Horizontal distribution: isobars and the world's pressure belts

Detail

1. Isobars: how pressure is shown on a map

• Isobar is a line on a map that joins places with the same air pressure. The word comes from Greek: iso means equal and baros means weight.
• Reduction to sea level: pressure falls as you go higher. So a reading taken in Shimla is lower than one taken in Chennai, even if the weather is the same in both places.
• Every station's reading is therefore first converted to what it would be at sea level.
• Only then can the map show real horizontal differences in pressure, without height getting in the way.

• Mean sea-level pressure is about 1,013.2 mb (millibar). This is the standard figure for comparing places.

• Pressure gradient: the rate at which pressure changes over a distance. It is the force that sets air moving from high pressure to low pressure.

Isobar spacing	Pressure gradient	Wind
Close together	Steep	Strong (e.g. near a cyclone's centre)
Far apart	Gentle	Light or calm

2. Pressure systems drawn with isobars

System	Isobar shape	Pressure	Typical weather
Low (depression or cyclone)	Closed rings	Lowest at the centre	Rising air → cloud and rain
High (anticyclone)	Closed rings	Highest at the centre	Sinking air → clear and dry
Trough	V- or U-shaped tongue reaching out from a low	Long, narrow belt of low pressure	Cloudy, often unsettled
Ridge	Tongue reaching out from a high	Long, narrow belt of high pressure	Mostly fair
Col	Saddle area	Lies between two highs and two lows set crosswise	Light winds that change direction

• Simple rule (Class 9 NCERT):
• Warm air → expands → becomes lighter → rises → low pressure → clouds and rain.

• Cold air → becomes denser → sinks → high pressure → clear, dry skies.

• Low pressure goes with "rainy and stormy weather". High pressure goes with "fair and dry/hot weather" [3].

3. The world's pressure belts on sea-level maps (Figs. 9.2 January, 9.3 July)

• Pressure belt: a zone of fairly even high or low pressure that runs east–west around the Earth, roughly along a line of latitude.
• From the equator to each pole there are four belts, so there are seven in all: one equatorial low, plus two each of subtropical highs, subpolar lows and polar highs.
• On a planet with no tilt and no continents, the belts would be smooth bands. Real maps show them broken into cells (see §5).

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: calm, still air; heavy rain from convection; the ITCZ lies here
Subtropical high	~30° N/S (30–35°) [6]	Dynamic	Hadley-cell air piles up, cools and sinks	Horse latitudes: light winds, clear skies [6]; hot deserts below
Subpolar low	~60° N/S (50–60°) [3]	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 the polar easterlies

• Thermal vs dynamic origin (a common exam question):
• Thermal origin: pressure is caused directly by temperature. Heat makes a low (equator). Cold makes a high (poles).

• Dynamic origin: 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. This is the classic "which of these is NOT thermal" trap.

• 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 surface wind. Sailing ships used to get stuck here.
• Horse latitudes: the calm subtropical high belts around 30°–35° N and S [6]. The usual story is that sailing ships stuck there threw horses overboard to save water.

4. Why the subtropical highs form (NCERT exercise 3(ii))

• How the Hadley cell builds the 30° high:
• Air rises at the equator. High in the sky it flows towards the poles [3].
• 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) also bends it until it moves almost east–west.
• The air can no longer move further towards the pole, so it piles up near 30°. This added weight of air sinks, and the result is a dynamic high.
• Part of this sinking air flows back towards the equator as the trade winds, which closes the loop called the Hadley cell [3].

• 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) [3].

• Why there is no rain:

• Subsidence (air sinking slowly over a large area) squeezes the air and warms it, so clouds cannot form.
• A temperature inversion (a layer where air gets warmer with height instead of colder) sits over the region like a lid. It stops moist air from rising.

• Result: dry belts and hot deserts. Deserts gather around 30° N/S: the American South-west, northern Africa and Australia [3].

• The great hot deserts (20–30°, on west sides of continents):

Hemisphere	Desert	Location
N	Sahara	North Africa
N	Arabian	Saudi Arabia, West Asia
N	Thar	Rajasthan (India) and Sindh (Pakistan)
N	Sonoran / Mojave (American South-west)	USA–Mexico [3]
S	Kalahari (with Namib)	Botswana, Namibia
S	Atacama	Chile, west coast of South America
S	Great Australian deserts	Western and central Australia

• Why the deserts are on the west sides: the eastern edge of each oceanic subtropical high, which lies next to the west coast of a continent, has the strongest sinking air. Cold ocean currents along these coasts make the air even drier. Examples are the Canary, Benguela, Peru (Humboldt) and West Australian currents.

5. Belts break into cells: the semi-permanent centres

• Why the belts break up: 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.
• Southern Hemisphere: it is mostly ocean, so its belts are more continuous. The subpolar low forms an almost unbroken trough around Antarctica.

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 [8]	Deepest in winter
Aleutian low	Subpolar low	Gulf of Alaska, near the Aleutian Islands [7]	Winter; moves north and almost disappears in summer [7]
Siberian high	Thermal high (land)	Northern Asia	Winter (very cold land)
Asian (Indian) thermal low	Thermal low (land)	North-west India, Pakistan and West Asia	Summer (very hot land)

• January map (Fig. 9.2): a strong Siberian high over Asia, with the Aleutian low over the North Pacific and the Icelandic low over the Atlantic [9]. Over the oceans, pressure is lower than over the cold land.
• July map (Fig. 9.3): the Siberian high is replaced by the deep Asian thermal low. The oceanic highs (Azores–Bermuda, Hawaiian) grow and spread.
• Link to India:
• Summer: the Asian low pulls in moist south-west winds from the Indian Ocean, giving the south-west monsoon.
• Winter: the Siberian high sends cold, dry air outwards, giving the dry north-east monsoon over most of India.

6. Shifting of pressure belts

• What it means: the belts are not fixed. They follow the apparent path of the Sun. They move about 5–10° north in the northern summer (June) and south in the northern winter (December).
• Why: 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 with it the ITCZ, moves after the Sun, and the other belts move with it.
• Effect 1: Mediterranean climate (about 30–45°, west sides of continents):
• Summer: the subtropical high moves north over the region → sinking air → dry summer.
• Winter: the belts move south → the westerlies and their depressions (cyclones) arrive → winter rain.

• Examples: the Mediterranean basin, California, central Chile, the Cape region of South Africa and south-west Australia.

• Effect 2: Monsoon lands (South and South-east Asia, northern Australia, West Africa):

• In summer the ITCZ moves far north, to about 20–25° N over India.
• The south-east trade winds cross the equator. The Coriolis force turns them into south-west winds.
• In winter everything moves back south and the winds reverse. This seasonal reversal is the heart of the monsoon. See indian-climate-monsoon and world-climate-koeppen.

7. Present-day link: the Pacific pressure see-saw (El Niño and La Niña)

• Southern Oscillation: a change in air pressure, like a see-saw, between Tahiti (French Polynesia, central South Pacific) and Darwin (northern Australia) [4].
• Normal year:
• Pressure is lower at Darwin and higher at Tahiti. This pushes air from east to west, which is the trade winds [4].
• The winds drive warm surface water westward.

• Cold, deep water full of nutrients comes up to the surface off north-west South America (Peru) [4].

• El Niño (the warm phase of ENSO; ENSO stands for El Niño–Southern Oscillation, the linked ocean and air pattern):

• Pressure at Darwin becomes higher than at Tahiti, so the see-saw reverses [4].
• The trade winds weaken, and the eastern Pacific becomes warmer than normal [4].

• For India this often means a weaker monsoon.

• La Niña (the cool phase): the eastern Pacific becomes cooler than normal [4].

• Southern Oscillation Index (SOI): the standardised difference in surface pressure between Tahiti and Darwin. It measures how strong the trade winds are [5].
• High SOI → stronger trade winds → La Niña [5].

• Low SOI → weaker trade winds → El Niño [5].

• Sea temperatures change by only about ±3 °C (and usually much less), yet the effects on weather are felt across the world [4].

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 [6]. Doldrums = equatorial low.
• Isobars close together = steep pressure gradient = strong wind. Pressure readings are reduced to sea level before isobars are drawn.
• Col = the saddle between two highs and two lows. Trough = a stretched-out low. Ridge = a stretched-out high.
• Aleutian low → Gulf of Alaska / North Pacific [7]. Icelandic low → between Iceland and southern Greenland [8]. Azores–Bermuda high → North Atlantic. Hawaiian high → North Pacific.
• Siberian high is a winter, thermal high. The Asian (Indian) low is a summer, thermal low.
• Trap: hot deserts such as 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.
• SOI = pressure difference between Tahiti and Darwin. A low or negative SOI goes with El Niño [5].

Mains Points

• Pressure belts control world climate zones: the equatorial low gives rainforests, the subtropical highs give hot deserts, the subpolar lows give stormy, wet mid-latitude coasts (50–60° N/S [3]), and the polar highs give cold deserts. This is a ready framework for GS-I answers on how climate is distributed.
• Thermal vs dynamic origin: the 30° high is hot yet high-pressure, and the 60° low is cold yet low-pressure. This shows that pressure depends on Earth's rotation and the three-cell circulation (Hadley, Ferrel, polar [3]), not on temperature alone.
• Shifting belts explain seasonal climates. The Mediterranean pattern of dry summers and wet winters, and the Indian monsoon's reversal between the Asian thermal low in summer and the Siberian high in winter, both come from the belts moving north and south with the Sun.
• Pressure changes reach India's farms and disaster planning. A change in the Tahiti–Darwin pressure gradient (ENSO) weakens or strengthens the trade winds [4][5]. El Niño years carry a risk of a weak monsoon and drought, which matters for monsoon forecasting, kharif crop planning and drought preparedness (GS-III).