All wind is powered by the sun. As air is warmed, it expands and rises, forming low pressure areas. And as it rises, nearby masses of denser, cooler air move in as wind to take its place. Many other factors come into play, such as the presence of mountains and deserts and the differential heating of land and sea. In addition to the global system of prevailing winds, persistant local wind patterns affect many areas of the world.

The globe is surrounded by several zones of prevailing surface winds. Hot air at the equator expands and rises, and the trade winds blow in to take its place. The air settles again at the horse latitudes* (30°N & 30°S), where it fuels both the trade winds and the westerlies. Closer to the poles, the air rises again at another low pressure zone, where the warm westerlies meet the cold polar air masses. This pattern of prevailing winds generally constant; however, it is not unchanging. Uneven heating of the earth's surface and changing seasons sometimes alter it.

Wind direction is the same in the southern hemisphere.

Footnotes:
* They are belts of high pressure located at about 30°N and 30°S lattitudes.

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