NCERT
Every NCERT book that matters for UPSC — open a book, then a chapter. Learn each chapter's concepts, then how UPSC tests them — with diagrams, flowcharts and the past questions to attempt.
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← Fundamentals of Physical Geography — all chaptersAtmospheric Circulation and Weather Systems
Start here: why this chapter matters
In simple wordsLet the air out of a bicycle tyre: it rushes from the high pressure inside to the lower pressure outside. The atmosphere does the same on a planetary scale. Wherever the sun heats air more, it gets lighter and the pressure drops; elsewhere pressure stays high. Air rushes from high to low — that is wind, and the spinning earth bends its path. This chapter explains that whole chain, from a gentle sea breeze to a cyclone.
Chapter 8 showed that the earth is heated unevenly — a surplus in the tropics, a deficit near the poles. This chapter shows how the atmosphere moves that heat around: through pressure differences, winds, great circulation cells, air masses and storms. Rain, droughts, monsoons and cyclones all start here, so it is one of the most heavily examined chapters in physical geography.
- Comes after Chapter 8 (uneven heating → pressure differences).
- Leads into Chapter 10 (rising air cools → clouds and rain) and Chapter 11 (climate types are shaped by the pressure and wind belts).
- Links to Chapter 13: winds drive ocean currents; El Niño is an ocean–atmosphere event.
- Links to India's monsoon (Class 11 ‘India — Physical Environment’), which this chapter only introduces as a seasonal wind.
- UPSC: GS1 Geography (world climatology, cyclones) and GS3 Disaster management (cyclones, warnings).
| Word | What it means here |
|---|---|
| Atmospheric pressure | The weight of the column of air above a place, measured in millibars (mb) |
| Isobar | A line on a map joining places of equal pressure |
| Pressure gradient | How quickly pressure changes over a distance — the ‘slope’ the wind runs down |
| Coriolis force | The apparent force from the earth's rotation that bends moving air |
| Cyclone / anticyclone | A low-pressure system with winds circling into it / a high-pressure system with winds circling out of it |
| Air mass | A huge body of air with nearly the same temperature and moisture throughout |
| Front | The boundary zone where two different air masses meet |
| ITCZ | Inter Tropical Convergence Zone — the low-pressure belt near the equator where the trade winds meet |
At a glance — the six ideas to carry away
- Wind blows from high to low pressure; isobars close together = strong wind.
- Belts: equatorial low · subtropical high (30°) · sub-polar low (60°) · polar high — they shift north in July, south in January.
- Coriolis deflects winds right in the NH, left in the SH — zero at the equator, maximum at the poles.
- NH lows spin anticlockwise, highs clockwise; the reverse in the SH.
- Three cells — Hadley, Ferrel, polar — give the trades, westerlies and polar easterlies.
- Tropical cyclones need sea above 27°C, Coriolis force, low wind shear, a pre-existing low, upper divergence.
1. Atmospheric pressure and how it changes with height
In simple wordsImagine a stack of mattresses. The one at the bottom is squashed by all the others; the one at the top carries almost nothing. Air is the same: at sea level it carries the weight of the whole atmosphere, so pressure is highest there; climb a mountain and there is less air above you, so pressure falls — which is why you feel breathless.
Atmospheric pressure is the weight of a column of air of unit area from mean sea level to the top of the atmosphere. It is measured in millibars (mb); the average at sea level is 1,013.2 mb. Gravity makes the air near the surface denser, so pressure is highest there. It is measured with a mercury barometer or an aneroid barometer. Differences in pressure from place to place are the primary cause of wind, which always blows from high to low pressure.
Vertical variation. In the lower atmosphere pressure falls fast — about 1 mb for every 10 m of height — but not at the same rate all the way up. The upward-pointing (vertical) pressure gradient force is much bigger than the horizontal one, but it is balanced by gravity pulling down, so we do not feel strong upward winds.
| Level | Pressure (mb) | Temperature (°C) |
|---|---|---|
| Sea level | 1,013.25 | 15.2 |
| 1 km | 898.76 | 8.7 |
| 5 km | 540.48 | −17.3 |
| 10 km | 265.00 | −49.7 |
Reading NCERT Table 9.1 — Standard pressure and temperature (page 76)
- At 5 km pressure is already about half its sea-level value; at 10 km only about a quarter.
- The first kilometre loses about 114 mb (roughly 1 mb per 10 m), but between 5 and 10 km the loss is only about 55 mb per km — pressure falls more slowly with height.
- Temperatures fall by exactly 6.5°C per km (15.2 → 8.7 → −17.3 → −49.7) — the normal lapse rate from Chapter 8.
- The text gives sea-level pressure as 1,013.2 mb and the table as 1,013.25 mb — the same value, rounded.
Exam trapNCERT exercise Q1-i: if surface pressure is 1,000 mb, the pressure 1 km up is about 900 mb (1 mb per 10 m × 1,000 m = 100 mb less). Atmospheric pressure exists because of gravity pulling the air down (UPSC 1989), not because of the earth's rotation. A falling barometer means a low is approaching — usually stormy weather (UPSC 1987, 2001).
2. Isobars and pressure systems
In simple wordsOn a hill map, contour lines show the shape of the land: rings for a hilltop, rings for a hollow, lines close together for a steep slope. Isobars do the same for pressure. A ‘hollow’ in pressure is a low, a ‘hill’ is a high, and where the lines crowd together the ‘slope’ is steep — and the wind is strong.
Small pressure differences matter a lot for wind speed and direction. Horizontal pressure is shown by drawing isobars — lines joining places of equal pressure. Because pressure falls with height, every station's reading is first reduced to sea level, so that a hill station and a coastal town can be compared fairly. A low-pressure system is enclosed by one or more isobars with the lowest pressure at the centre; a high-pressure system has the highest pressure at the centre.
Reading NCERT Figure 9.1 — Isobars, pressure and wind systems in the northern hemisphere (page 77)
- Isobars are drawn every 2 mb (1014, 1016, 1018, 1020 mb). Two LOWs have 1014 mb at the centre; two HIGHs have 1020 mb.
- The figure names three shapes that the text never explains:
- Trough — a long, narrow tongue of low pressure stretching out from a low (its centre line is the trough's axis).
- Ridge — a long tongue of high pressure stretching out from a high (drawn with a zig-zag line).
- Col — the saddle-shaped area of nearly even pressure between two highs and two lows; winds there are light and variable.
- Small wind arrows curve anticlockwise into the lows and clockwise out of the highs — the northern hemisphere pattern (see Table 9.2 below).
Beyond NCERTConcentric closed isobars with the lowest value at the centre and anticlockwise winds in the northern hemisphere = a cyclone (UPSC 1990). A daily weather map with isobars is a kind of isopleth map (a map of lines of equal value).
3. World pressure belts and how they shift
In simple wordsThink of the globe wearing stripes of high and low pressure, like the bands on a ball. The stripes are not painted on: they slide north in the northern summer and south in the northern winter, following the overhead sun — like a shadow following a lamp.
- Equatorial low — near the equator; low sea-level pressure (the ITCZ).
- Subtropical highs — along about 30°N and 30°S.
- Sub-polar lows — along about 60°N and 60°S.
- Polar highs — near the poles.
- The belts are not permanent: they move with the apparent movement of the sun — southwards in the northern winter, northwards in summer.
Reading NCERT Figure 9.2 — Distribution of pressure, January (page 77)
- A huge high (over 1,030 mb) sits over central Asia/Siberia — cold land in winter makes cold, heavy air. Smaller highs lie over North America, North Africa and the subtropical oceans of the northern hemisphere.
- Two deep lows lie over the northern oceans: near Iceland (about 1,000 mb) and near the Aleutian Islands in the North Pacific. These names are not in the text.
- In the southern hemisphere (summer) the subtropical highs are separate cells over the South Pacific, South Atlantic and South Indian Oceans; the continents between them are relatively low.
- The thick line marked ITCZ runs near the equator and dips south of it over the Indian Ocean and towards Australia.
- Around 50–60°S the isobars (1005, 1000, 995 mb) run in straight, closely spaced bands — the sub-polar low belt over an almost unbroken ocean, with arrows showing strong westerly winds.
Reading NCERT Figure 9.3 — Distribution of pressure, July (page 78)
- The Asian winter high is replaced by a deep low over north-west India–Pakistan (1,000 mb inside a 1,005 mb ring) — the summer heat low that pulls in the monsoon.
- Strong subtropical highs over the North Pacific and North Atlantic (above 1,020 mb).
- The ITCZ moves north of the equator; arrows show the south-east trades crossing the equator in the Indian Ocean and turning towards India.
- Southern hemisphere (winter): highs over the South Pacific, South Atlantic, South Indian Ocean and Australia (1,020 mb); the sub-polar low belt (995–1,000 mb) stays in place.
- The northern lows near Iceland and the Aleutians are much weaker (about 1,010 mb).
Beyond NCERT (asked by UPSC)Doldrums — the belt of calm, light, variable winds near the equator, i.e. the equatorial low (UPSC 1980, 1983). Horse latitudes — the calm belts over the oceans at about 30°–35° N and S: they are subtropical highs, not lows (UPSC 2007). The sub-polar lows at 60–65° form mainly because air from the subtropical highs and polar highs converges and rises there; they are stronger over oceans (UPSC 2002).
4. Forces that drive and steer the wind
In simple wordsRoll a ball across a spinning merry-go-round. To someone riding it, the ball seems to curve away even though nobody pushed it sideways. The earth is a giant merry-go-round, so moving air seems to curve too — that apparent push is the Coriolis force. Add the push from high to low pressure, and the drag of the ground, and you have the three forces that set every wind.
| Force | What it does | Key facts |
|---|---|---|
| Pressure gradient force | Starts the wind, from high to low pressure | Strong where isobars are close together, weak where they are far apart; acts at right angles to the isobars |
| Frictional force | Slows the wind | Greatest at the surface; felt up to about 1–3 km; minimal over the sea |
| Coriolis force | Changes the wind's direction | Deflects to the right in the NH, left in the SH; greater when the wind is faster; maximum at the poles, zero at the equator; named after the French physicist who described it in 1844 |
| Gravity | Acts downward | Balances the vertical pressure gradient |
The Coriolis force acts at right angles to the pressure gradient force. Because of the two together, air does not rush straight into a low; it circles around it. At the equator the Coriolis force is zero, so the wind blows straight across the isobars into a low and fills it up instead of deepening it — this is why tropical cyclones do not form near the equator.
Geostrophic wind. At 2–3 km above the surface, friction disappears. If the isobars are straight, the pressure gradient force and the Coriolis force balance each other, and the wind blows parallel to the isobars. This is the geostrophic wind.
Reading NCERT Figure 9.4 — Geostrophic wind (page 79)
- Two straight isobars labelled 704 mb and 708 mb — values found about 3 km up, where friction no longer matters (a detail only in the figure).
- PH (the horizontal pressure gradient) points across the isobars towards the lower pressure; C (Coriolis) points the opposite way; V (the wind) runs along the isobars.
- The two panels show the wind blowing in opposite directions in the two hemispheres for the same pressure pattern: in the north the low pressure is on the wind's left; in the south it is on the wind's right.
| Pressure system | Pressure at the centre | Northern hemisphere | Southern hemisphere |
|---|---|---|---|
| Cyclone | Low | Anticlockwise | Clockwise |
| Anticyclone | High | Clockwise | Anticlockwise |
Surface and upper winds are linked. Over a low, air converges and rises; over a high, air subsides from above and diverges at the surface. Besides convergence, air is also lifted by eddies, convection currents, orographic uplift (forced up a mountain) and uplift along fronts — and rising air is essential for clouds and rain.
Reading NCERT Figure 9.5 — Convergence and divergence of winds (page 79)
- Two linked loops sit over the surface line: over the Low, arrows meet at the surface (convergence), rise, and spread apart aloft (divergence); over the High, arrows meet aloft, sink, and spread apart at the surface.
- The loops share their sides: air that rises over the low sinks over the neighbouring high. Surface and upper-air circulation form one system.
⤵ From pressure difference to the wind you feel
Uneven heatingChapter 8
Pressure differenceswarm air → low; cold air → high
Pressure gradient forcepushes air from high to low
Coriolis forceturns it right (NH) / left (SH)
Aloft: geostrophic windparallel to isobars
Friction (up to 1–3 km)slows it near the ground
At the surfacecrosses isobars at an angle, into lows
Exam traps
- UPSC 2024: the Coriolis force increases with wind velocity and is maximum at the poles, absent at the equator — both true.
- UPSC 2005: ‘wind patterns are clockwise in the NH and anticlockwise in the SH’ was false as a blanket claim — the direction depends on whether it is a cyclone or an anticyclone; the Coriolis reason was true.
- NCERT exercise Q2-ii: the pressure gradient points from the subtropical high (north) to the equator (south), but the Coriolis force turns the wind to the right, so the trades blow from the north-east.
Beyond NCERT — a precise wordNCERT says the Coriolis force is ‘directly proportional to the angle of latitude’. More precisely, it is proportional to the sine of the latitude (and to wind speed): zero at 0°, half its maximum at 30°, maximum at 90°. The exam-ready meaning is the same — it grows towards the poles.
5. The general circulation of the atmosphere
In simple wordsHeat a pan of water and watch: water rises over the flame, spreads out on top, cools, sinks at the sides and flows back along the bottom — a loop. The atmosphere makes three such loops in each hemisphere, called cells. Their bottom halves are the steady planetary winds that sailors relied on for centuries.
The pattern of planetary winds depends on: (i) how atmospheric heating varies with latitude; (ii) the pressure belts; (iii) their migration with the sun; (iv) the distribution of continents and oceans; (v) the rotation of the earth. This large-scale pattern is the general circulation of the atmosphere. It also sets ocean currents in motion, and the oceans in turn feed energy and water vapour back to the air — slowly, over large areas.
Reading NCERT Figure 9.6 — Simplified general circulation (page 80)
- The globe is labelled with the belts: H at each pole, SPL (sub-polar low) at 60°, STH (subtropical high) at 30°, ITCZ at the equator.
- Surface winds: North East Trades and South East Trades blowing into the ITCZ; Westerlies between 30° and 60°, drawn as wavy arrows (they meander and carry storms); Polar Easterlies poleward of 60°.
- Arrows around the outside show the Hadley cell, Ferrel cell and Polar cell. A Polar Vortex is marked over the North Pole.
- Black dots mark the Polar Jet Stream and the Subtropical Jet Stream — fast upper-air winds at the boundaries between cells. The text does not mention jet streams; they appear only in the figure.
| Cell | Where | How it works | Surface wind |
|---|---|---|---|
| Hadley | 0°–30° | Air rises at the ITCZ by convection (high insolation), climbs to about 14 km, moves poleward, piles up and cools near 30°, sinks → subtropical high | Trade winds (easterlies) back to the ITCZ |
| Ferrel | 30°–60° | Sinking cold air from the poles meets warm air rising from the subtropical high | Westerlies |
| Polar | 60°–90° | Cold dense air subsides at the poles and flows towards the middle latitudes | Polar easterlies |
⤵ Why there is a high at 30° (NCERT exercise Q3-ii)
Intense heating at the equatorconvection → air rises at the ITCZ
Rises to the top of the troposphere (~14 km)then flows poleward
Air piles up near 30°accumulation
Air cools as it travelsbecomes denser
It sinks → subtropical highsinking air warms and dries → deserts lie here
Surface air returns to the equatoras the trade winds
Beyond NCERT (asked by UPSC)
- The trade winds are the surface flow back towards the equator (UPSC 1985); the monsoon is not a planetary wind — it is seasonal (UPSC 1991).
- The westerlies blow between about 30° and 60° in each hemisphere, not ‘from 30°N to 60°S’; moist westerly systems (western disturbances) bring winter rain to north-west India (UPSC 2015).
- Southern hemisphere westerlies are stronger and steadier because there is less land to slow them — the Roaring Forties (40°–50°S), Furious Fifties and Shrieking Sixties (UPSC 1984, 2000, 2011). The Coriolis force is not stronger in the south.
- Jet streams blow from west to east in the upper troposphere of both hemispheres (UPSC 2020).
6. El Niño, the Southern Oscillation and seasonal winds
In simple wordsPicture a long bathtub where a steady breeze keeps pushing the warm surface water to one end. If the breeze weakens, the warm water sloshes back to the other end. The Pacific Ocean is that bathtub. When its warm water sloshes east towards South America, rain moves with it — and weather changes around the world. That is El Niño.
Warming and cooling of the Pacific Ocean matters most for the general circulation. Sometimes warm water from the central Pacific drifts towards the South American coast and replaces the cool Peruvian current. This warm water off Peru is El Niño. It is linked to pressure changes between the Central Pacific and Australia, called the Southern Oscillation. Together they are ENSO. In strong ENSO years: the normally arid west coast of South America gets heavy rain, Australia suffers drought (and sometimes India), and China has floods. ENSO is closely watched for long-range forecasting.
Beyond NCERT (asked by UPSC)
- Normally the easterly trade winds pile warm water in the west Pacific; in an El Niño they weaken. The opposite phase, with unusually cold water in the eastern equatorial Pacific, is La Niña — both are Pacific events (UPSC 2011 put La Niña in the Indian Ocean: wrong).
- El Niño is linked to weak Indian monsoons (UPSC 2002) and to droughts and forest fires in Indonesia (1997 fires, UPSC 1998). El Niño Modoki warms the central Pacific rather than the east (UPSC 2010).
- The NCERT glossary adds that El Niño usually appears around Christmas and lasts weeks to months — hence the name (‘the child’).
Seasonal winds. The wind pattern changes with the seasons as the zones of maximum heating, pressure and wind belts shift. The most striking result is the monsoon, especially over South and Southeast Asia (studied in ‘India — Physical Environment’).
7. Local winds
In simple wordsOn a sunny beach the sand gets too hot to walk on while the water stays cool. By afternoon a cool breeze blows from the sea. At night the sand cools quickly but the water stays mild, and the breeze turns round. Local winds are these small, daily ‘breathing’ cycles caused by surfaces heating and cooling differently.
Land and sea breezes. By day the land heats faster than the sea; air over the land rises and pressure falls, while the cooler sea keeps higher pressure. The pressure gradient runs from sea to land, so a sea breeze blows onshore. At night the land loses heat faster and becomes cooler than the sea; the gradient reverses and a land breeze blows offshore.
Reading NCERT Figure 9.7 — Land and sea breezes (page 81)
- Each panel shows a closed loop, not just a surface arrow: in the sea-breeze panel air moves from sea to land near the ground, rises over the land, returns towards the sea aloft and sinks over the sea.
- In the land-breeze panel every arrow is reversed: surface air flows from land to sea and rises over the sea.
- The loops are drawn over a sloping coast — the circulation is shallow and local, reaching only a short distance inland or offshore.
Mountain and valley winds. By day the mountain slopes heat up, air moves up the slopes, and air from the valley flows up to replace it — the valley breeze. At night the slopes cool, and dense air slides down into the valley — the mountain wind. Cool air from high plateaus and ice fields draining into valleys is called a katabatic wind.
Warm leeward winds. Moist air forced over a mountain range drops its moisture as rain on the windward side. As it descends the leeward side, the now-dry air is warmed by compression (adiabatic warming) and can melt snow in a short time.
Beyond NCERT (asked by UPSC)
- Named local winds: Foehn (Alps), Chinook (Rockies, ‘snow-eater’), Zonda (Argentina), Santa Ana (California), Samun (Kurdistan/Iran) — UPSC 2001 asked for exactly these pairs. The loo of north India is a hot, dry summer wind (Chapter 8 example of advection) — UPSC Mains 2026 compared the Loo, Chinook and Foehn.
- The NCERT glossary defines a katabatic wind more broadly as any wind blowing down a mountain slope.
8. Air masses and fronts
In simple wordsLeave a sponge on a wet floor and it soaks up water; leave it in a hot oven and it dries and heats. Air that sits for days over one kind of surface takes on its character — hot or cold, wet or dry. Such a huge body of air is an air mass. When two different air masses bump into each other, they don't mix easily; the boundary is a front, and that is where the weather turns stormy.
An air mass is a large body of air with little horizontal variation in temperature and moisture. It forms when air stays long enough over a homogeneous surface — a vast ocean or a vast plain — called its source region.
| Source region | Air mass | Character |
|---|---|---|
| Warm tropical and subtropical oceans | Maritime tropical (mT) | Warm, moist |
| Subtropical hot deserts | Continental tropical (cT) | Hot, dry |
| Relatively cold high-latitude oceans | Maritime polar (mP) | Cool, moist |
| Very cold snow-covered continents in high latitudes | Continental polar (cP) | Cold, dry |
| Permanently ice-covered Arctic and Antarctica | Continental arctic (cA) | Extremely cold, dry |
Exam trapNCERT exercise Q1-iv: of the equatorial forest, the Himalayas, the Siberian Plain and the Deccan Plateau, only the Siberian Plain is a source region — a vast, flat, uniform surface. Mountains and varied plateaus are not.
Fronts. When two different air masses meet, the boundary zone is a front; its formation is frontogenesis. There are four kinds: cold (cold air advances into warm air), warm (warm air advances over cold air), stationary (neither moves) and occluded (the warm air is lifted completely off the ground). Fronts occur in the middle latitudes, have steep temperature and pressure gradients, bring abrupt temperature changes, and make air rise to form clouds and rain.
Reading NCERT Figure 9.8 — Vertical sections of fronts (page 82)
- The axes give the scale, which is only in the figure: height up to 4,000 m, distance from 600 km behind to 800 km ahead of the front.
- (a) Warm front: a gentle slope rising about 4 km over some 800 km; the arrow shows warm air advancing into the cold air.
- (b) Cold front: a steep, curved nose of cold air, rising about 4 km within a couple of hundred km; the arrow shows cold air advancing and lifting the warm air.
- (c) Occluded front: cold air on both sides at the ground and the warm air lifted above them; the occluded front is where the cold and warm fronts meet at the surface.
Read the NCERT line carefullyNCERT says ‘if an air mass is fully lifted above the land surface, it is called the occluded front’. More precisely, it is the warm air mass that is lifted completely off the ground when the faster cold front overtakes the warm front.
Beyond NCERT (UPSC Mains 2016)Air masses carry their source region's climate thousands of km away: cold cP outbreaks bring cold waves to North America and northern India; warm mT air brings humid heat and rain. Mid-latitude weather is largely the story of air masses meeting along the polar front.
9. Extra-tropical (temperate) cyclones
In simple wordsImagine two crowds walking past each other along a corridor — one warm, one cold. A small kink in the line where they meet grows into a swirl, with warm people pushing into the cold side and cold people curling round behind. Eventually the cold crowd surrounds the warm one and lifts it off its feet. That swirl is a temperate cyclone.
Cyclones that develop in the mid and high latitudes, beyond the tropics, are middle-latitude or extra-tropical cyclones. They form along the polar front. At first the front is stationary; in the northern hemisphere warm air blows from the south and cold air from the north. When pressure drops along the front, warm air moves north and cold air moves south, starting an anticlockwise circulation. A well-developed cyclone has a warm front and a cold front, with a warm sector wedged between cold air ahead and behind.
- Ahead of the warm front: warm air glides up over the cold air → a sequence of clouds appears and brings precipitation.
- Along the cold front: cold air pushes the warm air up from behind → cumulus clouds develop.
- The cold front moves faster and overtakes the warm front → the warm air is lifted completely → the front is occluded and the cyclone dissipates.
Reading NCERT Figure 9.9 — Extra-tropical cyclones (page 82)
- Small panels (a) and (b) show the start: a straight front, then a wave-shaped bend with a small closed low at the tip.
- The large plan view shows the mature storm: cold air to the north and west, the warm air tongue (warm sector) pushing north-east between the cold front (left) and the warm front (right), and a dashed arrow for the direction of motion of the centre — towards the north-east.
- Cross-sections show the warm air riding over cold air at both fronts, with shaded rain areas beneath the fronts. The vertical scale is about 9 km — a figure-only detail.
- Wide arrows above the sections show the upper westerly flow carrying the whole system eastward.
| Extra-tropical cyclone | Tropical cyclone | |
|---|---|---|
| Fronts | Clear frontal system | No fronts |
| Where it forms | Over land and sea | Only over warm seas; dies on reaching land |
| Area affected | Much larger | Smaller |
| Wind speed / damage | Lower | Much higher, more destructive |
| Direction of movement | West → east (with the westerlies) | East → west (with the trades) |
⤵ Life cycle of an extra-tropical cyclone
Stationary polar frontcold air north, warm air south
Pressure drops; a wave formsanticlockwise spin begins (NH)
Mature cyclonewarm front + cold front + warm sector
Cold front overtakes warm frontocclusion
Warm air lifted off the groundthe cyclone dissipates
Beyond NCERT (asked by UPSC)The heaviest, short-lived rain with thunder in a temperate cyclone falls along the cold front (UPSC 1999, which used a diagram of a cyclone). Surface winds spiral into a cyclone because air rises at its centre — UPSC 2002 marked ‘air descends in the centre of the cyclone’ as the false reason.
10. Tropical cyclones
In simple wordsA tropical cyclone is a giant engine fuelled by warm sea water. Water evaporates from the warm ocean; when the vapour rises and turns back into cloud droplets it releases heat; that heat makes the air rise faster, which draws in more moist air — and the engine speeds up. Cut off the fuel (move it over land or cold water) and the engine dies.
Tropical cyclones are violent storms that start over tropical oceans and move to the coasts, bringing violent winds, very heavy rain and storm surges. They are among the most devastating natural calamities. Names: Cyclones in the Indian Ocean, Hurricanes in the Atlantic, Typhoons in the western Pacific and South China Sea, Willy-willies in Western Australia.
- (i) A large sea surface with temperature above 27°C.
- (ii) The Coriolis force (so not right at the equator).
- (iii) Small variations in vertical wind speed (little wind shear).
- (iv) A pre-existing weak low or low-level cyclonic circulation.
- (v) Upper divergence above the sea-level system.
The storm's energy comes from condensation in the towering cumulonimbus clouds around its centre. A continuous supply of moisture from the sea keeps strengthening it; on land the supply is cut off and the storm dies. Where it crosses the coast is the landfall. Cyclones that cross 20°N generally recurve and are more destructive.
Reading NCERT Figure 9.10 — Vertical section of the tropical cyclone, after Rama Sastry (page 83)
- Height scale 0–15 km; the tropopause is pushed up over the storm's centre.
- At the centre is the eye, with subsiding warm air. Either side, the eye wall clouds rise all the way up, with arrows showing strong ascent.
- Outflow at about 12 km spreads away from the centre; above it a steering wind flow and undisturbed winds carry the storm along.
- Rain bands slope outward from the eye wall, with cumulus (Cu) and rain-cloud (Nb) symbols, out to about 200–300 km from the centre.
- Lines marked T₁, T₂ (temperature) bend upward near the centre and P₁, P₂ (pressure) bend downward — a warm core and very low central pressure.
- At the bottom: ‘direction of the storm’ pointing west, matching the east-to-west movement in the text.
| Part / measure | NCERT value |
|---|---|
| Eye | Calm, subsiding air |
| Eye wall | Strong spiralling ascent up to the tropopause; maximum winds, up to 250 km/h; torrential rain |
| Diameter of the circulating system | 150–250 km |
| Diameter of storms over the Bay of Bengal, Arabian Sea, Indian Ocean | 600–1,200 km |
| Speed of movement | 300–500 km per day |
NCERT inconsistency — two diametersThe text gives the ‘circulating system’ as 150–250 km across, and a few lines later the storm over Indian seas as 600–1,200 km. The smaller figure fits the core of intense circulation around the eye; the larger fits the whole storm with its rain bands. Quote each with its own label.
⤵ Life of a tropical cyclone
Warm sea (>27°C) + pre-existing low + Coriolisaway from the equator
Moist air converges and risescumulonimbus towers form
Condensation releases latent heatthe storm's fuel
Mature storm: eye, eye wall, rain bandsupper divergence keeps it going
Landfallviolent wind, rain, storm surge on the coast
Moisture supply cut off over landthe storm peters out
Exam traps
- NCERT exercise Q3-iii: torrential rain and the highest winds are in the eye wall, not the eye — the eye is calm.
- UPSC 2020: only some cyclones develop an eye (true); the eye is warmer, not 10°C colder, than its surroundings; jet streams are not limited to the NH.
- UPSC 2015: no cyclones form in the South Atlantic and south-eastern Pacific mainly because the sea surface is too cold (cold currents).
- Willy-willy = a tropical cyclone of north-west Australia (UPSC 1995). A hurricane's winds exceed — not fall below — 100 km/h (UPSC 1983).
Beyond NCERT (asked in Mains)
- Tropical cyclones are named from lists agreed by each region's member countries under the World Meteorological Organization; for the north Indian Ocean the India Meteorological Department's regional centre assigns names in turn (Mains 2013, ‘Phailin’).
- Rising sea-surface temperatures add energy and moisture, making cyclones intensify faster (Mains 2024).
- The Fujiwhara effect is when two nearby cyclones rotate around each other and can merge or change course (Mains 2026).
- IMD's colour-coded warnings run from green (no action) through yellow and orange to red (take action) (Mains 2022).
11. Thunderstorms and tornadoes
In simple wordsOn a hot, humid afternoon, warm air bubbles up like steam from a kettle. If it rises high enough, it builds a towering cloud that rumbles with thunder. Rarely, a part of that cloud starts spinning into a narrow, violent funnel that reaches the ground — like water swirling down a drain, but upside down. That funnel is a tornado.
Thunderstorms and tornadoes are short-lived and cover small areas, but they are violent. A thunderstorm comes from intense convection on hot, moist days: it is a well-grown cumulonimbus cloud producing thunder and lightning. Where the cloud reaches sub-zero heights, hail forms; with too little moisture, a thunderstorm can raise a dust storm. A strong updraft of warm air makes the cloud grow taller; later a downdraft brings cool air and rain to the ground.
From severe thunderstorms, a spiralling wind sometimes descends ‘like the trunk of an elephant’, with very low pressure at its centre, causing massive destruction — a tornado. Tornadoes mostly occur in the middle latitudes; over the sea they are waterspouts. These storms are the atmosphere's way of adjusting to uneven energy: potential and heat energy are turned into kinetic energy, and the atmosphere settles back to stability.
⤵ From a hot afternoon to a storm
Hot, moist air near the groundintense convection
Strong updraft builds a cumulonimbusthunder and lightning
Top reaches sub-zero heightshailstorm
Too little moisturedust storm
Downdraft brings cool air and rainthe storm decays
Rarely: a spiralling funneltornado (waterspout over sea)
Beyond NCERT (asked by UPSC)Thunder clouds need condensation, high temperature and humidity, orography and vertical winds — UPSC 2002 accepted all four. Thunder is the sound of air expanding violently when lightning heats it — UPSC 2013's answer was ‘none of the above’ for the three causes it offered.
NCERT exercise — answer outlines
Try each question yourself first, then open the outline to compare. The multiple-choice questions are in the self-check below.
What is the unit used in measuring pressure? Why is the pressure measured at station level reduced to the sea level in preparation of weather maps? (30 words)
Unit: the millibar (mb); sea-level average 1,013.2 mb. Pressure falls with height (about 1 mb per 10 m), so station readings are reduced to sea level to remove the effect of altitude and compare places fairly on isobar maps.
While the pressure gradient force is from north to south, i.e. from the subtropical high pressure to the equator in the northern hemisphere, why are the winds north easterlies in the tropics? (30 words)
The Coriolis force deflects moving air to the right in the northern hemisphere. Air heading south towards the equator is turned westward, so it arrives from the north-east — the north-east trades.
What are the geostrophic winds? (30 words)
Winds in the upper air (2–3 km and above), free of friction, where the pressure gradient force and Coriolis force balance across straight isobars, so the wind blows parallel to the isobars (low pressure on the left in the NH).
Explain the land and sea breezes. (30 words)
By day land heats faster → low pressure over land → cool sea breeze blows onshore. At night land cools faster → higher pressure over land → land breeze blows offshore. Caused by different heating of land and water.
Discuss the factors affecting the speed and direction of wind. (150 words)
Intro: wind is air moving from high to low pressure; three horizontal forces plus gravity act on it.
Pressure gradient force: sets the speed — strong where isobars are close; acts at right angles to isobars.
Friction: greatest at the surface, felt up to 1–3 km, minimal over the sea; slows wind and makes it cross isobars into lows.
Coriolis force: deflects right in the NH and left in the SH; increases with wind speed and latitude (zero at the equator, maximum at the poles); acts at right angles to the pressure gradient.
Results: aloft, balance → geostrophic wind parallel to isobars; around lows → cyclonic circulation (anticlockwise NH), around highs → anticyclonic; at the equator air fills lows, so no tropical cyclones form there.
Diagram: Figure 9.4 force sketch.
Conclude: other influences — pressure belts, land–sea distribution and relief — shape regional winds.
Pressure gradient force: sets the speed — strong where isobars are close; acts at right angles to isobars.
Friction: greatest at the surface, felt up to 1–3 km, minimal over the sea; slows wind and makes it cross isobars into lows.
Coriolis force: deflects right in the NH and left in the SH; increases with wind speed and latitude (zero at the equator, maximum at the poles); acts at right angles to the pressure gradient.
Results: aloft, balance → geostrophic wind parallel to isobars; around lows → cyclonic circulation (anticlockwise NH), around highs → anticyclonic; at the equator air fills lows, so no tropical cyclones form there.
Diagram: Figure 9.4 force sketch.
Conclude: other influences — pressure belts, land–sea distribution and relief — shape regional winds.
Draw a simplified diagram to show the general circulation of the atmosphere over the globe. What are the possible reasons for the formation of subtropical high pressure over 30°N and S latitudes? (150 words)
Diagram: globe with belts — ITCZ (0°), STH (30°), SPL (60°), polar highs; surface winds — NE/SE trades, westerlies, polar easterlies; three cells — Hadley, Ferrel, polar (as in Figure 9.6).
Reasons for the subtropical high: (1) air heated at the ITCZ rises by convection to about 14 km and flows poleward; it accumulates (piles up) near 30°; (2) the air cools on its journey and becomes denser; (3) so part of it sinks to the ground, raising surface pressure; (4) sinking air warms and dries, giving clear skies.
Consequences: surface air flows back to the equator as the trades and poleward as the westerlies; hot deserts lie under these highs on the western sides of continents.
Conclude: the belt shifts north in July and south in January with the sun.
Reasons for the subtropical high: (1) air heated at the ITCZ rises by convection to about 14 km and flows poleward; it accumulates (piles up) near 30°; (2) the air cools on its journey and becomes denser; (3) so part of it sinks to the ground, raising surface pressure; (4) sinking air warms and dries, giving clear skies.
Consequences: surface air flows back to the equator as the trades and poleward as the westerlies; hot deserts lie under these highs on the western sides of continents.
Conclude: the belt shifts north in July and south in January with the sun.
Why does tropical cyclone originate over the seas? In which part of the tropical cyclone do torrential rains and high velocity winds blow and why? (150 words)
Over the seas because: the storm needs a large sea surface above 27°C that supplies endless moisture; its energy is the latent heat released by condensation in cumulonimbus clouds. Other conditions — Coriolis force (away from the equator), little vertical wind shear, a pre-existing low, upper divergence — are also met over warm tropical oceans. Over land the moisture supply is cut and the storm dies.
Where: in the eye wall, the ring of towering clouds around the calm eye.
Why: here air spirals in fastest and rises strongly to the tropopause; the steep pressure gradient near the low centre gives winds up to 250 km/h; intense uplift and condensation give torrential rain. Rain bands spread outward from it.
Diagram: sketch of Figure 9.10 (eye, eye wall, rain bands, outflow).
Conclude: add storm surge and the 20°N recurving rule.
Where: in the eye wall, the ring of towering clouds around the calm eye.
Why: here air spirals in fastest and rises strongly to the tropopause; the steep pressure gradient near the low centre gives winds up to 250 km/h; intense uplift and condensation give torrential rain. Rain bands spread outward from it.
Diagram: sketch of Figure 9.10 (eye, eye wall, rain bands, outflow).
Conclude: add storm surge and the 20°N recurving rule.
What to remember
Numbers
Average sea-level pressure1,013.2 mb
Fall of pressure near the ground~1 mb per 10 m
Pressure at 1 / 5 / 10 km (Table 9.1)898.76 / 540.48 / 265 mb
Temperature at sea level / 10 km (Table 9.1)15.2°C / −49.7°C
Friction felt up to1–3 km
Winds free of friction above2–3 km (geostrophic)
Coriolis force described in1844
Pressure belts0° low · 30° high · 60° low · poles high
Hadley cell air rises to~14 km
Air-mass types5 (mT, cT, mP, cP, cA)
Front types4 (cold, warm, stationary, occluded)
Tropical cyclone sea temperature> 27°C
Eye-wall windsup to 250 km/h
Circulating system / whole storm diameter150–250 km / 600–1,200 km
Tropical cyclone movement300–500 km per day
Cyclones that recurvethose crossing 20°N
One-liners
- Wind = air moving from high to low pressure; isobars close = strong wind.
- Coriolis: right in the NH, left in the SH; zero at the equator, maximum at the poles.
- Geostrophic wind blows parallel to isobars, in the upper air.
- Low → converge and rise → clouds; high → sink and diverge → clear skies.
- Hadley → trades; Ferrel → westerlies; polar → polar easterlies.
- El Niño = warm water off Peru; Southern Oscillation = pressure see-saw; together ENSO.
- Extra-tropical cyclones move west → east; tropical cyclones east → west.
- The eye is calm; the eye wall has the fiercest winds and rain.
Traps to avoid
- NH cyclones turn anticlockwise; NH anticyclones clockwise — and the reverse in the SH.
- Horse latitudes (30°–35°) are high-pressure belts, not lows.
- No tropical cyclones form at the equator — Coriolis force is zero there.
- The monsoon is a seasonal wind, not a planetary wind.
- La Niña and El Niño are Pacific events, not Indian Ocean ones.
- SH westerlies are stronger because there is less land — not because Coriolis is stronger there.
- The occluded front lifts the warm air off the ground.
- Trough = tongue of low pressure; ridge = tongue of high; col = saddle between them (Figure 9.1).
- A falling barometer means stormy weather is coming.
How to strategise
Prelims — what UPSC asks
- Coriolis force and wind direction keep coming back: Coriolis facts (2024), direction around systems (2005), cyclones near the equator, and why SH westerlies are stronger (2011).
- Pressure belts and their names: doldrums (1980, 1983), horse latitudes (2007), sub-polar lows at 60–65° (2002), trade winds (1985), planetary vs seasonal winds (1991), westerlies and the Roaring Forties (1984, 2000, 2015).
- ENSO appears in every decade: weak monsoons (2002), Indonesian fires (1998), El Niño Modoki (2010), La Niña vs El Niño (2011). Know which ocean and which phase.
- Cyclones and storms: what a low on a weather map looks like (1990), barometer falls (1987, 2001), winds spiral in because air rises (2002), where rain falls in a cyclone (1999), cyclone eyes and jet streams (2020), why none form in the South Atlantic (2015), Willy-willy (1995), thunderstorms (2002, 2013), local winds matching (2001).
- Assertion–reason and statement pairs dominate — learn the reason behind each fact, not just the fact.
Mains — where it appears and how to answer
- GS1 cyclones: why they cluster in the South China Sea, Bay of Bengal and Gulf of Mexico (2014), naming (2013), sea-surface temperature (2024), the Fujiwhara effect (2026), IMD colour-coded warnings (2022).
- GS1 circulation: air masses and macro-climate (2016), El Niño and unusual weather (2014), local winds — Loo, Chinook, Foehn (2026).
- Answer frame: define → conditions or mechanism (draw the cell diagram, the cyclone cross-section or the front sketch) → distribution with world and India examples → impacts → mitigation (early warning, NDMA guidelines, mangroves, shelters).
- A labelled cross-section of a tropical cyclone, or Figure 9.6, earns marks in a 150-word answer — practise drawing them in under a minute.
Self-check
Tap an option. NCERT's own exercise questions come first; the rest test the figures and common confusions.
1. If the surface air pressure is 1,000 mb, the air pressure at 1 km above the surface will be:
2. The Inter Tropical Convergence Zone normally occurs:
3. The direction of wind around a low pressure in the northern hemisphere is:
4. Which one of the following is the source region for the formation of air masses?
5. Why do tropical cyclones not form right at the equator?
6. In a mature extra-tropical cyclone of the northern hemisphere, the warm sector lies:
7. Which statement about El Niño is correct according to NCERT?
UPSC questions from this chapter
Open any question to attempt it in the Question bank — your attempt is saved to your progress.