Pressure Belts of the Earth – Geography UPSC

• Atmospheric (air) pressure is the weight of the column of air above a unit area of the earth’s surface. It acts in all directions and is greatest at sea level.
• Pressure is unevenly distributed over the globe. Differences of pressure set the air moving as wind, so the pattern of pressure is the framework for the planetary winds, weather systems and rainfall belts.
• On a uniform, rotating earth the pressure would be arranged in seven alternating belts of high and low pressure. Land and sea break these belts into cells, and the whole pattern shifts north and south with the sun.

Air Pressure

Measuring Air Pressure

• Gas law: pressure = density × temperature × a constant, so a change in either density or temperature changes the pressure.
• Standard sea-level pressure is 1013.25 hectopascals (hPa) at 15 °C, equal to 760 mm or 29.92 inches of mercury.
  • 1 millibar (mb) = 1 hPa, roughly the weight of one gram per square centimetre; the whole atmosphere presses down with about 1 kg per cm².
• Instruments:
  • Mercury barometer, devised by Evangelista Torricelli (1643); its standard form is Fortin’s barometer
  • Aneroid barometer, the barograph (a self-recording aneroid), the altimeter, and the very sensitive microbarograph
  • Readings are reduced to sea level (corrections for elevation, temperature and gravity) so that stations can be compared on one map.
• Range on earth: sea-level pressure usually lies between about 980 and 1035 hPa.
  • Highest: 1083.8 hPa at Agata, Siberia (31 December 1968), under the winter Siberian High
  • Lowest: 870 hPa in the eye of Typhoon Tip (12 October 1979), western Pacific
  • The 1999 Odisha super cyclone brought the pressure down to about 912 hPa.
• Pressure and weather: the tendency matters more than the reading. Steadily rising pressure means fair, settled weather; steadily falling pressure means cloud, rain and unsettled weather.

Factors Controlling Pressure

• Thermal factors:
  • Heated air expands, its density falls and pressure falls; cooled air contracts, density rises and pressure rises.
  • The equatorial low and the polar highs are thermal in origin.
• Dynamic factors:
  • Air piling up aloft and sinking raises surface pressure; air diverging aloft and rising lowers it. Both are governed by the pressure gradient and the earth’s rotation (the Coriolis force).
  • The subtropical highs and the subpolar lows are dynamic in origin.
• Water vapour: moist air is lighter than dry air (a water-vapour molecule is lighter than the nitrogen and oxygen it replaces), so humid air has lower pressure.
• Altitude: pressure falls rapidly with height.

Vertical Distribution of Pressure

• The lower layers carry the weight of all the air above and are compressed, so they are densest and exert the highest pressure.
• Pressure falls rapidly with height, by about 1 hPa for every 8–10 m near the surface, and roughly halves every 5.5 km.
  • On the summit of Mount Everest the pressure is only about one-third of that at sea level; at Leh (about 3,500 m) it is about two-thirds, which is why visitors and troops need acclimatisation against altitude sickness.
• The rate of fall is not uniform: it varies with temperature, water vapour and gravity.
• The large vertical pressure gradient is almost exactly balanced by gravity (hydrostatic balance), so it does not produce strong upward winds.

Isobars and the Pressure Gradient

• Isobars are lines joining places of equal (sea-level) pressure, usually drawn every 2 or 4 hPa on weather charts.
• The pressure gradient is the rate of change of pressure per unit horizontal distance, measured at right angles to the isobars (in hPa per 100 km or per degree of latitude). It is also called the barometric slope.
  • Closely spaced isobars mean a steep gradient and strong winds; widely spaced isobars mean a gentle gradient and light winds.
• Basic pressure patterns on a chart:

• Cyclones and anticyclones differ in size and intensity: anticyclones are larger and gentler, cyclones smaller and more intense.
• Indian example: in May–June an intense heat low forms over north-west India and Pakistan, while pressure stays higher over the Indian Ocean. This gradient draws in the south-west monsoon; in winter the gradient reverses.

World Pressure Belts

• On the earth’s surface there are seven pressure belts: the equatorial low, two subtropical highs, two subpolar lows and two polar highs. Except the equatorial low, they form matching pairs in the two hemispheres.
• This ideal zonal pattern assumes a uniform surface. In reality:
  • In the northern (land) hemisphere, continents break the belts into separate cells, elongated east–west.
  • In the southern (water) hemisphere, the belts are nearly continuous; the subpolar low around Antarctica is the only truly zonal belt on earth.

Equatorial Low (Doldrums)

• Location: about 5° N to 5° S, but its mean position lies north of the equator, with the thermal equator.
• Origin: thermal. The overhead sun heats the surface intensely all year; warm, moist air expands and rises in convection currents.
  • Rising moist air releases huge amounts of latent heat, which powers the global circulation.
• Features:
  • Pressure is below 1013 hPa throughout, and lower over the eastern hemisphere.
  • The trade winds of both hemispheres converge here, so it is the Inter-Tropical Convergence Zone (ITCZ).
  • Surface winds are light and variable with frequent calms, hence doldrums: sailing ships were becalmed here for days.
  • The weather is hot, humid and oppressive, with afternoon thunderstorms from towering cumulonimbus.
  • The diurnal variation of pressure is more marked here than anywhere else.
• Indian example: in summer the ITCZ moves over north India as the monsoon trough, running from north-west India to the head of the Bay of Bengal.

Subtropical Highs (Horse Latitudes)

• Location: about 25° to 35° N and S.
• Origin: dynamic. Air that rose at the equator flows poleward aloft and subsides here because:
  • it cools by radiation aloft, grows dense and sinks
  • the Coriolis force, increasing with latitude, turns the poleward flow into a westerly flow near 25–30°, so air piles up aloft and descends
• Features:
  • Sinking air is warmed by compression and is dry, so skies are clear and rain is scarce. The hot deserts (Sahara, Arabian, Thar, Kalahari, Atacama, Australian) lie in this belt, on the western sides of continents.
  • Winds are light and variable. Sailing ships becalmed here are said to have thrown horses overboard to save water, hence horse latitudes.
  • The belt is broken into oceanic cells (centres of action): the Azores–Bermuda, North Pacific (Hawaiian), South Atlantic (St Helena), South Pacific and Mascarene (South Indian Ocean) highs.
  • The cells are stronger on their eastern sides, where subsidence and a low trade-wind inversion keep the air stable; their western sides are weaker, humid and stormy.
• Indian example: a strong Mascarene High strengthens the cross-equatorial flow that becomes the south-west monsoon.
• The trade winds blow from these highs towards the equator, and the westerlies towards the subpolar lows.

Subpolar Lows

• Location: about 60° to 65° N and S (shifting between 50° and 70°).
• Origin: dynamic. The westerlies from the subtropical highs and the polar easterlies from the polar highs converge; the warm, light westerly air rises over the cold polar air along the polar front.
  • Temperature cannot explain this belt, since it is cold; the low is produced by convergence and frequent cyclones.
• Features:
  • It is the zone of temperate cyclones and stormy weather, especially in winter.
  • In the southern hemisphere it forms an unbroken belt around Antarctica (the circumpolar trough).
  • In the northern hemisphere it survives as two oceanic cells, the Icelandic Low and the Aleutian Low, which deepen in winter when the oceans are much warmer than the frozen continents.

Polar Highs

• Location: about 70° to 90° N and S.
• Origin: thermal. Extremely low temperatures make the air cold, dense and subsiding.
• Features:
  • The highs are shallow; the Antarctic High, over a high, ice-covered plateau, is far stronger and more permanent than the Arctic High, which is weak and centred near Greenland and the Canadian islands rather than the pole.
  • Polar easterlies blow out of them towards the subpolar lows.
  • These are ice-cap regions with very little precipitation: polar deserts.

Pressure Belts and the Atmospheric Circulation

The Coriolis Effect

• The Coriolis force deflects moving air to the right in the northern hemisphere and to the left in the southern hemisphere; it is zero at the equator and greatest at the poles.
• It sets the number of circulation cells a planet’s atmosphere divides into: the faster a planet spins and the larger it is, the stronger the deflection and the more cells form.
  • Earth divides into three cells in each hemisphere.
  • Jupiter, about 11 times Earth’s diameter and spinning once in under 10 hours, has many alternating bands.

The Three-Cell Circulation

• Atmospheric circulation is the large-scale movement of air which, with ocean currents, carries surplus heat from the tropics to the poles. Each pressure belt sits at a rising or sinking limb of a circulation cell:
  • Hadley cell (named after George Hadley, 1735): air rises at the equatorial low, flows poleward aloft, sinks at the subtropical highs and returns as the trade winds. It is thermally direct, driven by equatorial heating and latent heat.
  • Ferrel cell (after William Ferrel, 1856): between about 30° and 60°, surface air flows poleward as the westerlies and rises at the subpolar low. It is thermally indirect, a weaker zone driven by the cells on either side and by travelling cyclones.
  • Polar cell: cold air sinks at the poles and flows out as the polar easterlies, rising again at about 60°.
• The jet streams blow at the tropopause above the cell boundaries: the subtropical jet over the Hadley–Ferrel boundary and the polar-front jet over the Ferrel–Polar boundary. The waves of the polar jet steer the cyclones and fronts of the middle latitudes.

Walker Circulation

• Besides the north–south (latitudinal) cells, the tropics have east–west (longitudinal) cells. Their cause is the contrast in heating between land and sea and between warm and cool parts of the ocean: water absorbs much heat but its temperature changes little.
• The Walker circulation of the equatorial Pacific (after Gilbert Thomas Walker, who described the Southern Oscillation in 1924):
  • High pressure over the cool eastern Pacific (off Peru–Chile) and low pressure over the warm western Pacific (Indonesia–Australia)
  • Easterly trade winds at the surface push warm water west; air rises with heavy rain over Indonesia and sinks over the dry eastern Pacific.
  • Off Peru, cold, nutrient-rich water upwells to replace the surface water. It feeds plankton, the anchoveta fishery and the sea birds whose droppings form guano.

• El Niño: when the trade winds weaken, warm water drifts east and replaces the cool Peru Current. The see-saw of pressure between the eastern Pacific and Australia–Indonesia is the Southern Oscillation; together they form ENSO.
  • Effects: heavy rain on the arid coast of Peru, drought in Australia and Indonesia, and often a weak Indian monsoon.
  • Indian example: the 2026 south-west monsoon ended 12.6% below normal, the weakest since 2015, largely because of strengthening El Niño conditions.

Seasonal Shifting of Pressure Belts

• The pressure belts are tied to the sun: they move north in July and south in January with the overhead sun.
  • The shift is about 5–10° of latitude, larger over continents than over oceans, and lags the sun by one to two months.
  • The shift is greater in the northern hemisphere, where land is extensive, and smaller in the southern, where water predominates.
• Land and sea modify the belts: in winter the continents are colder than the oceans and develop highs; in summer they are warmer and develop lows. The oceans do the reverse.

Pressure Belts in July

• The thermal equator and the ITCZ move well north of the equator: up to about 20° N in North Africa and beyond the Tropic of Cancer over Asia.
• Heat lows form over north-west India–Pakistan, Arabia, Asia Minor, North Africa and the south-western USA. The low over Asia merges with the equatorial trough, and the subpolar and equatorial lows join over the continents.
• The subtropical highs survive only over the oceans and strengthen: the Azores–Bermuda High and the Pacific High.
• In the southern hemisphere (winter), the subtropical high belt becomes more continuous, with a high over Australia.

Pressure Belts in January

• The ITCZ moves south of the equator, deepest over the heated southern continents, where lows form over Australia, southern Africa and South America.
• The cold northern continents develop strong highs: the Siberian High, the most intense on earth, and the weaker Canadian (North American) High.
• The Icelandic and Aleutian Lows deepen over the relatively warm northern oceans, while the Azores and Pacific Highs weaken slightly.
• The southern subtropical highs lie at about 30°–40° S as cells over the three oceans; the circumpolar low around Antarctica stays continuous.

Effects of the Shift

• Transitional zones come under different belts in different seasons:
  • 5°–15°: the ITCZ brings summer rain and the trade winds a dry winter, giving the wet-and-dry savanna and monsoon climates.
  • 30°–40° on the western sides of continents: the subtropical high brings dry summers and the westerlies rainy winters: the Mediterranean climate.
  • 60°–70°: polar air in winter, westerlies and cyclones in summer.
• Indian example: the Indian monsoon follows the northward march of the ITCZ in summer, while in winter the southward-shifted westerlies bring western disturbances and rain or snow to north-west India.

Pressure Aloft and Recent Changes

• The cellular pattern of surface pressure fades with height. By the middle troposphere (about 3 km), pressure simply falls from the equator towards the poles, and the gradient is steepest over the middle latitudes in winter.
• Tropical widening: observations since the late 1970s show the tropical belt and the subtropical highs expanding poleward. The IPCC Sixth Assessment Report (2021) finds that mid-latitude storm tracks have likely shifted poleward in both hemispheres since the 1980s, which tends to dry subtropical margins such as the Mediterranean.