Prevailing winds
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 reflect global patterns of movement in the
earth's atmosphere.
:Additional detail is available in a separate article,
Geostrophic wind.
The prevailing surface winds are calm at the
equator (the
doldrums), then 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 because they allowed early European ships to sail from east to west to the Americas. 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 that 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.
All of this is the general trend, but the prevailing wind in any specific location will depend not just on latitude but on local geography. For example, during the day, a large body of water may be cooler than the air above it, causing the air to sink and form higher pressure, then blow towards the land (where the surface is warmer so the pressure is lower). At night, the opposite happens, and the warm water causes air to rise, creating low pressure and sucking air off the cooler land. The result is an onshore wind called the
sea breeze during the day, and an offshore wind called the
land breeze during the night (which can be treacherous for recreational sailors caught too far from land after dark). A very severe version of the sea breeze is the
monsoon, which lasts for an entire summer instead of a day. In these cases, the direction of the prevailing wind may be away from or towards the water, no matter what the latitude.
