The tilt in the earth's axis causes the seasonal variations as different parts of the earth receives varied amount of the sunrays everyday. The Tropical Zone between 23.5 degrees north and 23.5 degrees south receives maximum heat throughout the year. This effects convection. This convection causes the hot air to rise up, form a low pressure and come down again as rainfall.

Until it reaches the tropopause (the juncture of the stratosphere and the troposphere) the hot air keeps moving up. It cannot cross the tropopause. It spreads towards the pole and then cools and sinks down generally in between the 30 degrees north and south. The sinking air produces a high pressure belt which causes dry and sunny climate. The deserts on the earth are located mostly in these zones.

In the zone between the 30 degrees north and south latitudes the sinking air displaces a considerable mass of air that was already there. Some of it moves towards the equatorial area as trade winds. By the time it reaches the equator, it dies out and this was called the doldrums by sailors.

Thus we can see that the global circulation of air is to a large extent a cause of the rise of air in the tropics, sinking of the same in between the 30 degrees north and south latitude and also the eventual flowing back of it to the equator. This whole process was first observed by English scientist George Hadley and thus it gets its name as the Hadley cells.

The whole of the rising equatorial wind do not sink in the 30 degrees' zone. Some continues to flow till they reach the poles. They meet the polar air at 60 degrees north and south. This zone is called the polar fronts. From here the warmer air moves towards the 30 degrees' area and this was first observed by William Ferrel in 1856. Thus this process earns its name: the Ferrel cells.

There are some air that rises at the 60 degree and flows towards the 60 degrees north and south zone. This is quite a weak process and is popularly called the polar Hadley cells.