BBC Bitesize – global circulation system
There are large-scale movements of air over the Earth’s surface. These large-scale movements of air form loops called cells. These cells have warm, rising air (creating low pressure) and cool, sinking air (creating high pressure). Winds are caused by differences in air pressure. Winds move from areas of high pressure to areas of low pressure. These differences in air pressures are caused by differences in temperature between the Equator and the poles.
- At the Equator, the sun warms the Earth’s surface which in turn warms the air above it. This warmed air is less dense so starts to rise creating low pressure at the surface. The moisture in the rising air cools and condenses to form clouds, eventually bringing rain.
As the air rises it cools and is pushed out to 30° north and south of the Equator. The cool air then starts to sink creating high pressure at the Earth’s surface. As the air is warming up no moisture can cool or condense meaning no clouds form and low rainfall.
- The cool air reaches the ground and moves as surface wind either back towards the Equator or towards the poles.
Surface winds that blow towards the Equator are known as trade winds. They blow from the southeast in the southern hemisphere and from the northeast in the northern hemisphere. They meet at the Equator in the intertropical convergence zone (BBC Bitesize).- Surface winds that blow towards the poles are called westerlies. They blow from the NW in the southern hemisphere and the SW in the northern hemisphere.
- At 60° north and south of the Equator, the warmer surface winds meet colder air from the poles. The warmer air is less dense than the colder air so it rises, creating low pressure. Any moisture in the rising air will cool and condense to form clouds and rain.
- At the poles, the cool air sinks creating high pressure. This air is then drawn back towards the Equator causing surface winds (polar easterlies).
The pressure belts caused by the global circulation system cause variations in climate.
- Around the Equator there is rising air which creates low pressure. Temperatures are hot all year round due to intense sunlight and creates very moist air due to high rates of evaporation. The moisture in the rising air cools and condense to form clouds which bring high amounts of rainfall.
- Towards 30° north and south of the Equator, there high pressure means that rainfall is very low for most of the year. This is because air is sinking and warming up so any moisture cannot condense to form clouds and rain. Temperatures are still warm in this arid (dry) zone).
- In the temperate zone, around 60° north and south of the Equator, there is low pressure. Air is rising and any moisture in the air is cooling and condensing meaning that rainfall is frequent. Temperatures are mild.
- In the polar zone, temperatures are low all year round due to weak sunlight. There is high pressure meaning air is sinking which bring dry conditions.