Global Atmospheric Circulation and Pressure Belts

Global atmospheric circulation is the large-scale pattern of air movement around the planet, organized into three cells in each hemisphere (Hadley, Ferrel and polar), which creates alternating belts of low and high pressure and the major wind systems: the trade winds, the westerlies and the polar easterlies. In brief: warm air rises at the equator (low pressure, rain), sinks at about 30° latitude (high pressure, deserts), rises again at about 60° (low pressure, stormy weather) and sinks at the poles (high pressure, cold and dry). Winds blow from high to low pressure and are deflected by Earth’s rotation (the Coriolis effect), to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

The circulation explains why rainforests lie near the equator, why deserts occur near 30° latitude and why mid-latitude weather is changeable. This guide explains the pressure belts, the three-cell model, the surface winds and jet streams, compares the cells in a table, works through simple calculations, outlines the history and corrects common mistakes.

What drives atmospheric circulation?

The circulation is driven by the uneven heating of Earth. The equator receives much more solar energy per unit area than the poles, so air near the equator becomes warm and light and rises, while air near the poles is cold and dense and sinks. If Earth did not rotate, a single large cell would carry air from the equator to the poles at high altitude and back at the surface. Because Earth spins, the flow is broken into three cells in each hemisphere, and the Coriolis effect turns the winds.

Pressure is the weight of the air above a point, measured in hectopascals (hPa) or millibars. Rising air leaves low pressure at the surface, and sinking air creates high pressure. Air flows from high to low pressure, and the greater the pressure difference over a distance (the pressure gradient), the stronger the wind.

What are the three cells?

Hadley cell (0° to about 30°)

Strong heating at the equator makes air rise, and it forms towering clouds and heavy rain in the Intertropical Convergence Zone (ITCZ), a belt of low pressure sometimes called the doldrums. At altitude, the air flows poleward, cools and sinks at about 30° latitude, creating the subtropical high-pressure belts with clear skies and dry conditions, where most of the world’s hot deserts lie (the “horse latitudes”). The air returns toward the equator at the surface as the trade winds.

Ferrel cell (about 30° to 60°)

This middle cell is driven by the neighboring cells, like a gear between two wheels. At the surface, air flows poleward from the subtropical highs and meets cold polar air at about 60°, where it is forced up. The mid-latitudes experience changeable weather, with depressions (low-pressure systems) and fronts moving from west to east.

Polar cell (about 60° to 90°)

Cold air sinks over the poles, creating the polar highs, and flows toward lower latitudes at the surface. At about 60°, it meets the warmer air from the Ferrel cell at the polar front, forming the subpolar low-pressure belts. Cold, dry air at the poles gives them low precipitation, so they are classed as polar deserts.

What are the main surface winds?

Winds are named after the direction they come from. In the Northern Hemisphere, the Coriolis effect deflects moving air to the right, and in the Southern Hemisphere, to the left, and there is no effect at the equator.

• Trade winds: blowing from the subtropical highs to the equatorial low, from the northeast in the Northern Hemisphere and from the southeast in the Southern Hemisphere. They were used by sailing ships to cross the oceans.
• Westerlies: blowing from the subtropical highs toward the subpolar lows, from the southwest in the north and from the northwest in the south. They are strong and consistent in the Southern Hemisphere, where there is little land, and are known as the “Roaring Forties” near 40° S.
• Polar easterlies: cold winds from the polar highs, blowing from the northeast in the north and southeast in the south.

What are jet streams and how does the pattern shift?

Jet streams are narrow bands of very fast westerly winds near the top of the troposphere (about 9–12 km), where the temperature contrast between air masses is large. The polar jet near 60° and the subtropical jet near 30° can have speeds of well over 100 km/h and sometimes above 300 km/h. They steer weather systems, and their meanders influence weather and long spells of heat, cold or rain. The whole pattern of pressure belts and winds shifts north and south with the seasons, following the overhead Sun by up to about 20–25° latitude. This causes the wet and dry seasons of the tropics and the Mediterranean climate, where summers are dry under the subtropical high and winters wet as the westerlies move south.

Pressure belts and cells compared

Worked example: pressure, direction and latitude

1. Pressure gradient: the pressure at a station on the subtropical high is 1,024 hPa and at a station 800 km away on the subpolar low it is 996 hPa. The gradient is (1,024 − 996) ÷ 800 = 28 ÷ 800 = 0.035 hPa per km, or 3.5 hPa per 100 km. A steeper gradient means stronger winds.
2. Wind direction in the Northern Hemisphere: air moving from the subtropical high (30° N) toward the equator would move south. The Coriolis effect turns it to the right (to the west), so the wind comes from the northeast, which is the northeast trade wind.
3. Wind in the Southern Hemisphere: air moving poleward (south) from 30° S is deflected to the left (east), so it comes from the northwest, which is the westerlies.
4. Seasonal shift: in July, the ITCZ is at about 15° N over parts of Africa, while in January it moves to about 5° S in the same region. A place at 10° N is under the ITCZ in July (wet season) and under the dry trade winds in January (dry season).
5. Height of the troposphere: the troposphere is about 17 km thick at the equator and about 8 km at the poles, because warm air rises higher in the tropics.
6. Time to circle the globe: a jet stream moving at 200 km/h around a latitude circle of 30° (circumference about 40,000 × cos 30° = 34,600 km) takes 34,600 ÷ 200 = 173 hours, about 7 days.

How was the circulation described?

Sailors used the trade winds and westerlies for centuries. In 1735, the English lawyer and amateur meteorologist George Hadley proposed that heated air at the equator flows poleward at altitude and that the Earth’s rotation deflects the surface return flow, which explained the trade winds. Gaspard-Gustave Coriolis described the deflection force in 1835, and William Ferrel in 1856 explained the mid-latitude westerlies. The polar front theory of the Bergen School in Norway after the First World War described depressions and fronts, and jet streams were discovered during the Second World War when high-flying bombers met very strong winds. Satellites and computer models now confirm the three-cell pattern as an average picture, while showing that the real circulation is more variable.

Which mistakes are common?

Mistake 1: Thinking that the cells are fixed and perfectly regular

The three-cell model is an average. The real atmosphere is turbulent and varies from day to day and between seasons.

Mistake 2: Naming winds by the direction they blow to

A westerly wind blows from the west. This convention is used in weather forecasts and navigation.

Mistake 3: Saying that high pressure always means good weather

High pressure usually brings clear, dry weather, but it can also bring cold fog and, in winter, very cold temperatures.

Mistake 4: Believing that the Coriolis effect makes bathtubs drain in a particular direction

It affects large-scale motion such as storms and winds, but is far too weak to determine the drain direction of a sink.