As discussed previously, the sun shines most intensely over the equator throughout the year, and least at the poles. Thus, the Earth has a strong temperature contrast between the poles and the equator.

Atmospheric circulation on a non-rotating Earth with this temperature contrast would look like this:

As we know the Earth does rotate. This produces a different general circulation. A rotating Earth causes changes in wind directions because of the Coriolis effect. The Earth rotates on its axis once every 24 hours. The speed each point on the Earth moves is based on its position.

So air at the equator is moving at a greater speed than air closer to the poles. If the equatorial air is allowed to move away from the equator it will appear to move faster than the ground below it, thus appearing to move to the right in the northern hemisphere and to the left in the southern hemisphere.

This produces a different model of the general circulation of the Earth. Instead of a single cell each hemisphere, we see three separate cells.

Another cell is produced by the spreading of the winds that sink near 30° N and S. This produces what is called the Hadley cell.

The final cell is named the Polar cell. Air rising around 60° N and S sinks over the poles producing northeasterly surface winds.