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In meteorology, an anticyclone is a weather phenomenon associated with atmospheric high pressure. In the northern hemisphere an anticyclone rotates in the clockwise direction, while it rotates counterclockwise in the southern hemisphere. The rotation is caused by the movement of colder higher pressure air that is moving away from the poles towards the equator being affected by the rotation of the earth.
Anticyclones generally bring fair weather and clear skies as the dynamics of an anticyclone lead to downward vertical movement which suppresses convective activity and generally lowers the mean relative humidity, in contrast to the upward vertical movement in a cyclone. However as the anticyclone moves over the earth surface it may heat up locally, acquire water from the land or oceans or encounter warmer wet air. Local geography may cause a range of localised weather phenomena specific to anticyclones, while the interaction of the different air masses, which occurs at weather fronts, may cause a range of weather events.
The foregone is a general introduction to an anticyclone.
Francis was here
An ingenious man who traveled widely and left his marks upon the world, Sir Francis Galton proposed the existence of the anticyclone. He wrote Meteorographica, or Methods of Mapping the Weather(1863). The discovery of the anticyclone enabled meteorologists to draw the modern weather map.
DRY AIR
All anticyclones are produced by dry air that settles to the surface of the earth and accumulates, forming air masses. The absence of aqueous vapor (water vapor) increases the density of the air which means that each volumetric unit of dry air weighs more than the same volumetric unit of humid air at the same temperature, and vapor pressure is why. The two commomest parts of the air are Nitrogen (roughly 78% of the total) and Oxygen (roughly 21% of the total). Together, the two components weigh more than 99% of the total weight of the atmosphere. When air takes on aqueous vapor (water vapor), vapor pressure displaces some of the heavier Nitrogen and Oxygen, thus, a mixture that is lighter in weight overall is created. Displacement by vapor pressure produces intense tropical storms called hurricanes, typhoons, or baguios. The weight of air is called its air pressure.
Cool or cold dry air type
Cool or cold dry air settles onto land and forms shallow anticyclones or high-pressure cells which often move across the terrain and create fair weather with little cloudiness or precipitation, then dissipate and vanish after reaching the open sea. The two types of anticyclones display different patterns of movement.
Warm dry air type
An anticyclone composed of warm dry air may be situated over much of the North Atlantic ocean during most of the year. The warm dry air type of anticyclone is tall and may be observed on weather charts above three miles (5km) in height. The warm dry air type of anticyclone is usually described as being semipermanent. Frontal activity is not associated with it. Transoceanic in extent, in Europe it is called the Azores High, and in the United States it is known by the name Bermuda High.
Since it has a tropical origin, its most proper name is extratropical anticyclone. It has a characteristic "vertical displacement" that shifts its center away from its surface position towards the equator and westwards, too. Far above the surface of the North Atlantic ocean at a height of 3-4 miles (5-7km), the center of the high-pressure cell may be seen about 3,000 miles (5,000km) southwestwards of its surface position (which is in the general vicinity of the Azores Islands).
Similar anticyclones that are built of warm dry air exist over other oceanic areas of the world, such as the South Atlantic ocean. The anticyclone that is located there is practically a mirror-image of the anticyclone that is located over the North Atlantic ocean. Its "vertical displacement" is also towards the equator and westwards, too. The warm dry air is continually being produced in the Intertropical convergence zone by thunderstorms.
External links
Intertropical Convergence Zone photo (http://rsd.gsfc.nasa.gov/rsd/images/goes8_lg.jpg)
- See also: cyclone, Coriolis force, atmospheric pressure, high-pressure cell, North American High, Siberian High, Atmospheric circulation, Earth's atmosphere, Pressure system.
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