Global wind patterns
Encyclopedia
Wind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
s are named after the direction from which they blow. The globe is encircled by six major wind belts, three in each hemisphere. From pole to equator they are the polar easterlies, the westerlies and the trade winds. All six belts move north in the northern summer and south in the northern winter.
Polar Easterlies: At about the latitude of Norway and northward (60-90 degrees) the Polar easterlies
Polar easterlies
The polar easterlies are the dry, cold prevailing winds that blow from the high-pressure areas of the polar highs at the north and south poles towards low-pressure areas within the Westerlies at high latitudes...
blow irregularly from the east and north.
Polar Front: Between the polar easterlies and the westerlies is the polar front
Polar front
In meteorology, the polar front is the boundary between the polar cell and the Ferrel cell in each hemisphere. At this boundary a sharp gradient in temperature occurs between these two air masses, each at very different temperatures....
.
Westerlies: At about the latitude of Western Europe and the United States (30-60 degrees) the Westerlies
Westerlies
The Westerlies, anti-trades, or Prevailing Westerlies, are the prevailing winds in the middle latitudes between 30 and 60 degrees latitude, blowing from the high pressure area in the horse latitudes towards the poles. These prevailing winds blow from the west to the east, and steer extratropical...
blow from the west, tending somewhat toward the north. This causes most weather in the United States to move from west to east.
Horse Latitudes: Where the Westerlies meet the trade winds at about 30 degrees (Jacksonville, Florida) is the Horse latitudes
Horse latitudes
Horse Latitudes or Subtropical High are subtropical latitudes between 30 and 35 degrees both north and south. This region, under a ridge of high pressure called the subtropical high, is an area which receives little precipitation and has variable winds mixed with calm.The consistently warm, dry...
, Variables of Cancer, Subtropical High or Subtropical ridge
Subtropical ridge
The subtropical ridge is a significant belt of high pressure situated around the latitudes of 30°N in the Northern Hemisphere and 30°S in the Southern Hemisphere. It is characterized by mostly calm winds, which acts to reduce air quality under its axis by causing fog overnight, and haze during...
. This is a region of high pressure, dry air and variable winds and is associated with deserts over land.
Trade Winds: South of about 30 degrees the northern or northeast trade winds blow mostly from the northeast toward the equator. These were the sailor's favorite winds since the weather was warm and the winds usually blew steadily in the right direction.
Doldrums: At about the equator is Intertropical Convergence Zone or doldrums
Doldrums
The doldrums is a colloquial expression derived from historical maritime usage for those parts of the Atlantic Ocean and the Pacific Ocean affected by the Intertropical Convergence Zone, a low-pressure area around the equator where the prevailing winds are calm...
, a region of light and irregular wind broken by occasional thunderstorms and squalls. The width and exact location of the doldrums is hard to predict. Sailing ships are sometimes becalmed here for many days waiting for a proper wind.
Southern hemisphere: In the southern hemisphere the belts are reversed. The southeast trade winds blow from the southeast toward the equator. The southern equivalent of the horse latitudes (Variables of Cancer) is called the Variables of Capricorn. The southern westerlies start somewhat south of South Africa. They tend to be stronger than the northern westerlies because they are mostly over water (roaring forties
Roaring Forties
The Roaring Forties is the name given to strong westerly winds found in the Southern Hemisphere, generally between the latitudes of 40 and 49 degrees. Air displaced from the Equator towards the South Pole, which travels close to the surface between the latitudes of 30 and 60 degrees south, combines...
). The southern polar easterlies are mostly over Antarctica.
Seasonal shifts: All of the belts move north during the northern summer and south during the northern winter. Because global heating and cooling lags behind the position of the sun they reach their northernmost latitude at or after the end of the northern summer. This brought the trade winds within reach of the Spain and Portugal and determined the sailing time of the Spanish treasure fleet
Spanish treasure fleet
The Spanish treasure fleets was a convoy system adopted by the Spanish Empire from 1566 to 1790...
. The northernmost position of the wind belts corresponds to the Atlantic hurricane season.
Land and sea breezes: Land gains and looses heat more rapidly than water. During the day the land warms more rapidly than the water. The air above land warms, becomes thinner and rises drawing cooler air landward from the sea. At night the process reverses and cool heavy air from the land flows out to sea. These land and sea breezes are important along the coast.
Monsoon: The annual equivalent of the daily land and sea breezes is the yearly monsoon
Monsoon
Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...
. During summer the continents heat more rapidly than the oceans. Air over the continents warms, thins and rises drawing cooler moist ocean air landward, producing a wet season
Wet season
The the wet season, or rainy season, is the time of year, covering one or more months, when most of the average annual rainfall in a region occurs. The term green season is also sometimes used as a euphemism by tourist authorities. Areas with wet seasons are dispersed across portions of the...
. During winter the process reverses and cold, dry heavy air flows outward from the continents, producing a dry season
Dry season
The dry season is a term commonly used when describing the weather in the tropics. The weather in the tropics is dominated by the tropical rain belt, which oscillates from the northern to the southern tropics over the course of the year...
. The monsoon is most striking in south Asia because of the size of the Eurasian landmass and because the Himalayas tend to bottle up the air above the continent. Approximations of the Indian monsoon exist in other places but they are poorly developed.
Coriolis force, Hadley cell and other things: In the northern hemisphere the Coriolis effect
Coriolis effect
In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...
causes wind and water currents to bend to the right (clockwise). Cold heavy air flows south from the north pole and is bent west, forming the polar easterlies. Warm air rises at the equator drawing air from the north which bends to the west, contributing to the trade winds. The Coriolis effect bends the westerlies and trade winds slightly clockwise in the northern hemisphere.
Hot air rises at the doldrums. As it rises it cools producing thunderstorms. The dry air flows north at a high altitude and descends at the horse latitudes and flows back to the equator with the trade winds. This is called the hadley cell
Hadley cell
The Hadley cell, named after George Hadley, is a circulation pattern that dominates the tropical atmosphere, with rising motion near the equator, poleward flow 10–15 kilometers above the surface, descending motion in the subtropics, and equatorward flow near the surface...
. There is also a Ferrel cell over the westerlies and a polar cell over the pole. There are other complexities, not all of which are properly understood.