The prevailing winds are the trends in speed and direction of wind over a particular point on the earth's surface. A region's prevailing winds often show global patterns of movement in the earth's atmosphere. Prevailing winds are the causes of waves as they push the ocean.

Geostrophic windsEdit

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The prevailing surface winds are calm at the equator (the doldrums). From the equator, they blow from the northeast immediately north of the equator and from the southeast immediately south of the equator. These are called the trade winds, a name which derives from the Middle English meaning of "trade" meaning "track" or "path." Around 25 degrees north and south, near the tropics, the winds calm again in the horse latitudes.

Most of this activity is due to air pressure. At the equator, the heat causes air to rise, creating a belt of low pressure in the doldrums. After the air rises, it flows north and south high in the atmosphere until it cools enough to subside, creating belts of high pressure in the horse latitudes. All of the extra air has to go somewhere, so it blows towards the equator as the trade winds, and towards the middle latitudes as the prevailing westerlies.

Meanwhile, at the poles, the cold causes air to subside, increasing the air pressure to cause the polar highs. As with the horse latitudes, the extra air has to flow somewhere, so it flows back in the direction of the equator as the polar easterlies, creating the far northern and southern climates of the world.