Prevailing winds

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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.

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.

Moving further towards the poles, between 30 and 60 degrees north and south there are the prevailing westerlies, blowing from the southwest in the northern hemisphere and from the northwest in the southern hemisphere. These are the typical winds for most of North America, Europe, Central Asia, China, and Japan, for example. Getting closer to the poles, the prevailing wind shifts to the polar easterlies, cold air blowing from the northeast in the northern hemisphere and from the southeast in the southern hemisphere.

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 tradewinds, 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.

Near sloping land like mountains and valleys, it is also common to have a cool wind called the katabatic wind or mountain breeze blowing downslope at night, and a warm wind called the anabatic wind or valley breeze blowing upslope during the day, particularly during the summer. Again, in cases like these the direction of the prevailing wind may be upslope or downslope, no matter what the latitude.Template:Climate-stub

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