General Circulation of the Atmosphere

At a Glance

• Hadley cell Equator to about 30 degrees; thermally direct
• Ferrel cell About 30 to 60 degrees; indirect
• Polar cell 60 degrees to the pole; direct
• Surface winds Trade winds, westerlies, polar easterlies
• Meeting zone Intertropical Convergence Zone
• Exam link UPSC Prelims, Geography: Pressure, Winds and Atmospheric Moisture

Where Does It Fit?

This is the fourth topic of the category on pressure, winds and atmospheric moisture. The previous topics described pressure belts and the forces on wind, and this one combines them into the global circulation. The next topic covers the planetary, seasonal and local winds. Previous topic: Forces Affecting Wind Movement. Next topic: Planetary, Seasonal and Local Winds.

What Is the General Circulation?

In short: the general circulation is the planet-wide system of winds that moves heat from the hot equator to the cold poles.

The Earth receives more heat at the equator than at the poles, as shown in Horizontal Distribution of Temperature and Latitudinal Heat Balance. To balance this, air moves in a giant circuit. If the Earth did not rotate, warm air would simply rise at the equator, flow toward the poles aloft, sink there and return along the surface, in a single huge cell in each hemisphere. The rotation of the Earth and the Coriolis force break this into three cells in each hemisphere, which together form the pattern of the general circulation of the atmosphere. The model is a simplification, but it explains the main belts of winds, pressure and rainfall.

What Are the Three Cells?

The Hadley and polar cells are thermally direct, since warm air rises and cold air sinks. The Ferrel cell is mechanically driven by the friction between the other two, as air circulates like a gear between them. The cells create the alternating belts of pressure described in Pressure Belts and Pressure Cells: low at the equator and 60 degrees where air rises, high at 30 degrees and at the poles where it sinks.

What Surface Winds Does the Circulation Produce?

• Trade winds: the steady winds blowing from the subtropical highs toward the equatorial low, from the northeast in the northern hemisphere and the southeast in the southern; deflected by the Coriolis force.
• Westerlies: winds blowing from the subtropical highs toward the subpolar lows, from the southwest in the north and the northwest in the south; stronger and steadier in the southern hemisphere, which has few land barriers.
• Polar easterlies: cold, dry winds from the polar highs toward the subpolar lows.

The trade winds from the two hemispheres meet near the equator in a belt of rising air called the Intertropical Convergence Zone, ITCZ, marked by clouds and heavy rain. It shifts north and south with the Sun and, over South Asia, moves far northward in summer, pulling in the monsoon, as in Mechanism of the Indian Monsoon.

How Is the Circulation Affected by Land, Sea and Change?

The three-cell model assumes a uniform Earth. In reality the distribution of land and sea, the seasonal movement of the Sun and the presence of mountains modify the circulation. The most important modification is the monsoon, a seasonal reversal of winds. The circulation also has east-west components, such as the Walker Circulation across the tropical Pacific, which is disturbed in El Niño years, as in Walker Circulation and Normal Conditions in the Pacific. At the top of the troposphere, narrow bands of fast winds, the jet streams, develop at the boundaries of the cells, as explained in Jet Streams. Observations show that the Hadley cells have widened by roughly half a degree to a degree of latitude per decade in recent decades, shifting the subtropical dry zones poleward, a change that scientists link to climate warming. As an analytical point, this is why changes in rainfall in the Mediterranean and southern Australia are being studied as part of the larger circulation, and not as local events.

Why Is This Topic Important?

• Model Three-cell model asked
• Winds Basis of climate zones
• Change Hadley cell expansion

What Came Next?

The next topic describes the planetary, seasonal and local winds. Next topic: Planetary, Seasonal and Local Winds.

Key People and Terms

• Hadley cell Tropical circulation cell
• Ferrel cell Mid-latitude cell
• Polar cell High-latitude cell
• Trade winds Easterlies of the tropics
• Westerlies Mid-latitude winds from the west
• ITCZ Equatorial convergence of trade winds

Exam Corner

Mains Angle

Questions ask for the model and its limits. These points add depth.

• Model Three cells
• Winds Trades, westerlies, easterlies
• Change Monsoon and widening tropics