The Global Atmospheric Circulation System

Why the atmosphere circulates

The Sun heats Earth unevenly. Near the equator, sunlight is more concentrated; towards the poles, it spreads over a larger area and provides less heating. This creates a surplus of heat in tropical regions and much colder conditions near the poles.

The atmosphere helps redistribute this heat. Air moves both vertically and horizontally, forming a global circulation system. This system helps explain why equatorial regions are generally hot and wet, many subtropical regions are dry, and the UK's weather is often changeable.

Weather means atmospheric conditions over a short period. Climate means the long-term pattern of those conditions. Global circulation influences both, but it does not determine every individual weather event.

Rising air, sinking air and surface winds

When air near the surface is heated, it expands, becomes less dense and rises. Rising air is associated with low pressure at the surface. As moist air rises, it cools, allowing water vapour to condense into cloud droplets. These can produce precipitation.

Sinking air is associated with high pressure at the surface. As it descends, it warms, making condensation less likely. High-pressure conditions therefore tend to have fewer clouds and little rainfall.

Near the surface, air moves from higher towards lower pressure: this movement is wind. The pressure difference over a distance is the pressure gradient. A large pressure difference over a short distance generally produces stronger winds. It is the pressure gradient, rather than a low pressure reading alone, that matters for wind strength.

The three-cell model

The simplified global model has three circulation cells in each hemisphere: the Hadley, Ferrel and Polar cells. Their boundaries and associated pressure belts are approximate and shift with the seasons.

The Hadley cell extends from the equator to roughly 30° north or south. Strong heating near the equator causes air to rise, creating an equatorial low-pressure belt. Moist rising air cools and forms clouds and frequent heavy rain. At altitude, air moves towards the subtropics before sinking around 30°, creating high-pressure belts. Some of this air then returns towards the equator at the surface.

The Ferrel cell lies between about 30° and 60° in each hemisphere. Surface air moves towards higher latitudes. Around 60°, warmer air meets colder air moving from polar regions and rises, producing a low-pressure belt. Air returns towards the subtropics at altitude. This cell circulates in the opposite direction to its neighbours in the simplified model.

The Polar cell extends from about 60° to the poles at 90°. Very cold, dense air sinks over the poles, creating high pressure. It moves towards lower latitudes at the surface, rises near 60° and returns towards the poles at altitude.

Why the wind belts curve

Earth's rotation deflects moving air through the Coriolis effect: to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Left and right are judged looking along the air's direction of travel, not relative to the page. Surface winds therefore do not travel in straight north–south lines.

The main wind belts are:

• Trade winds, blowing from the subtropical high-pressure belts towards the equatorial low-pressure belt.
• Westerlies, blowing from the subtropical high-pressure belts towards the low-pressure belts near 60°.
• Polar easterlies, blowing from polar high pressure towards lower pressure near 60°.

Winds are named for the direction they come from. Westerlies come from the west; easterlies come from the east.

How pressure belts create climatic zones

Near 0°, strong heating and rising moist air produce a hot, humid tropical climate with frequent rainfall. Singapore is an example of an equatorial location with these conditions.

Around 30° north and south, sinking air suppresses cloud formation and rainfall. This helps explain the location of many arid regions, including the Sahara. Clear skies allow strong daytime heating and substantial cooling at night, so desert temperatures can vary greatly within a day.

In the middle latitudes, the meeting of warmer and colder air encourages rising air and changing low-pressure weather systems. The UK has a temperate climate and frequently receives wet, windy weather carried from the Atlantic by south-westerly winds.

At the poles, weak solar heating creates very low temperatures. Sinking air also limits precipitation, so polar regions can be extremely cold and dry. High pressure does not always mean hot weather: its temperature depends on location and season.

From usual weather to weather extremes

Global circulation creates the background conditions in which extremes develop. Persistent high pressure can keep an area dry for an extended period, contributing to drought. Clear skies can also allow intense daytime heating, contributing to very high temperatures.

Where moist air rises, condensation can produce heavy precipitation. Repeated low-pressure systems can bring prolonged wet weather. Where pressure gradients are steep, winds can become powerful enough to cause damage.

Extremes are not confined to the boundaries between cells. They can also occur within a pressure belt, and their severity depends on local conditions as well as global circulation.

Contrasting extremes in the UK and Australia

An extreme must be judged against what is usual for that location. The UK and Australia provide a useful contrast, although both countries contain regional differences and Australia's climate is especially varied.

Temperature: London's average maximum summer temperature is around 23°C, compared with around 33°C in Darwin in northern Australia. A temperature above 30°C is notably hot for London, whereas temperatures above 40°C represent a much more severe level of heat in Darwin. These are local comparisons, not universal definitions of a heatwave.

Precipitation: approximate national annual rainfall averages are about 1,150 mm for the UK and 465 mm for Australia. Australia is therefore much drier overall, with extensive dry areas influenced by subtropical high pressure. However, its tropical north can receive very heavy rain.

Annual totals show how unusually wet and dry conditions differ between the two countries:

These figures illustrate contrasting wet and dry years; they are not universal definitions of extreme rainfall or drought. For example, 550 mm would represent a wet year in this Australian comparison but would be far below the UK's usual annual total. Annual national totals do not show the intensity of individual downpours, seasonal rainfall shortages or conditions in every region. Both countries can experience prolonged wet weather and drought.

Wind: the UK can experience damaging winds from Atlantic low-pressure systems. A sea-level gust exceeding 220 km/h was recorded at Fraserburgh, Scotland, in 1989. Parts of Australia experience tropical cyclones, whose sustained winds can exceed 118 km/h. During Cyclone Olivia in 1996, a gust of roughly 400 km/h was recorded on Barrow Island off north-western Australia. These gust records demonstrate possible extremes, not typical wind conditions.

Circulation pattern

Unequal solar heating drives circulation and redistributes heat. Each hemisphere has three cells:

• Hadley: approximately 0–30°.
• Ferrel: approximately 30–60°.
• Polar: approximately 60–90°.

Pressure belts and climate

• 0°: low pressure — rising moist air; hot, humid and rainy.
• 30° N/S: high pressure — sinking air; dry conditions and many deserts.
• 60° N/S: low pressure — rising air where warmer and colder air meet; unsettled weather.
• 90° N/S: high pressure — cold, sinking air; cold and dry.

Rising air cools → condensation → clouds and precipitation. Sinking air warms → less condensation → generally dry, clear conditions.

Winds and extremes

Surface air moves from higher towards lower pressure. A steeper pressure gradient generally means stronger winds.

Earth's rotation deflects winds right in the Northern Hemisphere and left in the Southern Hemisphere, relative to their direction of travel. Main belts: .