The atmosphere circulates because of unequal heating of the Earth by the Sun.

At the equator air is heated, rises and then flows towards the poles, creating a permanent low-pressure area (the doldrums) around the equator.

At about 30°N and 30°S some of the air sinks, giving rise to the zones of high pressure called the horse latitudes.

Continuing to move away from the equator, the air cools and sinks (creating high pressure) over the poles. It then flows back towards the equator. The overall effect of the atmosphere's circulation is to create a pattern of prevailing winds that blow from high- to low-pressure areas.