Temperature, Pressure Belts & Global Winds (UPSC Geography)

The Driving Logic: Unequal Heating Sets the Air in Motion

🎯 Exam priority: High-yield. This is one of UPSC's most dependable climatology grounds. Prelims has repeatedly mined it — the Coriolis force (2024), westerlies and western disturbances (2015), jet streams (2020), January isotherms (2024), the heat budget and atmospheric heating (2024), insolation (2023) and the link between trade winds/westerlies and ocean temperatures (2021). Go deepest on the Coriolis force, the seven pressure belts and the jet streams — these recur most.

The whole machinery follows one chain: the Sun heats the Earth unequally → unequal heating creates pressure differences → air flows from high to low pressure as wind, redistributing heat from the surplus tropics to the deficit poles so the planet as a whole keeps a steady temperature.

The driving logic: unequal heating sets the air in motion

• Short-wave in, long-wave out. The Earth receives almost all its energy from the Sun as insolation (incoming solar radiation, in short wavelengths), and radiates an equal amount back to space as long-wave terrestrial radiation — so over the long run it neither warms nor cools. The average insolation at the top of the atmosphere is the solar constant, 1.94 calories per sq. cm per minute.

• The atmosphere is heated from below, not above. The air is largely transparent to short-wave solar radiation, which mostly passes through to heat the ground; the heated Earth then re-radiates long-wave radiation that is absorbed by carbon dioxide and other greenhouse gases, warming the atmosphere indirectly. (This is exactly why the 2024 Prelims statement "the atmosphere is heated more by solar than by terrestrial radiation" is wrong — terrestrial radiation dominates.)

• The heat budget keeps the balance. Of 100 units of insolation, about 35 units are reflected back to space (the albedo — 27 from cloud-tops, 2 from snow/ice) and 65 units absorbed (14 by the atmosphere, 51 by the surface). The surface re-radiates its 51 units (17 straight to space, 34 to the atmosphere); the atmosphere then radiates 48 units out, so 17 + 48 = 65 units leave — exactly balancing the 65 received.

• The surplus–deficit engine. There is a net radiation surplus between 40° N and 40° S and a deficit near the poles. Winds (and ocean currents) carry this surplus heat poleward — which is why the tropics do not keep getting hotter and the poles do not freeze solid. Pressure belts and planetary winds are the visible machinery of that transfer.

Temperature: Distribution, Controls and Inversion

Before pressure and wind, you must read temperature — how it is distributed, what controls it, and the curious case when it runs backwards.

Temperature: distribution, controls and inversion

• Isotherms and the latitude rule. Temperature distribution is mapped with isotherms — lines joining places of equal temperature. Because insolation falls with latitude, isotherms generally run parallel to latitude. The deviation is sharper in January than in July, especially in the Northern Hemisphere, where the larger landmass makes the effects of continents and ocean currents more pronounced.

• Five controls of temperature. Air temperature at a place is set by latitude (insolation), altitude (temperature falls with height at the normal lapse rate of 6.5°C per 1,000 m, because the air is heated from below), distance from the sea (continentality — land heats and cools fast, sea slowly, so interiors have a large annual range), ocean currents and air masses (warm currents/air masses raise coastal temperature), and local aspect.

• The January isotherm pattern (a 2024 Prelims fact): isotherms bend north over oceans and south over continents. Over the North Atlantic the warm Gulf Stream and North Atlantic Drift push isotherms north (the ocean stays warm), while they swing sharply south over cold continental Europe and the Siberian plain — where continentality gives the world's highest annual range (over 60°C over north-eastern Eurasia). The least range, about 3°C, lies between 20° S and 15° N.

• Inversion of temperature. Normally temperature falls with height (the lapse rate); sometimes it is reversed, so air gets warmer with height — an inversion. The classic surface (radiation) inversion needs a long winter night, clear sky and still air: the ground radiates its heat away and by dawn is colder than the air above. Over polar areas inversion is normal all year.

• Effects of inversion. It promotes stability, trapping smoke and dust below the inversion layer and causing dense morning fog in winter (it lifts once the Sun warms the ground). In hills, air drainage sends cold dense night air sliding down the slope to pool in valley bottoms — which, by keeping the warmer air above, actually protects valley-floor plants from frost.

Atmospheric Pressure and the Seven Pressure Belts

Unequal heating shows up first as pressure — and the world organises itself into seven great belts of high and low pressure.

Atmospheric pressure and the seven pressure belts

• What pressure is. Atmospheric pressure is the weight of a column of air over a unit area from sea level to the top of the atmosphere, measured in millibars (mb) with a barometer. Average sea-level pressure is 1,013.2 mb. Pressure falls with height — about 1 mb for every 10 m of ascent near the surface — because the air thins. Horizontal pressure is mapped with isobars (lines of equal pressure), reduced to sea level so altitude does not distort the comparison.

• Seven belts, four kinds. From equator to pole the surface pressure organises into seven belts (alternating low and high across the two hemispheres): the equatorial low (ITCZ) at the equator; the subtropical highs at about 30° N and 30° S; the sub-polar lows at about 60° N and 60° S; and the polar highs at the poles.

• Thermal vs dynamic origin — the key distinction. The equatorial low is thermally induced (intense heating makes air expand, rise and create low pressure) and the polar high is thermal too (extreme cold makes air dense and heavy). But the subtropical high and sub-polar low are dynamically induced — they arise from the descent and ascent of air driven by circulation and the Earth's rotation, not directly from local temperature. (Air piling up and sinking at 30° makes the subtropical high; converging rising air makes the sub-polar low.)

• The belts shift with the Sun. The pressure belts are not permanent: they oscillate with the apparent movement of the Sun — moving north in the northern summer and south in the northern winter (roughly 5°). This migration is what swings the wind belts and drives the monsoon and the seasonal rainfall of places like the Mediterranean.

The Forces on Wind: Pressure Gradient, Friction and Coriolis

Air would simply rush straight from high to low pressure — but the spinning Earth bends it, and three forces together decide where wind goes and how fast.

The forces on wind: pressure gradient, friction and the Coriolis force

• Pressure gradient force. Wind is air moving from high to low pressure. The pressure gradient is the rate of change of pressure with distance; it is strong where isobars are close together (steep gradient, fast wind) and weak where they are far apart. This force acts at right angles to the isobars and is the prime mover of all wind.

• Frictional force. Friction affects wind speed, slowing it near the ground. It is greatest at the surface, extends up to about 1–3 km, and is least over the sea. Above about 2–3 km the air is essentially friction-free.

• The Coriolis force — go deep here. Caused by the Earth's rotation and described by a French physicist in 1844, it deflects wind to the RIGHT in the Northern Hemisphere and to the LEFT in the Southern (Ferrel's law). It is directly proportional to wind velocity and to the angle of latitude — maximum at the poles, zero at the equator (the 2024 Prelims double-statement). It acts perpendicular to the wind (and so to the pressure-gradient force), so instead of crossing isobars head-on, winds end up blowing around pressure systems.

• Why no cyclones at the equator. Because the Coriolis force is zero at the equator, there is nothing to set the air spinning; the wind blows straight across the isobars and a low simply gets filled rather than intensified. This is precisely why tropical cyclones do not form right at the equator (a recurring exam point).

• Geostrophic wind. In the upper atmosphere (about 2–3 km up), free of friction, when isobars are straight the pressure-gradient force is exactly balanced by the Coriolis force, and the resultant wind blows parallel to the isobars — the geostrophic wind. Around a low this gives cyclonic circulation (anticlockwise in the NH, clockwise in the SH) and around a high anticyclonic circulation (clockwise NH, anticlockwise SH).

Planetary Winds and the Three-Cell Circulation

Put the belts and the Coriolis deflection together and you get the great planetary winds — the steady global currents of air that ride between the pressure belts.

Planetary winds and the three-cell circulation

• The general circulation. The steady pattern of planetary winds is the general circulation of the atmosphere, organised into three cells per hemisphere — the Hadley, Ferrel and Polar cells. It transfers heat from low to high latitudes and even sets the great ocean currents in motion.

• Trade winds (the Hadley cell). Air rises at the heated ITCZ (convection lifting it to the top of the troposphere, about 14 km), flows poleward aloft, sinks at the subtropical high near 30°, and returns to the equator at the surface as the trade winds. Coriolis deflection makes them the north-east trades in the NH and south-east trades in the SH (they blow towards the equator but are bent westward) — which is why, in the tropics, they pile warm water on the western side of oceans (Prelims 2021).

• Westerlies (the Ferrel cell). Between the subtropical high and the sub-polar low (roughly 30°–60° in EACH hemisphere) blow the westerlies — from the south-west in the NH, north-west in the SH. (Prelims 2015: they blow 30°–60° in each hemisphere, NOT "30° N–60° S".) They are strong and persistent in the SH ("Roaring Forties"), carry the temperate cyclones, and bring the western disturbances that give North-West India its winter rain. In the temperate zone they warm the eastern side of oceans (Prelims 2021).

• Polar easterlies (the Polar cell). Cold dense air subsides at the polar high and blows out towards the sub-polar low as the polar easterlies (deflected from the east). Where they meet the warm westerlies at about 60°, the polar front forms — the birthplace of temperate (extra-tropical) cyclones.

• Seasonal shift. Because the belts migrate with the Sun, the wind belts shift too, modifying circulation by season — the monsoon over South-East Asia is the most dramatic example.

Local Winds, Named Winds and the Jet Streams

Beyond the global winds lie the everyday breezes of a coastline or a valley, the violent named winds of particular regions, and the high-altitude rivers of air that steer the world's weather.

Local winds, named winds and the jet streams

• Land and sea breezes (the daily reversal). Land heats and cools faster than the sea. By day the land is warmer → low pressure over land → the cool SEA BREEZE blows from sea to land; by night the land is cooler → the LAND BREEZE blows from land to sea. These moderate coastal temperatures.

• Mountain and valley breezes. By day the heated slopes pull air up as the VALLEY BREEZE (anabatic); by night the cooled dense air drains down as the MOUNTAIN WIND (katabatic). Draining cold air from plateaus and ice-fields is the katabatic wind.

• Named local winds. Hot/dry or warm winds with regional names recur in Prelims: the Loo (hot, dry summer wind of northern India — an example of advection); the Chinook ("snow-eater", a warm dry wind down the leeward Rockies); the Foehn (the same kind of warm leeward wind in the Alps — air sheds its moisture climbing the windward side and is warmed adiabatically on descent); and the cold dry Mistral (which blows down the Rhône valley to the Mediterranean in France).

• Jet streams — go deep here. Jet streams are fast, narrow ribbons of westerly wind near the top of the troposphere (around 9–14 km). The two main ones are the polar-front jet stream (over the polar front, ~60°, strongest in winter) and the subtropical jet stream (~30°, over the subtropical high). They occur in BOTH hemispheres (Prelims 2020 statement "only in the Northern Hemisphere" is wrong).

• Why jet streams matter. They steer surface cyclones and air masses, govern temperate-zone weather, and in India the seasonal march of the subtropical jet stream helps trigger the onset and withdrawal of the monsoon (its retreat north of the Himalaya allows the monsoon to burst), while the polar-front jet guides the western disturbances that bring winter rain. They are also exploited by aircraft for fuel-saving eastward flight.

Why this matters for UPSC

• This single chain — unequal heating → pressure belts → planetary winds → jet streams — underlies dozens of Prelims questions: Coriolis (2024), westerlies and their latitude range (2015), jet streams (2020), January isotherms (2024), atmospheric heating/heat budget (2024), insolation (2023), and the trade-wind/westerly control of ocean temperatures (2021). For Mains it underpins monsoon, cyclone, El Niño and temperature-inversion answers. Master the thermal vs dynamic origin of the belts and the Coriolis rule (right in NH, left in SH; max at poles, zero at equator) and most of these answer themselves.

Further Reading

• Standard NCERT-level geography textbooks and reference books, plus a good atlas (any UPSC reading list).

Test Yourself: Practice Questions & PYQs

Test yourself on temperature distribution and inversion, the seven pressure belts and their thermal/dynamic origin, the Coriolis force and geostrophic wind, the planetary winds, named local winds and the jet streams. Attempt each before revealing the answer.

Practice Questions

Q1. The subtropical high-pressure belts (around 30° N and 30° S) are best described as:

• (a) Caused by the convergence of trade winds

• (b) Thermally induced by intense local heating

• (c) Permanent belts that never shift

• (d) Dynamically induced, formed by descending air

Q2. The Coriolis force is:

• (a) Maximum at the equator and zero at the poles

• (b) Maximum at the poles and zero at the equator

• (c) Independent of wind speed

• (d) The same at all latitudes

Q3. The normal lapse rate — the rate at which temperature falls with height — is about:

• (a) 6.5°C per 100 m

• (b) 6.5°C per 1,000 m

• (c) 10°C per 1,000 m

• (d) 1°C per 1,000 m

Q4. The trade winds blow:

• (a) Parallel to the isobars in the upper atmosphere

• (b) From the subtropical highs towards the equatorial low, deflected as NE (NH) and SE (SH)

• (c) From the poles towards 60° as easterlies

• (d) From the equator towards the poles as westerlies

Q5. A temperature inversion (the normal lapse rate reversed) is MOST favoured by:

• (a) A long winter night with clear skies and still air

• (b) A summer day over the open ocean

• (c) Strong daytime convection

• (d) A hot, cloudy, windy afternoon

Q6. When, in the upper atmosphere with straight isobars, the pressure-gradient force is exactly balanced by the Coriolis force, the resulting wind that blows parallel to the isobars is the:

• (a) Katabatic wind

• (b) Anabatic wind

• (c) Trade wind

• (d) Geostrophic wind

Q7. Which of the following is a warm, dry wind that descends the leeward side of a mountain range?

• (a) Polar easterlies

• (b) Loo of the Indian plains

• (c) Chinook (and the Foehn of the Alps)

• (d) Mistral

Q8. Consider the seven pressure belts from equator to pole. Which sequence is correct?

• (a) Equatorial low → subtropical high → sub-polar low → polar high

• (b) Subtropical high → equatorial low → polar high → sub-polar low

• (c) Equatorial high → subtropical low → sub-polar high → polar low

• (d) Polar high → sub-polar low → subtropical low → equatorial high

UPSC Previous Year Questions (PYQs)

These are genuine UPSC Prelims questions on this theme — proof of exactly how it is tested.

Q9. Consider the following statements: (1) The winds which blow between 30°N and 60°S latitude throughout the year are known as westerlies. (2) The moist air masses that cause winter rains in the North-Western region of India are part of westerlies. Which of the statements given above is/are correct? (UPSC Prelims 2015)

• (a) 1 only

• (b) Neither 1 nor 2

• (c) Both 1 and 2

• (d) 2 only

Q10. Consider the following statements: (1) Jet streams occur in the Northern Hemisphere only. (2) Only some cyclones develop an eye. (3) The temperature inside the eye of a cyclone is nearly 10°C lesser than that of the surroundings. Which of the statements given above is/are correct? (UPSC Prelims 2020)

• (a) 1 and 3 only

• (b) 1 only

• (c) 2 and 3 only

• (d) 2 only

Q11. Consider the following statements: (1) In the tropical zone, the western sections of the oceans are warmer than the eastern sections owing to the influence of trade winds. (2) In the temperate zone, westerlies make the eastern sections of oceans warmer than the western sections. Which of the statements given above is/are correct? (UPSC Prelims 2021)

• (a) Neither 1 nor 2

• (b) 2 only

• (c) Both 1 and 2

• (d) 1 only

Q12. With reference to 'Coriolis force', which of the following statements is/are correct? (1) It increases with increase in wind velocity. (2) It is maximum at the poles and is absent at the equator. Select the answer using the code given below. (UPSC Prelims 2024)

• (a) 1 only

• (b) 2 only

• (c) Neither 1 nor 2

• (d) Both 1 and 2

Q13. Which of the following is/are correct inference/inferences from isothermal maps in the month of January? (1) The isotherms deviate to the north over the ocean and to the south over the continent. (2) The presence of cold ocean currents Gulf Stream and North Atlantic Drift make the North Atlantic Ocean colder and the isotherms bend towards the north. Select the answer using the code given below. (UPSC Prelims 2024)

• (a) Both 1 and 2

• (b) Neither 1 nor 2

• (c) 1 only

• (d) 2 only

Q14. Consider the following statements: Statement-I: The atmosphere is heated more by incoming solar radiation than by terrestrial radiation. Statement-II: Carbon dioxide and other greenhouse gases in the atmosphere are good absorbers of long-wave radiation. Which one of the following is correct in respect of the above statements? (UPSC Prelims 2024)

• (a) Both statements are correct and II explains I

• (b) Statement-I is incorrect, but Statement-II is correct

• (c) Both statements are correct but II does not explain I

• (d) Statement-I is correct, but Statement-II is incorrect

Mains Practice Questions

Use these to frame full-length answers. You don't have to answer one exactly — they show the angles UPSC tests, so let them guide which points you cover.

• Explain the phenomenon of temperature inversion and its effects. (UPSC Mains 2013)

• Account for the seven global pressure belts, distinguishing their thermal and dynamic origins, and explain why they shift seasonally.

• Discuss the Coriolis force and explain how it, together with the pressure-gradient and frictional forces, governs the direction and speed of wind.

• Describe the three-cell model of the general circulation and the planetary winds (trades, westerlies, polar easterlies) it produces.

• What are jet streams? Explain their types and their significance for global weather and the Indian monsoon.

UPSC Mains — Previous Years’ Questions on this topic (last 10 years)

These are the actual GS Mains questions UPSC has asked on this theme — real proof of how, and when, it is tested. Try to write a full answer to each.

• 2013: Explain the phenomenon of 'temperature inversion' (and its effects).

• 2014: Most of the unusual climatic happenings are explained as an outcome of the El-Niño effect. Do you agree?

• 2013: Major hot deserts in the northern hemisphere are located between 20–30 degrees north and on the western side of the continents. Why?