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**…