anticyclone, region of high atmospheric pressure; anticyclones are commonly referred to as "highs." The pressure gradient, or change between the core of the anticyclone and its surroundings, combined with the Coriolis effect, causes air to circulate about the core in a clockwise direction in the Northern Hemisphere and a counterclockwise direction in the Southern Hemisphere. Near the surface of the earth the frictional drag of the surface on the moving air causes it to spiral outward gradually toward lower pressures while still maintaining the rotational direction. This outward movement of air is fed by descending currents near the center of the anticyclone that are warmed by compression as they encounter higher pressures at lower altitudes. The warming, in turn, greatly reduces the relative humidity, so that anticyclones, or "highs," are generally characterized by few clouds and low humidity. Such weather characteristics may extend over an area from a few hundred to a few thousand miles wide. Many low-level anticyclones are swept generally eastward by the prevailing west-to-east flow of the upper atmosphere, usually traversing some 500 to 1,000 mi (800-1,600 km) per day. Other anticyclones are permanent or seasonal features of particular geographic regions. The term anticyclone is derived from the fact that the associated rotational direction and general weather characteristics of an anticylone are opposite to those of a cyclone.

In meteorology, an anticyclone (that is, opposite to a cyclone) is a weather phenomenon in which there is a descending movement of the air and a high pressure area over the part of the planet's surface affected by it. Anticyclonic flow spirals in a clockwise direction in the Northern Hemisphere and counter-clockwise in the Southern.

Terminology

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 used in the United States. Except for the wording, there is no difference; there are no separate or different types of warm dry air anticyclones being generated by the Intertropical Convergence Zone. Both extratropical cyclones and subtropical anticyclones are seen in the Northern Hemisphere, year-round. For an example of the Australian term, see: Equatorial trough

Origin

The notable scientist Sir Francis Galton proposed the existence of the anticyclone as a part of his work in weather and writing Meteorographica, or Methods of Mapping the Weather (1863). Galton's anticyclone hypothesis was eventually confirmed with the discovery of the anticyclone which enabled meteorologists to draw the modern weather map.Early versions of surface weather analysis charts produced prior to 1863 depicted cyclones, but not anticyclones.

Formation

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 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 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 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 or typhoons.

Anticyclogenesis

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 the air 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 "Terminology", above). 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 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.

Structure

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.

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's surface it may heat up locally, acquire water from the land or oceans or encounter warmer wet air.

Evolution

{link to Interactive Weather Satellite (NOAH GOES) and see the absence of water vapor in the global water vapor mosaic.}

Motion

Effects

In winter the 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 winds and fine weather. The low, sharp inversion can lead to areas of persistent stratocumulus or stratus cloud, colloquially known as Anticyclonic gloom. The type of weather brought about by an anticyclone depends on its origin. For example, extensions of the Azores high pressure may bring about anticyclonic gloom during the winter, as they are warmed at the base and will trap moisture as they move over the warmer oceans. High pressures that build to the north and extend southwards will often bring clear weather. This is due to being cooled at the base (as opposed to warmed) which helps prevent clouds from forming.

Local geography may cause a range of localized 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.

Extraterrestrial anticyclones

The Great Red Spot on Jupiter is an example of an extraterrestrial anticyclonic storm. Other storms include the recently formed Oval BA on Jupiter, Anne's Spot on Saturn, and the Great Dark Spot on Neptune.

References

See also

• Atmospheric circulation
• Atmospheric pressure
• Block (meteorology)
• Coriolis force
• Cyclone
• Earth's atmosphere
• Gyre
• High pressure area
• North American High
• Pressure system
• Siberian High
• Anticyclonic tornado
• Great Red Spot

External links

• Intertropical Convergence Zone photo - NASA Goddard Space Flight Center