The actual global pattern of atmospheric circulation is much more complicated than a direct flow between the equator and poles.

Instead of one large circulation in each hemisphere, there are three circulations in each due to the Earth’s rotation:

1 . Hadley cell

At low latitudes, air moves toward the equator, where it is heated and rises vertically.

In the upper atmosphere, air moves poleward.

This forms a convection cell that covers tropical and sub-tropical climates.

This cell is named for English physicist and meteorologist George Hadley, who proposed the single circulation for each hemisphere in 1735.

Though the Hadley cell is described as located at the equator, it shifts northerly (to higher latitudes) in June and July and southerly (toward lower latitudes) in December and January, as a result of the Sun’s heating of the surface.

The Hadley system provides an example of a thermally direct circulation.

2 . Ferrel cell

In this mid-latitude atmospheric circulation cell, air near the surface flows poleward and eastward, while air higher in the atmosphere moves equatorward and westward.

Proposed by William Ferrell in 1856, it was the first to account for westerly winds between 35° and 60° N/S, which are caused by friction, not heat differences at the equator and poles.

The Ferrel cell is weak, because it has neither a strong source of heat nor a strong sink, so the airflow and temperatures within it are variable. For this reason, the mid-latitudes are sometimes known as the “zone of mixing.”

3 . Polar cell

At higher latitudes, air rises and travels toward the poles. Once over the poles, the air sinks, forming areas of high atmospheric pressure called the polar highs.

At the surface, air moves outward from the polar highs, creating east-blowing surface winds called polar easterlies.

It is the smallest and weakest of the cells.

By acting as a heat sink, the polar cell moves the abundant heat from the equator toward the polar regions.

The Hadley cell and the polar cell are similar in that they are thermally direct; in other words, they exist as a direct consequence of surface temperatures. Their thermal characteristics drive the weather in their domain.

These three cells describe circulations in both the Northern and Southern Hemispheres, although the difference in landmasses result in varying degrees of uniformity.

Bands of high and low pressure

Between each of these circulation cells are surface-level bands of high and low pressure.

The high pressure band is located at about 30° N/S latitude and at each pole.

Low pressure bands are found at the equator and 50°-60° N/S.