A Passat (Portuguese passar, " pass, pass, pull - go " or Italian passata, " crossing " ) is a moderately strong and very resistant to wind, which occurs around the world in the tropics to about 30 ° latitude.
There are two trade-winds with different prevailing wind directions:
- Northeast trade winds in the northern hemisphere
- Southeast trade winds in the Southern Hemisphere
The direction from which a wind blows, giving it the name. Thus, the northeast trade winds blowing from the northeast, the southeast trade winds from the southeast.
Between the Passat zones is the intertropical convergence zone, where the wind usually blows from different directions and weak (see Walker circulation ).
The trade, depending on the nature of the swept surface have different properties. Blowing it over water, it can absorb a lot of moisture and brings as onshore wind coastal areas of high precipitation. However he Strokes large land masses, its moisture content is very low and it causes dry climate.
Emergence of the trade winds: Basic principles
Äquatornah the sun is at noon almost vertically (up to 90 ° angle of incidence, that is, stand at the zenith ) and thereby heats the air above the ground very strong, even though the days are only 12 to a maximum of 13.5 hours long. The heated air less dense and rises, so more (along the so-called Intertropical Convergence Zone - ITC or ITCZ ) is formed above the ground a " trough of low pressure ".
As you ascend, the air cools adiabatically, so that water condenses ( cloud formation ) and often come down heavy downpours. About the evaporation and the condensation on the floor in height, which releases heat, additional heat energy from the earth's surface is transported into the air. At the tropopause ( roughly 15-18 km altitude), the air flows away to the north and south of the equator. In this case, although the air is further cooled, but in comparison to the masses of the air is still relatively warm high latitudes. By the temperature stratification on the very warm air coming from the ITC to the comparatively less warm air to the higher latitudes formed stable Passat inversion, which largely prevents the vertical air exchange. With the movement towards the poles, the air masses are compressed into a smaller space, because the meridians from the equator to the poles ever closer to each other (for comparison: distance between two meridians at the equator around 111 km, at the 30th latitude about 96 km). This spatial confinement pushes together the air masses and forces them closer towards the floor. A large part of the air flowing towards the pole mass decreases by approximately 30 ° North and 30 ° South in the area. This results in these regions stable anticyclones. On descending, the air is heated.
The air flowing from the high pressure area air follows again the air pressure gradient, therefore main currents blow to the equatorial trough of low pressure. These winds are relatively stable, but are distracted due to the Earth's rotation to easily easterly winds, namely, on the northern hemisphere in the direction of flow to the right and to the left in the southern hemisphere. To the north-east, respectively, Southeast trade winds that move, however, during the year in north-south direction are formed (see below). These winds are so characteristic that it was especially early on referred and investigated their formation in their direction, strength and temperature and precipitation conditions.
The confluence of the trade winds are the intertropical convergence zone its name.
The classification of the trade winds in global wind systems is described in Planetary circulation.
Shifts during the year
Due to the obliquity of the ecliptic, the Zenitalstand the sun moves during the year, on the day of the summer solstice ( summer solstice ), the sun is directly above the Tropic of Cancer, the day of the winter solstice ( beginning of winter ) corresponding to the southern. Due to the seasonal variation of solar radiation shifted the Intertropical Convergence Zone, and with it also shifts the wind system of the Passat circulations between the two tropics. This fall many tropical regions in the course of regularly alternating under the influence of the north-east and the south-east trade wind. This twice yearly change in the trade wind direction is called a monsoon.
The course of the Intertropical Convergence Zone may also be affected due to the distribution of land and water masses and their different thermal behavior as well as by the mountain ranges of large mountain ranges. It so happens that, for example, moved across northern Pakistan, the Intertropical Convergence Zone in the summer unusually far beyond the 35 ° latitude beyond.
Since the moisture content of the trade winds depends on whether they draw mainly on water or land, the resulting rainfall of northeast and southeast trade can be highly variable. In such cases occur seasonal variations in rainfall, which are referred to as rain and dry seasons. For example, on the Gulf of Guinea in West Africa, the rainy season in the months of May to July ( south-east trade ), prevails in the rest of the year dry season ( northeast trade winds ). The formation of the Atacama Desert in northern Chile is due to (dry ) trade winds.
The Passat circulation, ie the globe for seamless flow of air of the trade winds, was first described in 1735 by George Hadley factually correct, some one hundred years before Gaspard Gustave de Coriolis Coriolis force recognized the general as the cause of motion distraction to the west. The air circulation system, which spring from the trade winds, is therefore called the Hadley cell.
Was chosen for its durability and is the trade of sailing ships used for rapid crossing of the oceans. This was true especially true to the time of sailing ships that could cross only with difficulty against the wind because of square sails, and achieved significantly higher speeds with sideways or winds. To date, place sailors their route because of the good predictable winds like in the trade regions. Although characterized as a rule extended to the sailing route will be shortened by taking advantage of the trade winds, the journey times.
The northeast trade winds also made the Atlantic triangular trade in the early modern period possible. European sailing ships sailed first on the west coast of Africa to the south, until they were captured by the northeast trade winds, with the aid of a rapid drive west was possible. Once there, they used the northbound ocean current to come from the Gulf of Mexico to the north, where together they returned the westerly winds in the northern hemisphere and the Gulf Stream to Europe again.
The change in wind direction over the Indian Ocean
The ITC moves very far on the northern hemisphere, as the south-east trade winds cross the equator and is then deflected by the Coriolis force is not more to the east, but west. The southeast trade winds thereby turns to south-west. This happens regularly over the Indian Ocean, because the ITC India and Pakistan at the beginning of summer unusually far north migrate ( to beyond 30 degrees north latitude ).