Air circulation

Wind is nothing other than moving air. But why does it move at all? The reason is again the sun: it does not heat the Earth equally strongly everywhere. Where the air becomes warm it rises, where it is cool it sinks. From this constant up and down, great air streams form that span the whole Earth.

Warm air rises, cold air sinks

At the same volume, warm air is lighter than cold air, so it rises upwards. At the equator the air is strongly heated and rises high into the atmosphere. Up there it flows off to the side, cools down slowly and sinks back to the ground in the subtropics, roughly 30 degrees of latitude north and south of the equator. At the ground it streams back towards the equator, and the circle closes. Such a loop of rising and sinking air is called a circulation cell.

The trade winds

The air that streams back to the equator at ground level blows very steadily. These constant winds are called trade winds. Because the Earth turns, they do not blow exactly to the north or south but are deflected to the side. In the past sailing ships used the trade winds to travel reliably across the oceans, because they could almost always be counted on.

Why the Earth's spin turns the wind

Because the Earth turns away beneath the air, the winds do not run straight ahead but in a curve. In the northern half of the Earth they are deflected to the right, in the southern half to the left. This effect is called the Coriolis effect, and it makes fixed wind belts form around the Earth. Each half has three of them: the trade winds in the tropics, the westerlies in the middle latitudes and the cold easterlies at the poles. Without the turning of the Earth the wind would blow quite differently.