Hadley cell

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

Meaning

The Hadley cell is the tropical loop of air in each hemisphere: air rises near the equator at the ITCZ, flows towards the poles high up, sinks near 30° N and S to form the subtropical highs, and flows back to the equator along the ground as the trade winds.

• It matters because it is the main engine of tropical climate. It explains the wet equator, the hot deserts near 30° and the trade winds. Its seasonal shift helps explain the Indian monsoon.

Explanation

How the loop works: four branches

• Rising branch (at the ITCZ)
• The ITCZ (Inter-Tropical Convergence Zone) is the low-pressure belt near the equator. The trade winds of the two hemispheres meet here.
• Strong sunshine heats the ground. The air above gets warm and light, so it rises. This rising of warm air is called convection. Low pressure forms at the surface.

• The trade winds flow in and meet. This meeting is called convergence. It pushes 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, and part of it sinks.
• Sinking air presses down on the surface. This forms the subtropical highs (belts of high pressure near 30°).

• Sinking air warms and dries, so clouds rarely form. This is why many great hot deserts lie in this belt.

• Surface return branch

• Air flows along the ground from the subtropical highs back to the ITCZ. These are the trade winds (NCERT calls them "easterlies").
• The Coriolis force is the turning effect of the Earth's spin. It turns moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
• So the winds become the north-east trades in the Northern Hemisphere and the south-east trades in the Southern Hemisphere.

Why it is called "thermally direct"

• In a thermally direct cell, warm air rises, cool air sinks, and heat moves from warm places to cool places [3].
• Heating drives the Hadley cell directly. It was named after George Hadley, an English scientist, who described it in 1735 [2].
• Its job in the planet's heat budget:
• The tropics gain more heat than they lose (a heat surplus).
• The Hadley cell carries part of this extra heat towards the poles.

It moves with the seasons

• The ITCZ follows the thermal equator, which is the line of greatest heating. This line moves with the overhead Sun, and it shifts more over land than over sea.
• In July the Sun is overhead near the Tropic of Cancer (23.5° N).
• South Asia gets very hot, so the ITCZ moves to about 20°–25° N over the Indo-Gangetic Plain.
• Here it forms the monsoon trough (a long low-pressure belt).

• 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.

It is widening (present-day change)

• What has been seen
• The edge of the Hadley cell has moved towards the poles by 0.1°–2.0° latitude per decade [4].

• Satellite data show a widening of about 2° latitude between 1979 and 2005 [4].

• Why it happens

• Greenhouse gases warm the troposphere and cool the stratosphere. This raises the tropopause (the top of the troposphere), so the air can travel farther before it sinks [4].

• High latitudes warm faster than low latitudes. This shrinks the temperature gap between the equator and the poles, which also lets the cell widen [4].

• What it causes

• The dry subtropical belt moves towards the poles [4].
• Tropical cyclones have started forming farther from the equator, and this is linked to the wider Hadley cell [5].

Where it occurs

• Belt covered: from about 0° to 30° in both hemispheres. There is one Hadley cell in each hemisphere.
• Rising branch: the ITCZ (equatorial low), near the equator for most of the year.
• India, July: the ITCZ lies over the Indo-Gangetic Plain as the monsoon trough. The trough runs from north-west India (Rajasthan, Punjab) to the Head Bay of Bengal.

• January: the ITCZ lies over southern Africa and northern Australia.

• Sinking branch (hot deserts near 30°):

• Sahara in North Africa
• Arabian Desert
• Thar in Rajasthan, India

• Kalahari in southern Africa

• Regions hit by the widening cell:

• More dryness in the American Southwest, Mediterranean Europe and southern China [4].
• Less rain over southern Australia and southern Amazonia [4].

Don't confuse with

• Ferrel cell (30°–60°): it is thermally indirect. Heating does not drive it. The cells on either side turn it, and mid-latitude eddies (large swirling weather systems) keep it going [3]. The Hadley cell is thermally direct. The Ferrel cell's surface winds are the westerlies, not the trades.
• Polar cell (60°–90°): this cell is also thermally direct. But air sinks at the poles (polar high) and rises near 60°, and its surface winds are the polar easterlies. It is the smallest and weakest cell [2]. The Hadley cell is not.
• Walker circulation: this is an east–west loop over the equatorial Pacific. Air rises over the warm western Pacific near Indonesia and sinks over the cool eastern Pacific off Peru. The Hadley cell is a north–south loop. The Walker loop, not the Hadley cell, is the one that changes during El Niño and La Niña.
• Thermal equator vs real equator: the ITCZ (the Hadley cell's rising branch) follows the thermal equator. It does not stay fixed at 0° latitude.

Prelims Hooks

• Hadley cell → George Hadley, 1735 [2]. Ferrel cell → William Ferrel, 1856 [2].
• Trap: the Hadley cell and the Polar cell are thermally direct. Only the Ferrel cell is thermally indirect.
• Air rises at the ITCZ to about 14 km (top of the troposphere). Air sinks at the subtropical high (~30°). The surface wind is the trade winds (NE trades and SE trades).
• Sahara, Arabian, Thar and Kalahari deserts lie under the sinking branch of the Hadley cell.
• Widening: about 2° latitude between 1979 and 2005, measured by satellite [4]. The rate is 0.1°–2.0° per decade [4].
• ITCZ in July: over the Indo-Gangetic Plain as the monsoon trough. The SE trades cross the equator and become the SW monsoon.

Mains Points

• It sets the tropical climate belts. Rising air at the ITCZ gives the wet equator. Sinking air near 30° gives the subtropical deserts. The surface branch gives the steady trade winds. Together these carry surplus tropical heat towards the poles and help drive the planetary ocean currents (GS-I).
• The Indian monsoon is a seasonal shift of the Hadley cell.
• The ITCZ follows the thermal equator north in summer and forms the monsoon trough over the Indo-Gangetic Plain.
• The south-east trades cross the equator and turn into the south-west monsoon.

• Any change in this circulation affects rainfall, farming and water supply in India (GS-I, GS-III).

• Climate change is widening the tropics.

• The Hadley cell is expanding towards the poles at 0.1°–2.0° per decade [4].
• This pushes the dry belt into the Mediterranean, the American Southwest, southern China and southern Australia [4].
• It also moves tropical cyclone formation farther from the equator [5].
• Water security planning and disaster-risk planning must adjust to this (GS-III, environment and disasters).

Related concepts

• General circulation of the atmosphere
• Ferrel cell
• Polar cell

Read more

• The general circulation of the atmosphere: three cells