Atmospheric Circulation and Weather Systems

Why this chapter matters for UPSC: This is Chapter 9 of Fundamentals of Physical Geography in the current rationalised NCERT (2026-27 reprint); it was Chapter 10 before "Minerals and Rocks" was dropped. It explains why air moves, how the planetary pressure and wind belts arise, and how air masses, fronts and cyclones make day-to-day weather. Mains GS1 has asked directly about air masses, the formation and naming of tropical cyclones, IMD's cyclone warnings, El Niño and the monsoon winds.

Contemporary hook: India's 2026 south-west monsoon ended at 87% of its Long Period Average (759.4 mm; LPA 87 cm, 1971–2020 base), a deficient season and the fourth lowest since 2001. IMD's end-of-season report records that a weak El Niño developed in June, strengthened through the season and was strong by its end, while the Indian Ocean Dipole stayed neutral throughout. IMD's forecasts of April (92%) and May (90%) had both correctly called a below-normal season (IMD, Salient Features of the 2026 Southwest Monsoon Season, 30 September 2026). A Pacific reading feeding straight into India's harvest is this chapter at work.

🧠 First Principles — Read This First

Air has weight. The weight of the column of air above a place, per unit area, is atmospheric pressure. Where air is heated it expands and rises, so surface pressure falls; where air cools it sinks, so surface pressure rises. Air then flows from higher to lower pressure, and that horizontal flow is wind. A ceiling fan pushes air around a room in the same basic way: air moves from where it is pushed to where there is room for it.

Three forces shape surface wind. The pressure gradient force sets the wind going; the closer the isobars on a weather map, the stronger the wind. Friction with the ground slows it. The Coriolis force, caused by the Earth's rotation, turns moving air to the right in the northern hemisphere and to the left in the southern hemisphere. Because of this turning, air does not flow straight into a low; it spirals around it.

Repeated over the whole planet, these forces produce belts of high and low pressure and the trade winds, westerlies and polar easterlies between them. Within those belts, large bodies of air with uniform temperature and moisture, called air masses, move about. Where two unlike air masses meet, a front forms, and fronts and warm seas give rise to cyclones.

The same chapter explains the monsoon, cyclones and their warnings, El Niño, and the winter weather of India's north-west.

PART 1 — Quick Reference

Table 1: Pressure at height (NCERT Table 9.1, standard atmosphere)

Source: NCERT FoPG (2026-27), Table 9.1. In the lower atmosphere pressure falls by about 1 mb for every 10 m of height, though not at a constant rate. NCERT's exercise uses this rule: 1,000 mb at the surface gives about 900 mb at 1 km.

Table 2: Pressure belts and wind belts

Source: NCERT FoPG (2026-27), Ch 9. The belts are not fixed: they shift south in the northern winter and north in the northern summer, following the apparent movement of the Sun.

Table 3: Wind direction around pressure systems (NCERT Table 9.2)

Over a low, air converges at the surface and rises; over a high, air subsides from above and diverges at the surface.

Table 4: Air masses (NCERT)

Source: NCERT FoPG (2026-27), Ch 9. The "warm/cold" and "moist/dry" column follows NCERT's rule that tropical air masses are warm and polar ones cold, and that maritime sources supply moisture.

Table 5: Extra-tropical vs tropical cyclones (NCERT comparison)

Source: NCERT FoPG (2026-27), pp. 82–83. NCERT names an energy source only for tropical cyclones; the extra-tropical entry restates its description of warm air gliding over cold air. Tropical cyclones move west in the trade winds and usually recurve poleward and eastward later; storms of the north Indian Ocean commonly move west, north-west or north.

Table 6: IMD classification of cyclonic disturbances (North Indian Ocean)

Source: IMD criteria as presented by A.K. Das (IMD), Synoptic Aspects: Monitoring & Prediction of Cyclonic Storms, WMO training, New Delhi, April 2022. IMD defines categories in knots; km/h values are conversions, so boundary figures differ by 1 km/h between documents.

PART 2 — Concepts & Narrative

Atmospheric pressure

NCERT defines atmospheric pressure as the weight of a column of air of unit area from mean sea level to the top of the atmosphere. It is expressed in millibars (mb); average sea-level pressure is 1,013.2 mb. It is measured with a mercury barometer or an aneroid barometer. Air near the surface is denser because of gravity, so pressure is highest there and falls with height (Table 1).

The vertical pressure gradient is much stronger than the horizontal one, but it is almost balanced by gravity acting the other way. That is why we do not feel strong upward winds.

On weather maps, pressure is shown by isobars, lines joining places of equal pressure. Station pressure is first reduced to sea level so that the effect of altitude is removed and places at different heights can be compared. A low is enclosed by isobars with the lowest value at the centre; a high has the highest value at the centre.

Forces that control wind

Surface wind is the net result of three horizontal forces, with gravity acting downward.

• Pressure gradient force. The rate of change of pressure with distance. It is strong where isobars are close and weak where they are far apart.
• Frictional force. It reduces wind speed. It is greatest at the surface and its influence usually reaches 1–3 km. It is smallest over the sea.
• Coriolis force. The Earth's rotation deflects wind to the right in the northern hemisphere and to the left in the southern hemisphere. NCERT states that the deflection is greater when wind speed is higher, that the force is directly proportional to the angle of latitude, maximum at the poles and absent at the equator, and that it acts perpendicular to the pressure gradient force. In physics the force is 2Ωv·sin(latitude): it grows with speed and with the sine of the latitude (zero at the equator, greatest at the poles), and it acts "normal to the velocity", at right angles to the wind itself (American Meteorological Society, Glossary of Meteorology). It balances the pressure gradient force only in the geostrophic wind described below.

Rising air and convergence

Over a low, surface air converges and rises; over a high, air sinks from above and diverges at the surface. Rising air is needed for clouds and precipitation. NCERT lists the ways air is made to rise: convergence, eddies, convection currents, orographic uplift (air forced over mountains) and uplift along fronts. The next chapter, Water in the Atmosphere, builds the three rainfall types on these mechanisms.

General circulation of the atmosphere

The pattern of planetary winds is called the general circulation. NCERT names five controls: (i) latitudinal variation in heating; (ii) the pressure belts; (iii) the migration of the belts with the apparent path of the Sun; (iv) the distribution of continents and oceans; and (v) the rotation of the Earth.

NCERT describes three cells in each hemisphere:

1. Hadley cell (tropics). At the ITCZ, strong insolation drives convection and creates low pressure. The converging air rises to the top of the troposphere, up to about 14 km, and moves poleward. Air piles up near 30° N and S. Part of it sinks, both because of this accumulation and because it has cooled, and forms the subtropical high. Near the surface the air returns to the equator as the easterlies (trade winds), which converge again at the ITCZ.
2. Ferrel cell (middle latitudes). NCERT: "the circulation is that of sinking cold air that comes from the poles and the rising warm air that blows from the subtropical high". Read it with the pressure belts: air sinks at the subtropical high, blows poleward along the surface as the westerlies, and rises at the sub-polar low, where it meets the cold polar air.
3. Polar cell. Cold, dense air subsides near the poles and blows towards the middle latitudes as the polar easterlies.

The poleward transfer of heat from low to high latitudes keeps this circulation going. The circulation also drives the large, slow ocean currents, and the oceans in turn supply heat and water vapour to the air.

El Niño, the Southern Oscillation and ENSO

NCERT's account: warm water from the central Pacific drifts towards the South American coast and replaces the cool Peruvian current. This appearance of warm water off Peru is El Niño. It is linked with pressure changes between the central Pacific and Australia, called the Southern Oscillation. The two together are ENSO. In strong ENSO years the arid west coast of South America gets heavy rain, Australia has drought, India sometimes has drought, and China has floods. ENSO is monitored for long-range forecasting.

NCERT's IPE chapter on climate adds that the Southern Oscillation can be measured by the pressure difference between Tahiti (French Polynesia, East Pacific) and Darwin (northern Australia), and that the name El Niño ("Child Christ") comes from the current appearing around Christmas, which is summer in Peru.

La Niña is the opposite, cool phase of the same cycle (eastern and central Pacific cooler than normal). It is not described in the rationalised NCERT text.

The Indian Ocean Dipole (IOD) is a related see-saw of sea-surface temperature between the western and eastern parts of the tropical Indian Ocean. It is not in the NCERT chapter. A positive IOD (warmer west) tends to help the Indian monsoon and can offset an El Niño; both of the IOD's poles are in the Indian Ocean. IMD forecasts ENSO and the IOD as separate inputs (see the Contemporary hook above).

Seasonal winds: the monsoon

NCERT FoPG treats the monsoon as the strongest example of how the general circulation changes with the seasons, as the zones of maximum heating, pressure and wind shift, especially over South-east Asia. It refers the details to India: Physical Environment (IPE), Chapter 4. The points from that chapter that depend on this one are:

• Differential heating. In April–May the landmass north of the Indian Ocean heats strongly, and an intense low forms over north-west India. Pressure over the ocean to the south stays higher because water heats slowly.
• ITCZ shift. The low draws the south-east trades across the equator and helps the ITCZ move north. In July the ITCZ lies around 20°–25°N over the Gangetic plain, where it is called the monsoon trough.
• Coriolis turning. The southern-hemisphere trades cross the equator between 40°E and 60°E and are deflected to the right, so they blow from the south-west: the south-west monsoon. In winter the ITCZ moves south and the winds reverse to the north-east monsoon.
• Jet streams. The westerly jet stream lies over the north Indian plain, south of the Himalaya, in winter. Its withdrawal is linked to the ITCZ's northward shift. Only then does the easterly jet set in along about 15°N, and NCERT holds this easterly jet responsible for the "burst" of the monsoon.
• Onset. NCERT: the monsoon usually reaches the Kerala coast by 1 June, Mumbai and Kolkata between 10 and 13 June, and the whole country by mid-July. IMD revised the normal dates from 1 June 2020 (onset normals from 1961–2019 data): Kerala stays at 1 June, Mumbai and Kolkata are now 11 June, New Delhi 27 June, and the whole country is covered by 8 July (the old date was 15 July). In 2026 the monsoon reached Kerala on 4 June and covered the country on 9 July.

Local winds

Local winds come from differences in heating and cooling of surfaces over a day or a year.

• Land and sea breezes. By day the land heats faster than the sea. Air rises over the land, pressure there falls, and wind blows from sea to land: the sea breeze. At night the land cools faster, the gradient reverses, and the land breeze blows from land to sea.
• Valley and mountain breezes. By day the mountain slopes heat up, air moves upslope, and air from the valley flows up to replace it: the valley breeze. At night the slopes cool and dense air drains down into the valley: the mountain wind.
• Katabatic wind. Cold air from high plateaus and ice fields draining into valleys.
• Warm leeward wind. Moist air crossing a mountain range drops its moisture on the windward side. On the leeward side it descends, warms adiabatically and arrives dry and warm, and can melt snow quickly. NCERT does not name it. The US National Weather Service calls such winds foehn winds and uses chinook for the foehn on the lee of the Rocky Mountains (NWS glossary, "Chinook").

India's own local winds and storms of the hot season, from NCERT IPE Chapter 4:

Source: NCERT India: Physical Environment (2026-27), Ch 4, p. 35.

Air masses

When air stays over a uniform surface long enough, it takes on that surface's temperature and humidity. NCERT defines an air mass as a large body of air with little horizontal variation in temperature and moisture. The uniform surfaces where air masses form are source regions: vast oceans or vast plains. NCERT recognises five source regions and five air-mass types (Table 4).

NCERT's exercise asks which of these is a source region: the equatorial forest, the Himalaya, the Siberian plain or the Deccan Plateau. The answer is the Siberian plain, a vast uniform cold surface where continental polar air forms. Mountains and varied plateaus are not uniform enough.

Fronts

When two different air masses meet, the boundary zone between them is a front, and the process of forming one is frontogenesis. Fronts occur in the middle latitudes. They have steep gradients of temperature and pressure, bring abrupt changes of temperature, and force air to rise, which produces clouds and precipitation.

Western disturbances

NCERT IPE describes the winter western disturbances as shallow cyclonic depressions that form over the east Mediterranean Sea and travel east across West Asia, Iran, Afghanistan and Pakistan to north-west India, picking up moisture from the Caspian Sea and the Persian Gulf on the way. NCERT links their movement to the westerly jet stream. They bring light rain to Punjab, Haryana, Delhi and western Uttar Pradesh, which helps the rabi crop, and snow in the lower Himalaya, which feeds the rivers in summer. The rain decreases from west to east in the plains and from north to south in the mountains. NCERT gives Delhi's average winter rainfall as about 53 mm.

Tropical cyclones

Tropical cyclones are violent storms that form over tropical oceans and move to the coasts, causing destruction through violent winds, very heavy rain and storm surges. NCERT gives their regional names: cyclones in the Indian Ocean, hurricanes in the Atlantic, typhoons in the western Pacific and South China Sea, and willy-willies in western Australia.

Conditions for formation and intensification (NCERT):

1. A large sea surface with temperature above 27°C.
2. The presence of the Coriolis force.
3. Small variation in vertical wind speed (low vertical wind shear).
4. A pre-existing weak low or low-level cyclonic circulation.
5. Upper-level divergence above the sea-level system.

NCERT gives 27°C. NOAA's Hurricane Research Division gives a threshold of about 26.5°C, which must extend through a layer about 50 m deep (NOAA AOML, TC FAQ A16). Use NCERT's figure in an NCERT-based answer and add the NOAA value as the current scientific figure.

Energy and life cycle. The energy comes from condensation in the towering cumulonimbus clouds around the centre. A steady supply of moisture from the sea strengthens the storm. On reaching land the moisture supply is cut off and the storm dies out. The point where a cyclone crosses the coast is its landfall. NCERT notes that cyclones that cross 20°N generally recurve, and that these are more destructive.

Structure of a mature tropical cyclone (NCERT):

Storm surges flood the low coastal lands.

Thunderstorms and tornadoes

These are severe local storms: short-lived and confined to small areas, but violent.

• Thunderstorm. Caused by intense convection on moist, hot days. It is a well-grown cumulonimbus cloud producing thunder and lightning. A strong updraft of warm air makes the cloud grow taller and produces precipitation; later a downdraft brings cool air and rain to the ground. If the cloud reaches heights where temperatures are below freezing, hail forms and falls as a hailstorm. If there is not enough moisture, a thunderstorm can raise a duststorm.
• Tornado. From severe thunderstorms, a spiralling wind sometimes descends "like a trunk of an elephant with great force", with very low pressure at its centre, causing massive destruction along its path. Tornadoes occur mostly in the middle latitudes. A tornado over the sea is a waterspout.

NCERT ends the chapter by explaining that these storms are how the atmosphere adjusts to uneven energy distribution. Potential and heat energy are turned into kinetic energy, and the atmosphere returns to a stable state.

PART 3 — UPSC Integration

Mains frameworks

• Cyclone questions. Use NCERT's five formation conditions as the skeleton, the sea-surface temperature threshold (NCERT 27°C; NOAA 26.5°C to about 50 m depth) for questions on warming seas, and the IMD category table for questions on warnings.
• El Niño "Do you agree?". Agree partly. Use IMD's own table: 7 of 17 El Niño years had a deficit of 10% or more, 1997 and 1994 did not, and the 2002 drought came in a weak El Niño year. Add the IOD as a second control.
• Air masses. Define the air mass and source region, list the five NCERT types, then show the "macro-climatic" role: fronts and extra-tropical cyclones form where they meet, and western disturbances bring winter rain to north-west India.

Exam Strategy

Prelims fact-traps from this chapter

• Coriolis direction. Right in the northern hemisphere, left in the southern; zero at the equator, maximum at the poles. A cyclone (low) turns anticlockwise in the north, an anticyclone (high) clockwise. Questions often swap the two systems.
• Geostrophic wind blows parallel to the isobars, not across them, and only above the friction layer (2–3 km).
• Source regions. A uniform surface (ocean, Siberian plain, desert, ice sheet), not mountains.
• Front types. Four: cold, warm, stationary, occluded. An occluded front forms when the cold front overtakes the warm front.
• Direction of travel (NCERT). Extra-tropical cyclones move west to east; tropical cyclones move east to west, and recurve after crossing about 20°N.
• SST threshold. NCERT: above 27°C. An option of 26.5°C reflects NOAA's figure; read the question for which source it follows.
• Names. Typhoon: western Pacific and South China Sea. Willy-willy: western Australia. Hurricane: Atlantic.
• Western disturbances are steered by the westerly jet stream that lies south of the Himalaya in winter (NCERT IPE), not by the polar-front jet.
• IOD is an Indian Ocean phenomenon. Any statement placing one of its poles in the Pacific is wrong.

Mains question patterns

• "Why?" questions on where cyclones form: answer from the formation conditions and the Coriolis argument.
• "Do you agree?" on El Niño: needs both supporting and contrary data.
• Concept-plus-application (air masses; sea-surface temperature): define first, then apply to India.

Practice Questions

Practice (UPSC-pattern, not past papers). Questions 1 to 4 are NCERT's own exercise MCQs.

Prelims:

1. If the surface air pressure is 1,000 mb, the air pressure at 1 km above the surface will be:
   (a) 700 mb
   (b) 1,100 mb
   (c) 900 mb
   (d) 1,300 mb
   About 1 mb is lost for every 10 m of rise near the surface (NCERT).

2. The Inter Tropical Convergence Zone normally occurs:
   (a) near the Equator
   (b) near the Tropic of Cancer
   (c) near the Tropic of Capricorn
   (d) near the Arctic Circle

3. The direction of wind around a low pressure in the northern hemisphere is:
   (a) clockwise
   (b) perpendicular to isobars
   (c) anti-clockwise
   (d) parallel to isobars

4. Which one of the following is a source region for the formation of air masses?
   (a) the Equatorial forest
   (b) the Himalayas
   (c) the Siberian Plain
   (d) the Deccan Plateau

5. With reference to the Indian Ocean Dipole (IOD), consider the following statements: 1. It is a difference in sea-surface temperature between the western Indian Ocean and the eastern Pacific Ocean. 2. A positive IOD can offset the effect of an El Niño on the Indian monsoon. Which of the statements given above is/are correct?
   (a) 1 only
   (b) 2 only
   (c) Both 1 and 2
   (d) Neither 1 nor 2
   Both poles of the IOD are in the Indian Ocean.

6. Consider the following statements: 1. The Coriolis force is maximum at the equator. 2. Geostrophic winds blow parallel to the isobars. 3. Extra-tropical cyclones move from west to east. Which of the statements given above are correct?
   (a) 1 and 2 only
   (b) 2 and 3 only
   (c) 1 and 3 only
   (d) 1, 2 and 3

Mains (practice, 150 words each):

1. Discuss the factors that control the movement of air and explain the general circulation of the atmosphere. (NCERT Ex. 3, adapted)
2. Why do tropical cyclones form over warm seas and weaken after landfall? Distinguish them from extra-tropical cyclones.
3. Does El Niño explain most unusual monsoon years in India? Use IMD's record of El Niño years.

Short answers (NCERT Ex. 2, about 30 words): pressure is measured in millibars (hPa); station pressure is reduced to sea level so that the effect of altitude is removed and maps compare like with like. The trade winds blow from the north-east, not from the north, because the Coriolis force deflects them to the right. Geostrophic winds blow parallel to the isobars above the friction layer, where the pressure gradient and Coriolis forces balance. Land heats faster by day, so the sea breeze blows from sea to land; at night the land cools faster and the land breeze blows out to sea.

📦 Revision Capsule

Sources

• NCERT, Fundamentals of Physical Geography, Class XI, Chapter 9 "Atmospheric Circulation and Weather Systems", Reprint 2026-27: https://ncert.nic.in/textbook/pdf/kegy209.pdf
• NCERT, India: Physical Environment, Class XI, Chapter 4 "Climate", Reprint 2026-27: https://ncert.nic.in/textbook/pdf/kegy104.pdf
• IMD, Southwest monsoon rainfall (percentage departure in LPA) during El Niño years (accessed 1-Oct-2026): https://mausam.imd.gov.in/responsive/el_nino_ismr.php
• PIB (Ministry of Earth Sciences), Updated Long Range Forecast for the Southwest Monsoon Seasonal Rainfall during June–September, 2026, 29-May-2026: https://www.pib.gov.in/PressReleasePage.aspx?PRID=2266479&reg=3&lang=1
• WMO, El Niño weakens but impacts continue, 5-Mar-2024: https://public.wmo.int/node/22609
• NOAA AOML Hurricane Research Division, TC FAQ A16 (26.5°C threshold): https://www.aoml.noaa.gov/hrd/tcfaq/A16.html
• NOAA JetStream, Tropical Cyclone Introduction: https://www.noaa.gov/jetstream/tropical/tropical-cyclone-introduction
• US National Weather Service, Flight Environment training page (jet streams, katabatic winds): https://www.weather.gov/source/zhu/ZHU_Training_Page/winds/Wx_Terms/Flight_Environment.htm
• US National Weather Service glossary, "Chinook": https://forecast.weather.gov/glossary.php?word=chinook
• A.K. Das (IMD), Synoptic Aspects: Monitoring & Prediction of Cyclonic Storms, WMO training, New Delhi, April 2022 (IMD classification): https://severeweather.wmo.int/TCFW/NewDelhi_Training2022/4_Synoptic-Aspects-TCMonitoringPrediction-07-04-2022.pdf
• AMS Glossary of Meteorology, "Coriolis force": glossary.ametsoc.org.
• C.-P. Chang, C.-H. Liu and H.-C. Kuo, "Typhoon Vamei: An equatorial tropical cyclone formation", Geophysical Research Letters (2003): doi:10.1029/2002GL016365; NASA Earth Observatory, "A Rare Cyclone on the Equator", 9 May 2003: earthobservatory.nasa.gov.
• IMD, RSMC New Delhi, "Frequently Asked Questions on Tropical Cyclones" (classification table): rsmcnewdelhi.imd.gov.in.
• Deshpande, M. et al., "Changing status of tropical cyclones over the north Indian Ocean", Climate Dynamics (2021): https://doi.org/10.1007/s00382-021-05880-z