Anticyclone

In meteorology, an anticyclone (i.e. opposite to a cyclone) is a weather phenomenon in which there is a descending movement of the air and a relative increase in barometric pressure over the part of the earth's surface affected by it. (This meteorogical phenomenon can also be observed on Jupiter, and can quite possibly exist on other planets). At the surface the air tends to flow outwards in all directions from the central area of high pressure, and is deflected on account of the earth's rotation (see Coriolis effect) so as to give a spiral movement. 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. Since the air in an anticyclone is descending, it becomes warmed and dried, and therefore transmits radiation freely whether from the sun to the earth or from the earth into space. Hence in winter anticyclonic weather is characterized by clear air with periods of frost, causing fogs in towns and low-lying damp areas, and in summer by still cloudless days with gentle variable airs and fine weather.

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.

Origin

Sir Francis Galton proposed the existence of the anticyclone. An ingenious man who traveled widely and left his marks upon the world, 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 vapour) increases the density of 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 vapour pressure. The two most common 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 vapour (water vapour), vapour 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.

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 types of anticyclones display different patterns of movement.

High-latitudes maritime type

In the months of winter, many strong cyclones appear at high latitudes. Rising air in them eventually descends to form anticyclones. Tall anticyclones appear at some places each year during the coldest months. They may exceed 35,000 feet or 10,200 meters in height. The position of each anticyclone is at about the same place on the surface as it is far above the surface. The sea-level pressure may exceed 1040 millibars (hectopascals) (hPa) (SI). They tend to linger close to the place at which they had appeared.

The Denmark Strait along the east coast of Greenland is a place where they often appear, particularly during the winter. They form part of the North Atlantic Oscillation that significantly influences the weather in that region of the Northern Hemisphere. The Beaufort Sea is an arm of the Arctic Ocean that exists north of northwestern Canada. An anticyclone called the Arctic High or the Beaufort High forms there. NSIDC

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 (but see the last paragraph, below). 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 (5,000 km) southwestwards of its surface position (which is in the general vicinity of the Azores Islands).

The maximum sea-level pressure in this type of anticyclone is not very high. It may reach, perhaps, 1025 millibars (hectopascals) or thereabouts during the summertime, which is a mere twelve millibars above the average sea-level pressure of 1013 millibars.

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 (ITCZ) by thunderstorms.

Confusion may be created by the fact that the term "subtropical anticyclone" is used by meteorologists in Australia in place of "extratropical anticyclone", which is the term that used in the United states. Except for the wordage, there is no difference; there are no separate or different types of warm dry air anticyclones being generated by the ITCZ. Both extratropical cyclones and extratropical anticyclones are seen in the Northern Hemisphere, year-round. For an example of the Australian term, see: Equatorial trough

References

External links

Glossary Definition: Anticyclonic rotation - StormWiki
Intertropical Convergence Zone photo - NASA Goddard Space Flight Center

Weather | Tropical cyclone meteorology

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