Global Atmospheric Circulation

Why the atmosphere circulates

The Sun does not heat the Earth evenly. Near the Equator, sunlight strikes the surface more directly, concentrating energy over a smaller area. Towards the poles, sunlight arrives at a lower angle and spreads its energy over a larger area. This creates a surplus of heat in equatorial regions compared with the poles.

Global atmospheric circulation is the worldwide movement of air that redistributes some of this heat towards higher latitudes. It also transports water vapour, helping to explain why some parts of the world are usually wet while others are dry.

These long-term patterns are part of a place's climate. Weather is the condition of the atmosphere from day to day: a generally dry region can still experience a rainy day.

Rising and sinking air create pressure belts

Atmospheric pressure is the force exerted by the weight of air on the Earth's surface. Heating makes air expand and become less dense, so it rises. Rising air is associated with low pressure at the surface. Colder, denser air sinks, producing high pressure.

As moist air rises, it cools. Water vapour can condense into droplets, forming clouds and sometimes rain. Descending air warms and becomes relatively drier, making cloud formation less likely. This gives a useful connection:

• Rising air → low pressure → cloudier, often wetter conditions.
• Sinking air → high pressure → generally clearer, drier conditions.

At the surface, air moves from areas of high pressure towards areas of low pressure. This movement is wind. The pressure belts therefore help determine both rainfall patterns and the directions of surface winds.

Three linked cells in each hemisphere

The circulation model contains three cells in the northern hemisphere and three in the southern hemisphere. A cell is a large circulation loop: air moves along the surface, rises, returns at altitude and sinks again.

The Hadley cell extends approximately from the Equator to 30° north or south. Strong heating causes air to rise near the Equator, creating a low-pressure belt. As moist air rises and cools, clouds and heavy convectional rainfall develop. This helps explain the wet climate associated with equatorial rainforests.

The air then travels away from the Equator at altitude. It cools and eventually descends around 30°, creating subtropical high-pressure belts. Descending air discourages cloud formation, so these regions are generally dry. Many of the world's hot deserts occur in these belts. Some surface air returns towards the Equator, completing the Hadley circulation.

The Ferrel cell lies approximately between 30° and 60°. At the surface, air moves from the subtropical high-pressure belt towards lower pressure around 60°. Here it meets colder air moving away from the poles and rises. This favours cloud, rainfall and unsettled weather. Air returns towards 30° at altitude. The Ferrel cell circulates in the opposite sense to its neighbouring cells, rather like the middle cog between two others.

The Polar cell extends approximately from 60° to 90°. Very cold air sinks over the poles, producing high pressure. Surface air flows towards 60°, where it meets warmer air and rises, before returning towards the poles at altitude. Polar regions receive little precipitation: they are cold deserts, showing that a dry climate is not necessarily a hot climate.

Why surface winds curve

Surface winds do not travel directly north or south between the pressure belts. The Earth's rotation deflects moving air through the Coriolis effect: to the right of its movement in the northern hemisphere and to the left in the southern hemisphere.

Together, the pressure belts and this deflection produce three main surface wind belts in each hemisphere:

Connecting the model to climate and UK weather

The model explains a broad alternating pattern: wet near 0°, dry near 30°, wetter near 60° and dry near 90°. Surface winds also matter because they transport heat and moisture between regions.

The UK is influenced by the westerly wind belt and low-pressure systems arriving from the Atlantic. Prevailing south-westerly winds bring moist air, helping to explain its frequently wet, windy and changeable weather.

This is a simplified model, not a prediction of every day's weather. The pressure belts move north and south during the year, changing rainfall patterns in some regions. Their approximate positions explain broad climate patterns rather than sharp boundaries between climates.

Pressure and rainfall

• Uneven solar heating drives circulation and redistributes heat towards the poles.
• Rising air → low pressure → cooling, condensation, clouds and rain.
• Sinking air → high pressure → generally clear, dry conditions.
• Surface wind moves from high to low pressure.

Recall the three-cell model

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

Surface winds

• Trade winds: 30° towards 0°; north-easterly in the north, south-easterly in the south.
• Westerlies: 30° towards 60°; from the west.
• Polar easterlies: 90° towards 60°; from the east.
• Coriolis effect: deflects moving air right in the northern hemisphere, left in the southern.

Climate connections

Equatorial low pressure supports wet rainforest climates; subtropical high pressure helps explain hot deserts. Polar high pressure produces dry conditions despite low temperatures.