Global Atmospheric Circulation

From daily weather to global climate patterns

Weather describes atmospheric conditions from day to day; climate describes their long-term pattern. A rainy afternoon is weather, whereas the tendency for a region to be warm and wet throughout the year is part of its climate.

The world’s climatic regions are not scattered randomly. They form broad patterns related to latitude, although oceans, continents and mountains modify those patterns. Global atmospheric circulation helps explain them by moving heat and moisture between the equator and the poles.

Where the main climatic regions occur

Tropical climates lie close to the equator, including parts of South America, central Africa and South-east Asia. Temperatures are high throughout the year. Equatorial rainforest areas have rainfall in every month, while other tropical areas have distinct wet and dry seasons.

Dry climates are widespread immediately north and south of the tropical regions. Many hot deserts lie around 30° north and south, including the Sahara in North Africa and deserts across much of Australia. However, not all deserts are hot or subtropical: the Gobi in eastern Asia is a cold desert. The defining feature is dryness, not heat.

Temperate climates occur mainly in the middle latitudes. They include much of western Europe, parts of the USA and eastern China. Temperatures are generally less extreme than in continental interiors. The UK has a temperate climate, with rainfall throughout the year. Mediterranean climates are a type of temperate climate with dry summers and mild, wet winters; they occur around the Mediterranean Sea and in places such as California and parts of Chile.

Continental climates occur mainly in the interiors of northern continents, especially Canada and Russia. They have greater seasonal temperature contrasts than ocean-influenced temperate climates because they are farther from the moderating influence of the sea.

Polar climates occur at high latitudes in the Arctic and Antarctic regions, including Greenland and the northernmost parts of Canada and Russia. They are cold throughout the year and generally receive little precipitation.

These regions are broad patterns, not continuous strips with sharp boundaries. For example, the UK is much milder than many parts of Canada at a similar latitude because of the influence of the North Atlantic and warm ocean currents.

Why air circulates between the equator and the poles

The Sun heats Earth unevenly. Near the equator, sunlight strikes the surface more directly, concentrating its 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 temperature difference between low and high latitudes.

As air warms, it expands and becomes less dense than surrounding cooler air, so it rises. Rising air is associated with low pressure at the surface. Cold, dense air tends to sink, producing high pressure at the surface.

Surface air moves from higher towards lower pressure. This movement is wind. A larger pressure difference over the same distance produces stronger winds. Earth’s rotation deflects moving air through the Coriolis effect, so global winds do not travel directly north or south.

Together, uneven heating, pressure differences and Earth’s rotation produce a circulation system that transfers heat towards the poles. Air does not travel in one uninterrupted loop from the equator to each pole: a simplified model shows three linked cells in each hemisphere.

Following the three circulation cells

The Hadley cell extends from the equator to roughly 30° north or south. Strong heating near the equator causes warm, moist air to rise. As it rises, it cools and water vapour condenses into cloud droplets, producing frequent heavy rain and thunderstorms. Trade winds meet in this rising-air belt, called the Inter-Tropical Convergence Zone, or ITCZ.

At altitude, air moves away from the equator. It eventually sinks in the subtropics, creating high pressure. Descending air warms, making condensation and cloud formation less likely. Some air then returns towards the equator at the surface as the trade winds, completing the cell.

The Ferrel cell lies roughly between 30° and 60°. At the surface, air moves from subtropical high pressure towards lower pressure nearer 60°. Earth’s rotation helps turn this flow into prevailing westerly winds: winds blowing from the west. Near 60°, warmer air meets colder polar air and rises. Air returns towards the subtropics at altitude. The Ferrel cell therefore circulates in the opposite direction to its neighbours.

The Polar cell extends roughly from 60° to 90°. Cold, dense air sinks over the poles, producing high pressure. It flows towards lower latitudes at the surface as polar easterlies. Near 60°, it meets warmer air in a region of rising air and low pressure. Air returns towards the poles at altitude.

The model shows average patterns. In reality, cell boundaries move with the seasons, and individual weather systems interrupt the neat loops.

How circulation shapes precipitation extremes

Where warm, moist air rises near the equator, cooling and condensation produce frequent, sometimes intense rainfall. This helps explain the wet climate of equatorial rainforest regions, including central Africa and parts of South-east Asia.

The opposite occurs beneath the descending branches of the Hadley cells. Subtropical high pressure suppresses cloud formation, helping explain the very low rainfall of the Sahara and much of inland Australia. Persistent sinking air can maintain long dry periods.

Around the boundary between the Ferrel and Polar cells, contrasting warm and cold air meet at the polar front. Rising moist air helps form travelling low-pressure systems. These bring unsettled weather to places such as the UK, including periods of heavy rain.

How circulation shapes wind and temperature extremes

Large pressure differences can produce strong winds. In the middle latitudes, low-pressure systems associated with the polar front can bring stormy weather. High above them, the polar front jet stream helps guide their movement. If the jet stream meanders and weather systems become slow-moving, a region can experience prolonged rain and strong winds rather than a brief spell of unsettled weather.

Temperature patterns also reflect latitude and circulation. Equatorial regions receive strong solar heating throughout the year. Beneath subtropical high pressure, clear skies allow strong daytime heating, helping hot deserts reach very high temperatures. Clear skies also allow rapid heat loss at night, producing a large daily temperature range; some deserts can fall below freezing.

At the poles, weak solar heating and cold air produce very low temperatures. Movement of polar air towards lower latitudes can also bring unusually cold weather elsewhere, including snow in the UK during winter.

Global circulation therefore helps explain both usual climate patterns and the conditions that favour extremes. It does not mean every place within a climate region has identical weather, or that an extreme event happens continuously.