OCR GCSE Geography

Why the Earth has a circulation system

The Sun heats the Earth unevenly. Near the equator the Sun's rays arrive almost overhead, so a given amount of energy is spread over a small area of surface and the ground gets very hot. Towards the poles the same rays arrive at a low angle, are spread over a much larger area and pass through more atmosphere, so they warm the surface far less. This difference in heating is the engine of the global atmospheric circulation, the large-scale movement of air that carries heat from the tropics towards the poles and so reduces what would otherwise be an even greater contrast in temperature.

Air moves because of differences in pressure. Warm air is less dense, rises and leaves a region of low pressure at the surface. Cool air is denser, sinks and creates high pressure at the surface. Wind is air moving at the surface from high pressure to low pressure, and the Earth's rotation bends the path of that wind to the right in the northern hemisphere and to the left in the southern hemisphere.

The three cells

In each hemisphere the circulation is organised into three linked loops, called cells.

• The Hadley cell lies between the equator and about 30° north and south. Air heated at the equator rises, cools as it climbs and forms towering clouds and heavy rain. At height the air spreads towards the poles, cools further and sinks at about 30°, then returns to the equator along the surface as the trade winds.
• The Ferrel cell lies between about 30° and 60°. At the surface air moves towards the pole, forming the south-westerly winds that dominate the weather of the UK in the northern hemisphere. It is driven by the cells on either side of it rather than by direct heating.
• The Polar cell lies between about 60° and the pole. Cold air sinks over the poles, flows outwards along the surface and meets the warmer air of the Ferrel cell at about 60°, where it is forced to rise.

Pressure belts and climatic zones

Where air rises there is a belt of low pressure at the surface, and where it sinks there is a belt of high pressure.

These belts explain the pattern of the world's climatic zones. The equatorial zone is hot and wet, the subtropical high-pressure belts hold most of the world's great hot deserts (the Sahara, the Arabian desert, the Australian outback), and the zones near 60° have mild, wet, changeable weather. Sinking air warms as it descends and can hold more water vapour, so it does not form cloud, which is why high pressure brings dry weather.

The belts do not stay still. They shift north and south with the seasons, following the overhead Sun, so many places in the tropics have a wet season when the low-pressure belt moves over them and a dry season when the high-pressure belt does.

How circulation causes extreme weather

Extreme weather happens where the circulation concentrates energy and moisture, or where it fails to deliver any.

• Intense rain and storms occur where air is rising strongly over very warm ocean, as in the equatorial low-pressure belt, because the rising air carries huge amounts of water vapour that condenses into cloud and rain.
• Drought is most likely under the subtropical high-pressure belts, where air sinks and suppresses cloud. If the belt sits over a place for longer than usual, or shifts away from its normal position, the rains fail.
• Very strong winds are found where large differences in pressure lie close together, for example in the depressions that cross the North Atlantic at about 60° north, and in tropical storms.
• Extreme temperatures follow from the same pattern. Places beneath persistent high pressure, with clear skies, heat up rapidly by day in summer, and cool rapidly at night in winter. Cold air flowing out of the polar cell can reach mid-latitudes and bring severe cold.

Extremes in contrasting countries

Different countries are exposed to different extremes because of where they sit in the circulation.

Wind, temperature and precipitation each have their own extremes. A useful way to describe a country's exposure is to name the extreme (a wind speed, a daily temperature, or the rainfall in a day or a season), say where it occurs and explain it by the circulation, the season and the local geography, such as a coast, mountains or a warm sea.

Extremes that are changing

Records of extreme weather suggest that heat waves have become more frequent and intense in many regions as global temperatures have risen, and that heavy rainfall events are more intense because warmer air holds more water vapour. Changes in the number of storms and droughts are more complicated, and are covered with tropical storms and El Niño.