Pressure belts

Also called: Global pressure belts · Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"

Meaning

Pressure belts (also called global pressure belts) are zones of fairly even high or low air pressure that run east–west around the Earth, roughly along lines of latitude. There are seven: the equatorial low near the equator, a subtropical high near 30°–35° N and S [5], a subpolar low near 60° N and S, and a polar high at each pole. Together they move about 5–10° north and south with the apparent movement of the Sun.

They matter because they set the direction of the planetary winds and decide where the world's rainforests, hot deserts, stormy mid-latitude coasts and cold deserts are found. Their seasonal shift explains the Mediterranean climate and the Indian monsoon.

Explanation

How pressure belts are shown and measured

• Isobar: a line on a map that joins places with the same air pressure (iso = equal, baros = weight).
• Reduction to sea level: air pressure falls as you go higher.
• Every station's reading is first converted to what it would be at sea level.
• Only then does the map show real horizontal differences in pressure.

• Mean sea-level pressure is about 1,013.2 mb (millibar).

• Pressure gradient (how fast pressure changes over a distance) pushes air from high pressure to low pressure.

• Isobars close together → steep gradient → strong wind.

• Isobars far apart → gentle gradient → light or calm wind.

• The world pattern of belts appears on NCERT's sea-level pressure maps: Fig. 9.2 (January) and Fig. 9.3 (July).

The seven belts: thermal and dynamic origin

• From the equator to each pole there are four belts. That gives seven in all, because the equatorial low is shared by both hemispheres.

Belt	Latitude	Origin	How it forms	Name / notes
Equatorial low	0–5° N/S	Thermal	Strong sun heats the air → air expands and rises	Doldrums; ITCZ lies here; heavy convectional rain
Subtropical high	~30° N/S (30–35°) [5]	Dynamic	Hadley-cell air piles up, cools and sinks	Horse latitudes; light winds, clear skies [5]; hot deserts below
Subpolar low	~60° N/S (50–60°) [2]	Dynamic	Warm westerlies meet cold polar easterlies → warm air forced up along the polar front	Icelandic and Aleutian lows; stormy
Polar high	90° N/S	Thermal	Very cold, dense air sinks over ice	Small, shallow highs; source of polar easterlies

• Thermal origin: the pressure is caused directly by temperature.
• Heat makes a low (the equator).

• Cold makes a high (the poles).

• Dynamic origin: the pressure is caused by air movement and Earth's rotation, not by local temperature.

• The 30° belt is hot but still has high pressure.

• The 60° belt is cold but still has low pressure.

• ITCZ (Inter Tropical Convergence Zone): the low-pressure belt near the equator where the trade winds of the two hemispheres meet and are forced to rise.

• Doldrums: the calm zone at the equatorial low. Air here mostly moves upward, so there is little wind at the surface.
• Horse latitudes: the calm subtropical high belts at about 30°–35° N and S [5].

Why the subtropical high forms: the Hadley cell

• Air rises at the equator and moves towards the poles:
• High in the sky, the rising air flows towards the poles [2].
• On the way it cools by giving off heat.

• The Coriolis force (the turning effect of Earth's rotation, which bends moving air to the right in the Northern Hemisphere and to the left in the Southern) bends it until it moves almost east–west.

• The air piles up near 30° and sinks:

• The air can no longer move further towards the pole, so it piles up near 30°.
• This extra weight of air sinks, and the result is a dynamic high.

• Part of the sinking air flows back to the equator as the trade winds. This closes the loop called the Hadley cell [2].

• Hadley cell: the low-latitude circulation loop. Air rises at the equator, moves towards the poles high up, sinks near 30° and flows back to the equator near the ground. It is named after George Hadley (1735) [2].

• Why the belt is dry:
• Subsidence (air sinking slowly over a large area) squeezes and warms the air, so clouds cannot form.
• A temperature inversion (a layer where air gets warmer with height instead of colder) acts like a lid and stops moist air from rising.

• Result: dry belts and hot deserts around 30° N/S [2].

• The three-cell circulation is made up of the Hadley, Ferrel and polar cells [2].

Belts break into cells, and belts shift with the Sun

• Breaking into cells:
• The Northern Hemisphere has large landmasses. Land heats and cools much faster than sea.
• So the continuous belts split into separate cells called centres of action.

• The Southern Hemisphere is mostly ocean, so its belts stay more continuous. Its subpolar low forms an almost unbroken trough around Antarctica.

• Shifting of belts:

• The Sun is overhead at the Tropic of Cancer (23.5° N) around 21 June and at the Tropic of Capricorn (23.5° S) around 22 December.
• The heat equator and the ITCZ follow the Sun, and the other belts move with them.
• The belts move about 5–10° north in the northern summer (June) and south in the northern winter (December).

Where it occurs

• Semi-permanent cells (centres of action):

Cell	Type	Location	Season
Azores–Bermuda high	Subtropical high	North Atlantic	All year; stronger in summer
Hawaiian (North Pacific) high	Subtropical high	North-east Pacific	All year; stronger in summer
Icelandic low	Subpolar low	North Atlantic, between Iceland and southern Greenland [7]	Deepest in winter
Aleutian low	Subpolar low	Gulf of Alaska, near the Aleutian Islands [6]	Winter; moves north and almost disappears in summer [6]
Siberian high	Thermal high (land)	Northern Asia	Winter
Asian (Indian) thermal low	Thermal low (land)	North-west India, Pakistan, West Asia	Summer

• January map (Fig. 9.2): a strong Siberian high over Asia, the Aleutian low over the North Pacific and the Icelandic low over the Atlantic [8].
• July map (Fig. 9.3): the Siberian high is replaced by the deep Asian thermal low. The Azores–Bermuda and Hawaiian highs grow and spread.
• Hot deserts under the subtropical highs (west sides of continents, 20–30°):
• Northern Hemisphere: Sahara (North Africa), Arabian (Saudi Arabia, West Asia), Thar (Rajasthan in India and Sindh in Pakistan), Sonoran and Mojave (USA–Mexico) [2].
• Southern Hemisphere: Kalahari with Namib (Botswana, Namibia), Atacama (Chile), Great Australian deserts (western and central Australia).

• Cold currents along these west coasts make the air even drier: the Canary, Benguela, Peru (Humboldt) and West Australian currents.

• Mediterranean climate regions (shift of belts): the Mediterranean basin, California, central Chile, the Cape region of South Africa and south-west Australia.

• India:
• Summer: the Asian low over north-west India pulls in moist south-west winds from the Indian Ocean. This gives the south-west monsoon. The ITCZ moves to about 20–25° N over India.

• Winter: the Siberian high sends cold, dry air outwards. This gives the dry north-east monsoon over most of India.

• Pacific Ocean: the Southern Oscillation, a see-saw in pressure between Tahiti (French Polynesia) and Darwin (northern Australia) [3].

Don't confuse with

• Thermal belts vs dynamic belts: only the equatorial low and the polar highs are thermal. The subtropical highs and subpolar lows are dynamic. The usual trap is "Which of these is NOT of thermal origin?"
• Doldrums vs horse latitudes: the doldrums are the calm zone at the equatorial low, where air rises. The horse latitudes are the calm subtropical highs at 30°–35° N/S [5], where air sinks.
• Permanent planetary belts vs seasonal thermal cells: the Siberian high (winter) and the Asian thermal low (summer) come from land heating and cooling. They are not part of the seven planetary belts, but they change those belts over Asia.
• Trough vs ridge: a trough is a long, narrow belt of low pressure reaching out from a low. A ridge is a long, narrow belt of high pressure reaching out from a high.

Prelims Hooks

• Thermal belts: equatorial low and polar highs. Dynamic belts: subtropical highs (~30°) and subpolar lows (~60°).
• Horse latitudes = subtropical high belts at about 30°–35° N/S [5]. Doldrums = equatorial low, where the ITCZ lies.
• Aleutian low → Gulf of Alaska / North Pacific [6]. Icelandic low → between Iceland and southern Greenland [7]. Azores–Bermuda high → North Atlantic. Hawaiian high → North Pacific.
• Siberian high = winter thermal high. Asian (Indian) low = summer thermal low.
• Trap: the Sahara, Thar, Kalahari and Atacama lie under the dynamic subtropical highs on the west sides of continents, not on east coasts.
• The belts move about 5–10° with the Sun: north in June, south in December. This explains the Mediterranean winter rain and summer drought.

Mains Points

• Pressure belts set the world's climate zones (GS-I):
• The equatorial low gives rainforests, and the subtropical highs give hot deserts.
• The subpolar lows give stormy, wet mid-latitude coasts (50–60° N/S [2]), and the polar highs give cold deserts.

• The 30° high is hot and the 60° low is cold. This shows that pressure depends on Earth's rotation and the three-cell circulation (Hadley, Ferrel, polar [2]), not on temperature alone.

• Shifting belts explain seasonal climates (GS-I):

• In the Mediterranean climate, the subtropical high moves over the region in summer and brings dry weather. In winter the belts move south, and the westerlies bring rain from their depressions.

• In the Indian monsoon, the ITCZ moves to about 20–25° N in summer. The south-east trades cross the equator and the Coriolis force turns them into south-west winds. In winter the winds reverse.

• Pacific pressure changes affect India's farms and disaster planning (GS-III):

• In a normal year, pressure is lower at Darwin and higher at Tahiti, which drives the trade winds [3].
• In El Niño years, the see-saw reverses and the trade winds weaken [3]. A low SOI (Southern Oscillation Index, the standardised Tahiti–Darwin pressure difference) goes with El Niño [4].
• This raises the risk of a weak monsoon and drought. It matters for monsoon forecasting, kharif crop planning and drought preparedness.

Related concepts

• Isobar
• Subtropical high
• Subpolar low
• Polar high
• Shifting of pressure belts

Read more

• Horizontal distribution: isobars and the world's pressure belts