankment: in most years, dry fields and a protected village. The village outside or downstream: the walled river can no longer spread, so it runs higher — and when a breach comes, it floods harder. Both agree: the embankment changes where the floodwater goes. What would you need to know to decide? The flood record on each side, before and after the wall, and who lives where.

5.7 Reading Temperature and Rainfall Graphs

Everything in this chapter — the maritime and continental contrast, the monsoon regime, the desert and the cold desert — can be read off a single small diagram drawn for one place. That diagram is the temperature and rainfall graph, and learning to read it is learning to read a climate.

5.7.1 The Combined Line-and-Bar Graph

A temperature and rainfall graph, often called a climograph or a combined line-and-bar graph, shows both elements of a station’s climate for all twelve months at once. The months run along the base. The left-hand axis carries temperature; the right-hand axis carries rainfall.

The two elements are drawn differently, and for a reason. Temperature exists at every moment and changes smoothly, so it is shown as a continuous line. Rainfall comes and goes and changes abruptly from month to month, so it is shown as separate bars.

Diagram

Fig. 5.7-1 Caption: How to read a climograph — temperature as a line on the left axis, rainfall as bars on the right axis. Brief: An annotated sample climograph. Show the twelve months January–December along the base; a left vertical axis labelled ‘temperature (°C)’; a right vertical axis labelled ‘rainfall (mm)’. Draw a smooth temperature line rising to a summer peak, and a set of rainfall bars clustered in one wet season. Add labels with pointer lines to: the temperature line (read on the left axis), the rainfall bars (read on the right axis), the highest and lowest points of the line marked as the ‘annual range’, and the tallest bars marked as the ‘wet season’. This is a teaching illustration of the graph’s parts, not a specific station.

Fig. 5.7-1 The line, read off the left axis, is temperature; the bars, read off the right axis, are rainfall. The gap between the line’s highest and lowest points is the annual range; the tallest bars mark the wet season. But every value here is a monthly average — it hides the hottest single afternoon, the coldest night, and how far this year strayed from the last.

Two readings matter most. The annual range of temperature — the difference between the warmest and coldest monthly means — tells you at once whether a place is maritime or continental. The timing of the tall rainfall bars tells you which season is wet, and therefore which part of the monsoon system the place belongs to.

5.7.2 Five Stations Compared

Five Indian stations, read side by side, show how location writes itself into climate.

Mumbai, on the Maharashtra coast, has a small annual range of only about 6°C, from around 24°C in January to about 30°C in May. Its rain is concentrated fiercely in the south-west monsoon, from June to September, and it is almost dry from December to April. This is the maritime, monsoon-fed climate of the west coast.

Graph

Fig. 5.7-2 Caption: Mumbai — a maritime west-coast climate with a small temperature range and heavy summer monsoon rain. Brief: Combined line-and-bar climograph. X-axis: months January–December. Left Y-axis: mean monthly temperature (°C). Right Y-axis: mean monthly rainfall (mm). Line: temperature, nearly level near 24–30°C with a shallow peak before the monsoon. Bars: rainfall, very low December–April, rising steeply to a tall peak in July, tailing off by October. Data: climate normals for Mumbai, India Meteorological Department.

Delhi, far inland in the north-west, is the opposite. Its annual range is around 20°C, from about 14°C in January to the mid-30s in June, the mark of a continental climate. Its rain arrives with the south-west monsoon in July, and the temperature falls sharply as the rains break; its winters are almost dry, with only a little rain from western disturbances.

Graph

Fig. 5.7-3 Caption: Delhi — a continental interior climate with a large temperature range and monsoon rain in July to September. Brief: Combined line-and-bar climograph. X-axis: months January–December. Left Y-axis: mean monthly temperature (°C). Right Y-axis: mean monthly rainfall (mm). Line: temperature with a wide swing, low near 14°C in January, rising to a June peak, dipping as the monsoon breaks. Bars: rainfall very low most of the year, concentrated in July–September. Data: climate normals for Delhi, India Meteorological Department.

Chennai, on the Tamil Nadu coast, is coastal like Mumbai, with a low annual range, but its rain regime is unique. Its heaviest rain falls not in the south-west monsoon but in October and November, brought by the retreating north-east monsoon after the winds have crossed the Bay of Bengal.

Graph

Fig. 5.7-4 Caption: Chennai — a coastal east-coast climate whose heaviest rain comes late, in October and November. Brief: Combined line-and-bar climograph. X-axis: months January–December. Left Y-axis: mean monthly temperature (°C). Right Y-axis: mean monthly rainfall (mm). Line: temperature warm and even, around 25–33°C. Bars: modest rain in the middle of the year but the tallest bars in October and November. Data: climate normals for Chennai, India Meteorological Department.

Q. Why does a coastal station like Mumbai show such a small annual range of temperature?

A. Because the sea beside it warms and cools slowly, holding both the summer heat and the winter cold in check.

Jaisalmer lies in the heart of the Thar Desert. Its extreme continental climate gives it a large annual range and meagre rain — in most years the total barely exceeds 12 cm, because the monsoon winds are weak and all but spent by the time they reach it.

Graph

Fig. 5.7-5 Caption: Jaisalmer — a hot desert climate with a large temperature range and very little rain. Brief: Combined line-and-bar climograph. X-axis: months January–December. Left Y-axis: mean monthly temperature (°C). Right Y-axis: mean monthly rainfall (mm). Line: temperature with a wide swing, cool in winter, very hot near mid-year. Bars: rainfall tiny throughout, with only slight bars in the monsoon months. Data: climate normals for Jaisalmer, India Meteorological Department.

Leh, high in the Union Territory of Ladakh, is a cold desert. Ringed by mountains and cut off from the sea, it stays below freezing through the winter and does not exceed about 17°C even in summer, and no month receives more than a few millimetres of rain.

Graph

Fig. 5.7-6 Caption: Leh — a cold desert climate with freezing winters and almost no rain. Brief: Combined line-and-bar climograph. X-axis: months January–December. Left Y-axis: mean monthly temperature (°C), with the scale extending below 0°C. Right Y-axis: mean monthly rainfall (mm). Line: temperature well below freezing in winter, rising to a modest summer peak around 17°C. Bars: rainfall tiny in every month. Data: climate normals for Leh, India Meteorological Department.

Read together, the five make the argument of the whole chapter. Mumbai and Chennai, both by the sea, have small temperature ranges but opposite rain seasons — one wet in June, one in November — because one faces the Arabian Sea and the other the Bay. Delhi and Jaisalmer, both inland, have wide ranges, but Jaisalmer, further from the sea and off the monsoon’s path, is far drier. And Leh shows that height can starve a place of rain as surely as distance can. In every case, the graph is not describing the weather so much as recording where the place sits, and which winds can reach it.

5.8 Summary

  1. Weather is the atmosphere at one moment; climate is its long-run average and rhythm. India’s climate is called the monsoon type because its winds reverse completely with the season, concentrating the rain into the summer.

  2. Latitude splits the country in two: a tropical south, warm all year, and a subtropical north with a real winter. The Tropic of Cancer runs across the middle.

  3. Distance from the sea decides the temperature range. Coastal India is maritime and equable; the interior is continental and extreme, because the sea heats and cools slowly.

  4. The northern mountain wall does two jobs — it blocks cold Central Asian winds so winters stay mild, and it traps the moist monsoon so the rain falls on India rather than escaping north.

  5. Physiography decides where rain lands: air forced up a slope rains on the windward side (orographic rainfall) and leaves a dry rain shadow on the leeward side.

  6. The monsoon winds are the master control. The moist south-west monsoon brings about four-fifths of India’s rain in summer; the dry north-east monsoon of winter brings rain only to the Tamil Nadu coast.

  7. The monsoon is the seasonal migration of pressure and wind belts following the overhead sun. Halley explained it by the differential heating of land and sea; Flohn corrected this with the northward shift of the ITCZ, into which the southern trades are drawn and turned by the Coriolis force.

  8. High-level jet streams set the timing: a split winter westerly jet steers western disturbances into the north-west, and a summer easterly jet accompanies the burst of the monsoon.

  9. Far-off ocean conditions matter too: an El Niño warming of the eastern Pacific tends to weaken the monsoon, while La Niña tends to strengthen it — a tendency, not a rule.

  10. The monsoonal regime has four stages: onset over Kerala about 1 June and advance across the country by mid-July; rainfall delivered by two currents and five sub-currents in wet-and-dry spells; a break when the monsoon trough shifts; and a gradual retreat from the north-west in September to the south-east by December.

  11. The year runs through four seasons — a cold, dry winter; a hot summer of the loo, mango showers and Nor’westers; the wet rainy season; and the retreating monsoon of the ‘October heat’ that wets Tamil Nadu.

  12. Rainfall is very uneven: over 200 cm on the wet coasts and north-east, under 20 cm in western Rajasthan, declining from the coasts toward the interior. Its year-to-year reliability is measured by the coefficient of variation, standard deviation divided by mean; heavy-rain areas are steady, scanty-rain areas are erratic.

  13. The same monsoon causes both disasters. Drought strikes the dry, high-variability regions when the rain fails; floods strike the river plains of the north and north-east when it is excessive. Both are met with programmes — irrigation and watershed work for drought, embankments, reservoirs and forecasting for floods.

  14. A temperature and rainfall graph records a climate: the line is temperature, the bars are rainfall; the annual range tells maritime from continental, and the timing of the wet bars tells which monsoon reaches the place.

Flowchart

Fig. 5.8-1 Caption: The monsoon year is a single closing cycle, driven round by the migrating overhead sun. Nodes: Winter — sun over the southern hemisphere → high pressure over north-west India → dry north-east monsoon (Tamil Nadu wetted from the Bay; western disturbances wet the north-west) → Summer — sun moves north over the Tropic of Cancer → the north-west heats, a thermal low forms, the ITCZ shifts north → the westerly jet withdraws and the easterly jet forms → southern trades cross the equator, deflected right by the Coriolis force → burst of the south-west monsoon over Kerala about 1 June → Rainy season — the monsoon advances to cover India by mid-July, giving about four-fifths of the year’s rain in wet-and-dry spells → Retreat — the sun moves south, the ITCZ retreats, the monsoon withdraws from the north-west in September to the south-east by December → back to winter.

What This Chapter Cannot Tell You

A rainfall map shows a state’s average and hides the district and the village inside it, where a dry pocket can sit within a wet region. A monthly mean on a climograph smooths away the flooding cloudburst and the killing dry spell that can share the same month. And the whole account assumes the ITCZ and the jet streams behave as they usually do — in an El Niño year, or when the depressions fail, the average tells us little, which is why forecasts, however sharpened, still carry real uncertainty.

5.9 Key Terms

  1. break in the monsoon — a dry spell of two weeks or more within the rainy season.

  2. climate — the long-run average of weather at a place, together with its seasonal rhythm.

  3. climatic divide — a barrier, such as the Himalayas, that separates one climatic region from another.

  4. climograph — a combined line-and-bar graph showing a station’s monthly temperature as a line and rainfall as bars.

  5. coefficient of variation — the standard deviation divided by the mean, times 100; a measure of how unreliable rainfall is from year to year.

  6. continental climate — an interior climate with a large difference between summer and winter temperatures.

  7. Coriolis force — the deflection the Earth’s rotation gives to moving air, to the right in the northern hemisphere and to the left in the southern.

  8. diurnal range — the difference between the day’s and the night’s temperature.

  9. drought — a distress situation caused by the failure of rainfall, when surface water runs short and groundwater is drawn down.

  10. El Niño — a periodic warming of the eastern equatorial Pacific that tends to weaken the Indian monsoon.

  11. embankment — a raised earthen bank built along a river to keep it within its channel.

  12. flood — a state of high water along a river or coast that submerges land normally kept dry.

  13. insolation — the incoming solar radiation received at a place.

  14. Inter-Tropical Convergence Zone (ITCZ) — the low-pressure belt near the equator where the trade winds of the two hemispheres meet and rise.

  15. isohyet — a line on a map joining places of equal rainfall.

  16. isotherm — a line on a map joining places of equal temperature.

  17. jet stream — a narrow ribbon of very fast wind high in the atmosphere.

  18. La Niña — a cooling of the eastern equatorial Pacific that tends to strengthen the Indian monsoon.

  19. latitude — the angular distance of a place north or south of the equator.

  20. leeward — the sheltered side of a hill, facing away from the wind.

  21. mango showers — pre-monsoon showers in Kerala and coastal Karnataka that help ripen the mangoes.

  22. maritime climate — a coastal climate that is equable, with a small annual range of temperature.

  23. monsoon — a tropical wind system whose direction reverses completely between summer and winter.

  24. monsoon trough — the low-pressure axis of the ITCZ, lying over the northern plains in summer.

  25. Nor’westers — dreaded evening thunderstorms of Bengal and Assam in the hot season, known locally as kalbaisakhi.

  26. orographic rainfall — rain that falls when moist air is forced to rise over high ground.

  27. rain shadow — the dry belt on the leeward side of a mountain, where the air has already lost its moisture.

  28. south-east trade winds — the steady tropical surface winds of the southern hemisphere, drawn across the equator to become the south-west monsoon.

  29. Southern Oscillation — the seesaw of surface air pressure between the Indian Ocean and the Pacific.

  30. trade winds — the steady tropical surface winds that blow toward the equatorial low-pressure belt.

  31. tropical cyclone — an intense low-pressure storm forming over warm tropical seas.

  32. tropical depression — a moderate low-pressure system, weaker than a cyclone, that steers monsoon rain up from the Bay of Bengal.

  33. weather — the state of the atmosphere at one place at one moment.

  34. western disturbances — winter low-pressure systems from the Mediterranean and West Asia that bring rain to north-west India.

  35. windward — the side of a hill facing the wind, which receives the rain.

5.10 Points to Ponder

  1. If the monsoon burst arrived a month late every year but the year’s total rainfall stayed exactly the same, would India’s farmers be better off, worse off, or unaffected?

  2. The Himalayas keep India’s winters warm and hold its monsoon rain in. If the range were only half as high, which of the two effects do you think would fade first — the warmth, or the rain?

  3. A coastal town and a desert town can record the same average annual rainfall, yet only the desert suffers droughts. What is the average hiding, and why does it hide it more cruelly in the desert?

  4. Chennai receives its heaviest rain just as most of India is drying out. Is the retreating monsoon best thought of as a second, separate monsoon, or as the tail end of the first?

  5. An embankment protects the fields behind it but can send a worse flood beyond it. Is a river easier to live with when it is walled into its channel, or when it is left free to spread across its plain?

  6. Suppose an El Niño could be forecast a full year in advance with certainty. Would that end drought in India, or only change the way the country prepares for it?

5.11 Objective Questions

  1. Choose the correct option. The word ‘monsoon’ comes from the Arabic word mausim, which means:
    1. rain
    2. season
    3. wind
    4. storm [1 mark]
  2. Choose the correct option. About what share of India’s total annual rainfall is delivered by the south-west monsoon?
    1. one-quarter
    2. one-half
    3. two-thirds
    4. four-fifths [1 mark]
  3. Choose the correct option. In the northern hemisphere, the Coriolis force deflects moving air to its:
    1. left
    2. right
    3. upward
    4. downward [1 mark]
  4. Choose the correct option. The hot, dry, oppressive wind that blows across the northern plains in summer is the:
    1. Nor’wester
    2. mango shower
    3. loo
    4. kalbaisakhi [1 mark]
  5. Assertion (A): Western Rajasthan receives very little rain from the south-west monsoon. Reason (R): The Aravali range runs parallel to the Arabian Sea current and offers it no slope to climb.
    1. Both A and R are true, and R is the correct explanation of A.
    2. Both A and R are true, but R is not the correct explanation of A.
    3. A is true but R is false.
    4. A is false but R is true. [1 mark]
  6. Assertion (A): A coastal station has a smaller annual range of temperature than an interior station. Reason (R): The sea heats and cools more slowly than the land, and so moderates the temperature beside it.
    1. Both A and R are true, and R is the correct explanation of A.
    2. Both A and R are true, but R is not the correct explanation of A.
    3. A is true but R is false.
    4. A is false but R is true. [1 mark]
  7. Match each term in Column I with its meaning in Column II. [1 mark]
Column I Column II
a. isohyet i. a low-pressure belt near the equator
b. isotherm ii. a line joining places of equal rainfall
c. coefficient of variation iii. a line joining places of equal temperature
d. ITCZ iv. a measure of how unreliable rainfall is from year to year

  1. Fill in the blank. A dry spell of two weeks or more within the rainy season is called a ________. [1 mark]

  2. State whether the following is true or false. An El Niño warming of the eastern Pacific tends to strengthen the Indian monsoon. [1 mark]

5.12 Check Your Concepts

  1. Define a monsoon, and state why India’s climate is described as the monsoon type. [2 marks]

  2. What is meant by a ‘break’ in the monsoon? [2 marks]

  3. Differentiate between the south-west monsoon and the north-east monsoon with respect to their season, their direction, and the rain they bring. [3 marks]

  4. Give reasons why the areas of heavy rainfall in India tend to have low variability of rainfall, while the areas of scanty rainfall have high variability. [3 marks]

  5. Explain the two ways in which the northern mountain ranges influence the climate of India. [3 marks]

  6. Explain the mechanism by which the south-east trade winds of the southern hemisphere arrive over India as the south-west monsoon. [4 marks]

  7. With one Indian example each, explain orographic rainfall and the rain shadow it produces. [4 marks]

  8. Explain how the jet streams help to control the timing and the sudden onset of the Indian monsoon. [4 marks]

  9. Describe the four stages of the Indian monsoonal regime — the onset and advance, the rain-bearing system, the break, and the retreat. [6 marks]

  10. Discuss the causes and the affected areas of droughts and of floods in India, and outline the main measures taken to cope with each. [6 marks]

5.13 Map, Diagram & Data Interpretation

  1. Study Fig. 5.2-1, the flowchart of the monsoon mechanism, and answer: [4 marks]

    1. Name the belt of low pressure that shifts north over India in summer.
    2. Which force turns the south-east trade winds into south-west winds?
    3. In two or three steps, trace how the northward movement of the sun leads to the burst of the monsoon.

  2. Study Map 5.2 (the advance of the monsoon) and Map 5.3 (its retreat), and answer: [4 marks]

    1. On about which date does the monsoon normally reach the Kerala coast?
    2. Which part of India is reached last by the advancing monsoon, and which part is vacated first by the retreating monsoon?
    3. Using the two maps, explain why Punjab has a much shorter rainy season than the Coromandel coast.

  3. The annual rainfall at a station over five years was 90, 110, 130, 70 and 100 cm. Using the coefficient-of-variation formula from this chapter, calculate the mean, the standard deviation and the coefficient of variation, showing each step of your working. [4 marks]

  4. Study Table 5.1, the regional frequency of severe drought, and answer: [3 marks]

    1. Which of the regions shown suffers severe drought most often?
    2. Which suffers it most rarely?
    3. State one thing this table cannot tell you about the drought experienced within a single region.

  5. Study the climographs of Delhi (Fig. 5.7-3) and Chennai (Fig. 5.7-4), and answer: [4 marks]

    1. Which of the two has the larger annual range of temperature?
    2. In which months does each station receive its heaviest rainfall?
    3. Account for the difference in the timing of their rain.