The general circulation of the atmosphere: three cells

Detail

1. What the general circulation is

• General circulation of the atmosphere means the planet-wide pattern of winds. It is the set of large, lasting wind belts (planetary winds) that cover the whole Earth.

• Five factors shape it (NCERT): 1. Uneven heating by latitude. The equator gets more sunshine than the poles. 2. The pressure belts. These are bands of high and low pressure that go around the globe. 3. The belts shift with the Sun. They move north in June–July and south in December–January. 4. How land and sea are spread out. Land heats and cools faster than water. 5. The Earth's rotation. It produces the Coriolis force, which turns moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

• What the circulation does:

• Moves heat. The tropics gain more heat than they lose (a heat surplus). The poles lose more than they gain (a heat deficit). Winds carry the extra heat towards the poles. Without this, the tropics would keep getting hotter and the poles colder.
• Drives the oceans. Winds push the large, slow ocean currents.

• Gets energy back from the oceans. The oceans give heat and water vapour back to the air. This is why the atmosphere and oceans are treated as one linked system.

• The three-cell model. In each hemisphere the north–south circulation forms three cells: the Hadley cell, the Ferrel cell and the Polar cell. Together they shape the Earth's main climate patterns [3].

• A cell is a closed loop of air: it rises in one place, flows sideways high up, sinks somewhere else, and flows back along the ground.

2. Hadley cell (0°–30°)

• Named after George Hadley, an English scientist, who described it in 1735 [2].
• Hadley cell: the tropical loop of air. It rises near the equator, flows towards the poles high up, sinks near 30° N and S, and comes back to the equator as the trade winds.
• How it works:
• Rising branch (at the ITCZ). The ITCZ (Inter-Tropical Convergence Zone) is the low-pressure belt near the equator where the trade winds from the two hemispheres meet.
  • Strong sunshine heats the ground → the air above gets warm and light → it rises (convection) → low pressure forms at the surface.
  • The trade winds flow in and meet here (convergence). They push the air up to the top of the troposphere, about 14 km high.
• Upper branch. High up, the air flows towards the poles.
• Sinking branch (near 30° N and S). The air piles up and cools. Part of it sinks. Sinking air presses down on the surface and forms the subtropical highs (high-pressure belts near 30°).
  • Sinking air warms and dries. So clouds rarely form here, and many of the world's great hot deserts lie in this belt, for example the Sahara (North Africa), Arabian, Thar (Rajasthan, India) and Kalahari (southern Africa).

• Surface return branch. Air flows back along the ground from the subtropical highs to the ITCZ. These are the trade winds (NCERT: "easterlies").

  • The Coriolis force turns them. They become the north-east trades in the Northern Hemisphere and the south-east trades in the Southern Hemisphere.

• Thermally direct cell. The Hadley cell is thermally direct: warm air rises, cool air sinks, and heat moves from warm areas to cool ones [4]. Heating drives it directly.

• Poleward expansion (present-day change):
• What has been seen. Observations show the edge of the Hadley cell has moved towards the poles by 0.1°–2.0° latitude per decade. Satellite data show a widening of about 2° latitude between 1979 and 2005 [5].
• Why it happens:
  • Greenhouse gases warm the troposphere and cool the stratosphere. This raises the tropopause (the top of the troposphere), so Hadley air can travel farther before sinking [5].
  • High latitudes warm faster than low latitudes. This cuts the temperature gap between the equator and the poles, which also lets the cell widen [5].
• What it causes. The dry subtropical belt moves towards the poles. This is linked to more dryness in the American Southwest, Mediterranean Europe and southern China, and less rain over southern Australia and southern Amazonia [5]. Tropical cyclones have also started forming farther from the equator, and this is linked to the wider Hadley cell [6].

3. The ITCZ and the thermal equator

• Thermal equator: the line of greatest heating on Earth. It moves with the overhead Sun, and the Sun's heat shifts it more over land than over sea.
• Normal position. The ITCZ normally lies near the equator.
• Seasonal shift. The ITCZ follows the thermal equator:
• In July the Sun is overhead near the Tropic of Cancer (23.5° N) → land in South Asia gets very hot → the ITCZ moves far north, to about 20°–25° N over the Indo-Gangetic Plain.
• Here it forms the monsoon trough: a long low-pressure belt running from north-west India (Rajasthan, Punjab) to the Head Bay of Bengal.

• The south-east trades cross the equator. The Coriolis force turns them to the right, so they reach India as the south-west monsoon winds.

• In January the ITCZ moves south of the equator, over the Southern Hemisphere continents (southern Africa, northern Australia).

4. Ferrel cell (30°–60°)

• Named after William Ferrel, an American meteorologist, who described it in 1856 [2].
• Ferrel cell: the mid-latitude loop. Air sinks near 30° and rises near 60°.
• Air flow:
• At the surface, air flows towards the poles and is turned eastward. These are the westerlies. "Westerly" means the wind blows from the west.
• Higher up, air flows back towards the equator and westward [2].

• Near 60° the warm westerlies meet cold polar air and rise. This forms the subpolar low.

• NOAA's range. NOAA puts the Ferrel cell at about 35°–60° N and S, with the low-pressure belt at 50°–60° (NCERT: 30°–60°) [2].

• Thermally indirect cell:
• Meaning. Direct heating does not drive this cell. The Hadley cell on one side and the Polar cell on the other turn it, like a gear turned by two gears beside it.
• Why it looks "backwards". Cooler air is forced to rise near 60°, and warmer air sinks near 30°. This is the opposite of a heat-driven cell.

• What really drives it. Large swirling weather systems (eddies) in the mid-latitudes keep it going [4].

• The real heat carriers. In the real atmosphere, travelling cyclones (low-pressure systems) and anticyclones (high-pressure systems) carry most of the heat in the middle latitudes. Examples are the temperate cyclones of the North Atlantic and the western disturbances that bring winter rain to north-west India.

5. Polar cell (60°–90°)

• Polar cell: the high-latitude loop. Air sinks at the poles and rises near 60°.
• How it works:
• At the poles, very cold, dense air sinks → this forms the polar highs [2].
• Air flows out along the ground towards about 60°. The Coriolis force turns it, and these winds are the polar easterlies [2].
• Near 60° the air meets the warmer westerlies and rises at the subpolar low.

• High up, the air flows back towards the poles, which closes the loop.

• Size. It is the smallest and weakest of the three cells [2]. It reaches from about 60°–70° N and S to the poles [3].

6. All three cells together (draw NCERT Fig. 9.6)

Feature	Hadley cell	Ferrel cell	Polar cell
Latitude	0°–30°	30°–60°	60°–90°
Year described	1735 (George Hadley)	1856 (William Ferrel)	—
Type	Thermally direct	Thermally indirect	Thermally direct
Where air rises	ITCZ / equatorial low	Subpolar low (~60°)	Subpolar low (~60°)
Where air sinks	Subtropical high (~30°)	Subtropical high (~30°)	Polar high (90°)
Surface wind	Trade winds (NE and SE trades)	Westerlies	Polar easterlies

• Pressure belts, from equator to pole: equatorial low (0°) → subtropical high (~30°) → subpolar low (~60°) → polar high (90°). High pressure lies near 30° and at each pole. Low pressure lies at the equator and near 50°–60° [2].
• Diagram tip: draw one hemisphere as a vertical cross-section with the three loops. Mark the four pressure belts on the surface line. Draw the surface wind arrows bent by the Coriolis force. Write "ITCZ" at the equator.

7. The Walker circulation: an east–west cell (El Niño and La Niña)

• Walker circulation: an east–west loop over the equatorial Pacific. The three cells above are north–south loops.

• Air rises over the very warm waters of the western Pacific (near Indonesia). It flows east high up and sinks over the cooler eastern Pacific (off Peru). It returns west at the surface as the trade winds.

• El Niño (warm phase):

• The trade winds weaken → the rising branch moves east of the International Date Line (180°) → sinking air moves over the Maritime Continent (Indonesia) and northern South America [7].
• Result: more thunderstorms near the Date Line, and fewer over the western Pacific [7].

• The 2023-24 El Niño peaked at about 2 °C above average. It was the fifth strongest since 1950, and by February 2024 it was past its peak [7].

• La Niña (cool phase): the Walker loop gets stronger, the trade winds get stronger, and rain over Indonesia increases.

• Link to India. During El Niño, sinking air over the Indonesia–Indian Ocean side tends to weaken the Indian south-west monsoon.

Prelims Hooks

• Hadley cell → George Hadley, 1735. Ferrel cell → William Ferrel, 1856.
• Thermally indirect cell: only the Ferrel cell. The Hadley and Polar cells are thermally direct. This is a common "which of the following" trap.
• Surface winds: Hadley → trade winds (easterlies). Ferrel → westerlies. Polar → polar easterlies.
• Where air sinks: subtropical high (~30°) and polar high. Where air rises: ITCZ (0°) and subpolar low (~60°).
• Height of the Hadley cell's rising air at the ITCZ: about 14 km (top of the troposphere).
• Smallest and weakest cell: the Polar cell.
• ITCZ in July lies over the Indo-Gangetic Plain as the monsoon trough. It follows the thermal equator, not the real equator.
• Hot deserts of the trade-wind belt (Sahara, Thar, Kalahari) lie under the sinking branch of the Hadley cell.
• Hadley cell widening: about 2° latitude between 1979 and 2005, measured by satellite.
• El Niño: trade winds weaken and the Walker rising branch moves east of the Date Line. La Niña: trade winds strengthen and Indonesia gets more rain.

Mains Points

• Heat transfer and climate zones. The three-cell circulation carries surplus tropical heat towards the poles. It sets the main climate belts: wet equator (rising air at the ITCZ), subtropical deserts (sinking air near 30°), stormy mid-latitudes (westerlies and cyclones) and cold polar highs. It also drives the planetary ocean currents.
• Indian monsoon as a seasonal shift of the Hadley cell. The ITCZ follows the thermal equator → in July it forms the monsoon trough over the Indo-Gangetic Plain → the south-east trades cross the equator and become the south-west monsoon. El Niño weakens the Walker circulation and links Pacific conditions to monsoon failure and drought in India (GS-I and GS-III, disasters).
• Climate change widens the tropics. The Hadley cell is expanding towards the poles at 0.1°–2.0° per decade. This pushes the dry subtropical belt into the Mediterranean, the American Southwest, southern Australia and southern China. It moves tropical cyclone formation farther from the equator, so planning for water security and disaster risk must change (GS-III).
• The model and the real atmosphere differ. The Ferrel cell is a statistical average, not a heat-driven loop. In the mid-latitudes, travelling cyclones, anticyclones and jet streams do most of the work. This explains the changeable weather of temperate lands and the western disturbances that bring winter rain to north-west India.