General circulation of the atmosphere

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

Meaning

The general circulation of the atmosphere is the planet-wide pattern of large, lasting wind belts (the planetary winds). Five things shape it: uneven heating by latitude, the pressure belts, the seasonal shift of those belts with the Sun, the way land and sea are spread out, and the Earth's rotation.

It matters because it carries extra heat from the tropics towards the poles and pushes the large ocean currents. It also sets the Earth's main climate belts, from the wet equator to the hot deserts and the stormy mid-latitudes.

Explanation

Why the circulation exists: causes and work

• Five factors shape it (NCERT): 1. Uneven heating by latitude. The equator gets more sunshine than the poles. 2. Pressure belts. Bands of high and low pressure circle the globe. 3. Seasonal shift. The belts move north in June–July and south in December–January, following the Sun. 4. Land and sea. Land heats and cools faster than water. 5. Earth's rotation. It produces the Coriolis force (a turning effect of the spinning Earth). It bends moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

• It moves heat.

• The tropics gain more heat than they lose. This is a heat surplus.
• The poles lose more heat than they gain. This is a heat deficit.

• Winds carry the extra heat towards the poles. Without this, the tropics would keep getting hotter and the poles colder.

• It drives the oceans. Winds push the large, slow ocean currents.

• It gets energy back from the oceans. The oceans return heat and water vapour to the air. So the atmosphere and the oceans are studied as one linked system.

The three-cell model

• A cell is a closed loop of air. Air rises in one place, flows sideways high up, sinks somewhere else, and flows back along the ground.
• Each hemisphere has three north–south cells: the Hadley, Ferrel and Polar cells. Together they shape the Earth's main climate patterns [3].
• Hadley cell (0°–30°): named after George Hadley, who described it in 1735 [2].
• Rising branch: Strong sunshine heats the ground near the equator, so warm, light air rises (convection). The trade winds meet here (convergence) at the ITCZ (Inter-Tropical Convergence Zone, the low-pressure belt where the trade winds of both hemispheres meet). They push the air up to about 14 km, the top of the troposphere.
• Upper branch: High up, the air flows towards the poles.
• Sinking branch: Near 30° N and S the air piles up, cools and partly sinks. This forms the subtropical highs. Sinking air warms and dries, so clouds rarely form there.
• Surface branch: Air flows back to the ITCZ as the trade winds (NCERT: "easterlies"). The Coriolis force turns them into the north-east trades (Northern Hemisphere) and the south-east trades (Southern Hemisphere).

• It is thermally direct: warm air rises, cool air sinks, and heat moves from warm areas to cool ones [4].

• Ferrel cell (30°–60°): named after William Ferrel, who described it in 1856 [2].

• At the surface, air flows towards the poles and is turned eastward. These are the westerlies. A westerly 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 puts this cell at about 35°–60° N and S, with the low-pressure belt at 50°–60° (NCERT: 30°–60°) [2].

• It is thermally indirect. Heating does not drive it. The Hadley and Polar cells turn it, like a gear turned by the gears beside it. Cooler air rises near 60° and warmer air sinks near 30°, which is the opposite of a heat-driven cell. Large swirling weather systems (eddies) in the mid-latitudes keep it going [4].

• Polar cell (60°–90°):

• Very cold, dense air sinks at the poles and forms the polar highs [2].
• Air flows out along the ground towards about 60°. The Coriolis force turns it into the polar easterlies [2].
• It rises at the subpolar low and flows back to the pole high up.
• It is the smallest and weakest cell [2]. It reaches from about 60°–70° N and S to the poles [3].

Feature	Hadley	Ferrel	Polar
Latitude	0°–30°	30°–60°	60°–90°
Type	Thermally direct	Thermally indirect	Thermally direct
Air rises at	ITCZ / equatorial low	Subpolar low (~60°)	Subpolar low (~60°)
Air sinks at	Subtropical high (~30°)	Subtropical high (~30°)	Polar high (90°)
Surface wind	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°) [2].

Seasonal shift, the Walker loop and present-day change

• ITCZ follows the thermal equator. The thermal equator is the line of greatest heating. It moves with the overhead Sun and shifts more over land than over sea.
• In July the Sun is overhead near the Tropic of Cancer (23.5° N). The ITCZ moves to about 20°–25° N and forms the monsoon trough (a long low-pressure belt).
• The south-east trades cross the equator. The Coriolis force turns them right, so they reach India as the south-west monsoon.

• In January the ITCZ moves south of the equator.

• Walker circulation: an east–west loop over the equatorial Pacific. Air rises over the warm western Pacific, flows east high up, sinks over the cooler eastern Pacific, and returns west as the trade winds.

• El Niño (warm phase): the trade winds weaken. The rising branch moves east of the International Date Line (180°), and sinking air moves over the Maritime Continent and northern South America [7].

• La Niña (cool phase): the loop and the trade winds get stronger.

• The Hadley cell is getting wider.

• Its edge has moved towards the poles by 0.1°–2.0° latitude per decade. Satellites show it widened by 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 the air travels farther before it sinks [5]. High latitudes also warm faster than the tropics, so the temperature gap between the equator and the poles shrinks [5].
• Result: Tropical cyclones now form farther from the equator [6].

Where it occurs

• Hot deserts under the sinking Hadley air (~30°): Sahara (North Africa), Arabian, Thar (Rajasthan, India) and Kalahari (southern Africa).
• India, monsoon trough (July): the ITCZ lies over the Indo-Gangetic Plain. The trough runs from north-west India (Rajasthan, Punjab) to the Head Bay of Bengal.
• ITCZ in January: over Southern Hemisphere land, such as southern Africa and northern Australia.
• Mid-latitude weather systems (Ferrel belt): temperate cyclones of the North Atlantic. Western disturbances bring winter rain to north-west India.
• Walker circulation: air rises near Indonesia (western Pacific) and sinks off Peru (eastern Pacific). 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].
• Drying from Hadley widening: the American Southwest, Mediterranean Europe and southern China. Rain has also fallen over southern Australia and southern Amazonia [5].

Don't confuse with

• Hadley/Polar cell vs Ferrel cell: Hadley and Polar are thermally direct, so heat drives them. Only the Ferrel cell is thermally indirect. The cells beside it and the mid-latitude eddies turn it.
• Walker circulation vs the three cells: Walker is an east–west loop over the equatorial Pacific. Hadley, Ferrel and Polar are north–south loops.
• Thermal equator vs the geographical equator: the ITCZ follows the thermal equator (the line of greatest heating). In July this lies over the Indo-Gangetic Plain, not at 0°.
• Trade winds/polar easterlies vs westerlies: both easterly winds blow from the east. Trades flow from about 30° towards 0°, and polar easterlies flow from the poles towards about 60°. Westerlies blow from the west, between 30° and 60°.

Prelims Hooks

• Hadley cell → George Hadley, 1735. Ferrel cell → William Ferrel, 1856 [2].
• Only thermally indirect cell: the Ferrel cell. This is a common "which of the following" trap.
• Surface winds: Hadley → trade winds. Ferrel → westerlies. Polar → polar easterlies. The smallest and weakest cell is the Polar cell [2].
• Air rises at the ITCZ (0°) and the subpolar low (~60°). Air sinks at the subtropical high (~30°) and the polar high (90°). Rising air at the ITCZ reaches about 14 km.
• Sahara, Thar and Kalahari lie under the sinking branch of the Hadley cell.
• The Hadley cell widened by about 2° latitude between 1979 and 2005 (satellite data) [5]. El Niño: trades weaken and the Walker rising branch moves east of the Date Line [7].

Mains Points

• Heat transfer and climate belts (GS-I):
• The three cells carry surplus tropical heat towards the poles and drive the planetary ocean currents.
• They set the climate belts: the wet equator (rising air at the ITCZ), subtropical deserts (sinking air near 30°), stormy mid-latitudes (westerlies and cyclones) and cold polar highs.

• The Ferrel cell is only an average picture. In the mid-latitudes, travelling cyclones, anticyclones and jet streams do most of the work. This explains the changeable temperate weather and the western disturbances over north-west India.

• Indian monsoon as a seasonal shift of the Hadley cell (GS-I, GS-III disasters):

• The ITCZ follows the thermal equator and forms the monsoon trough over the Indo-Gangetic Plain in July. The south-east trades cross the equator and turn into the south-west monsoon.

• In El Niño years, sinking air over the Indonesia–Indian Ocean side tends to weaken the monsoon. This links Pacific conditions to monsoon failure and drought in India.

• Climate change widens the tropics (GS-III):

• The Hadley cell is spreading towards the poles at 0.1°–2.0° per decade [5]. This pushes the dry belt into the Mediterranean, the American Southwest, southern China and southern Australia [5].
• Tropical cyclones now form farther from the equator [6]. So plans for water security and disaster risk need to change.

Related concepts

• Hadley cell
• Ferrel cell
• Polar cell

Read more

• The general circulation of the atmosphere: three cells