An anticyclone is a
weather phenomenon defined as a large-scale circulation of
winds around a central
region of high atmospheric pressure, clockwise in the
Northern Hemisphere and counterclockwise in the
Southern Hemisphere as viewed from above (opposite to a
cyclone). Effects of surface-based anticyclones include clearing skies as well as cooler, drier air.
Fog
Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...
can also form overnight within a region of higher pressure.
Mid-tropospheric systems, such as the
subtropical ridge
The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges, or highs. It is a high-press ...
, deflect tropical cyclones around their periphery and cause a
temperature inversion inhibiting
free convection near their center, building up surface-based
haze under their base. Anticyclones aloft can form within warm-core lows such as
tropical cyclones, due to descending cool air from the backside of upper
troughs such as
polar highs, or from large-scale sinking such as a
subtropical ridge
The horse latitudes are the latitudes about 30 degrees north and south of the Equator. They are characterized by sunny skies, calm winds, and very little precipitation. They are also known as subtropical ridges, or highs. It is a high-press ...
.
The evolution of an anticyclone depends upon variables such as its size, intensity, and extent of moist
convection, as well as the
Coriolis force.
History
Sir Francis Galton first discovered anticyclones in the 1860s. Preferred areas within a
synoptic flow pattern in higher levels of the dips in the
Rossby wave pattern. High-pressure systems are alternatively referred to as anticyclones. Their circulation is sometimes referred to as
cum sole. Subtropical high-pressure zones form under the descending portion of the
Hadley cell circulation. Upper-level high-pressure areas lie over tropical cyclones due to their warm core nature.
Surface anticyclones form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a
synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as
confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up the troposphere. Because they weaken with height, these high-pressure systems are cold.
Subtropical ridge
Heating of the earth near the equator forces upward motion and convection along the
monsoon trough or
intertropical convergence zone. The divergence over the near-equatorial trough leads to air rising and moving away from the equator and to the poles aloft. As air moves towards the mid-latitudes, it cools and sinks leading to subsidence near the 30° parallel of both hemispheres. This circulation known as the
Hadley cell forms the subtropical ridge. Many of the world's deserts are caused by these climatological
high-pressure areas. Because these anticyclones strengthen with height, they are known as warm core ridges.
Formation aloft
The development of anticyclones aloft occurs in warm core cyclones such as
tropical cyclones when latent heat caused by the formation of
clouds is released aloft increasing the air temperature; the resultant thickness of the atmospheric layer increases high pressure aloft which evacuates their outflow.
Structure
In the absence of rotation, the
wind tends to blow from areas of high pressure to
areas of low pressure. The stronger the pressure difference (pressure gradient) between a high-pressure system and a low-pressure system, the stronger the wind. The
coriolis force caused by
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's sur ...
's rotation gives winds within high-pressure systems their clockwise circulation in the northern hemisphere (as the wind moves outward and is deflected right from the center of high pressure) and anticlockwise circulation in the southern hemisphere (as the wind moves outward and is deflected left from the center of high pressure). Friction with land slows down the wind flowing out of high-pressure systems and causes wind to flow more outward (more
ageostrophically) from the center.
Effects
Surface-based systems
High-pressure systems are frequently associated with light winds at the surface and
subsidence of air from higher portions of the
troposphere. Subsidence will generally warm an air mass by
adiabatic (compressional) heating. Thus, high pressure typically brings clear skies. Because no clouds are present to reflect sunlight during the day, there is more incoming
solar radiation and heating so temperatures rise rapidly near the surface. At night, the absence of clouds means that
outgoing longwave radiation (i.e. heat energy from the surface) is not blocked, allowing the escape of heat and giving cooler
diurnal low temperatures in all seasons. When surface winds become light, the subsidence produced directly under a high-pressure system can lead to a buildup of particulates in urban areas under the high pressure, leading to widespread
haze. If the surface level
relative humidity rises towards 100 percent overnight,
fog
Fog is a visible aerosol consisting of tiny water droplets or ice crystals suspended in the air at or near the Earth's surface. Reprint from Fog can be considered a type of low-lying cloud usually resembling stratus, and is heavily influ ...
can form.
The movement of continental arctic air masses to lower latitudes produces strong but vertically shallow high-pressure systems. The surface level, sharp temperature
inversion
Inversion or inversions may refer to:
Arts
* , a French gay magazine (1924/1925)
* ''Inversion'' (artwork), a 2005 temporary sculpture in Houston, Texas
* Inversion (music), a term with various meanings in music theory and musical set theory
* ...
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 because they pick up moisture as they move over the warmer oceans. High pressures that build to the north and move southwards often bring clear weather because they are cooled at the base (as opposed to warmed) which helps prevent clouds from forming.
Once arctic air moves over an unfrozen ocean, the air mass modifies greatly over the warmer water and takes on the character of a maritime air mass, which reduces the strength of the high-pressure system. When extremely cold air moves over relatively warm oceans,
polar low
A polar low is a mesoscale, short-lived atmospheric low pressure system (depression) that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres, as well as the Sea of Japan. The systems usu ...
s can develop. However, warm and moist (or maritime tropical) air masses which move poleward from tropical sources are slower to modify than arctic air masses.
Mid-tropospheric systems
The circulation around mid-level (altitude) ridges, and the air subsidence at their center, act to steer tropical cyclones around and out their periphery. Due to the subsidence within this type of system, a ''
cap'' can develop which inhibits free convection and hence mixing of the lower with the middle level troposphere. This limits
thunderstorms and other low-pressure weather activity near their centers and traps low-level pollutants such as
ozone
Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the l ...
as
haze under their base, which is a significant problem in large urban centers during summer months such as
Los Angeles, California and
Mexico City
Mexico City ( es, link=no, Ciudad de México, ; abbr.: CDMX; Nahuatl: ''Altepetl Mexico'') is the capital and largest city of Mexico, and the most populous city in North America. One of the world's alpha cities, it is located in the Valley o ...
.
Upper tropospheric systems
The existence of upper-level (altitude) high pressure allows upper level
divergence
In vector calculus, divergence is a vector operator that operates on a vector field, producing a scalar field giving the quantity of the vector field's source at each point. More technically, the divergence represents the volume density of ...
which leads to surface
convergence. If a capping mid-level ridge does not exist, this leads to
free convection and the development of showers and thunderstorms if the lower atmosphere is humid. Because a
positive feedback loop develops between the convective tropical cyclone and the upper level high, the two system are strengthened. This loop stops once ocean temperatures cool to below ,
[ Chris Landsea]
Subject: A15) How do tropical cyclones form?
National Hurricane Center. Retrievon 2008-06-08. reducing the thunderstorm activity, which then weakens the upper-level high-pressure system.
Importance to global monsoon regimes
When the subtropical ridge in the Northwest Pacific is stronger than in other areas, it leads to a wet
monsoon season for
Asia
Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an are ...
. The subtropical ridge position is linked to how far northward monsoon moisture and
thunderstorms extend into the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country Continental United States, primarily located in North America. It consists of 50 U.S. state, states, a Washington, D.C., ...
. Typically, the subtropical ridge across
North America
North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and th ...
migrates far enough northward to begin monsoon conditions across the
Desert Southwest
The Southwestern United States, also known as the American Southwest or simply the Southwest, is a geographic and cultural region of the United States that generally includes Arizona, New Mexico, and adjacent portions of California, Colorado, ...
from July to September. When the subtropical ridge is farther north than normal towards the
Four Corners, thunderstorms of the
New Mexican Monsoon
The North American monsoon, variously known as the Southwest monsoon, the Mexican monsoon, the New Mexican monsoon, or the Arizona monsoon is a pattern of pronounced increase in thunderstorms and rainfall over large areas of the southwestern Un ...
can spread northward into
Arizona
Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...
and
New Mexico
)
, population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano)
, seat = Santa Fe, New Mexico, Santa Fe
, LargestCity = Albuquerque, New Mexico, Albuquerque
, LargestMetro = Albuquerque metropolitan area, Tiguex
, Offi ...
. When suppressed to the south, the atmosphere dries out across the Desert Southwest, causing a break in the monsoon regime.
Depiction on weather maps
On weather maps, high-pressure centers are associated with the letter H in English, within the
isobar with the highest pressure value. On constant-pressure upper-level charts, anticyclones are located within the highest height line contour.
[Glossary of Meteorology (2009)]
High
. American Meteorological Society. Retrieved on 2009-02-16.
Extraterrestrial versions
On Jupiter, there are two examples of an extraterrestrial anticyclonic storm; the
Great Red Spot and the recently formed
Oval BA on
Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousand ...
. They are powered by smaller storms merging unlike any typical anticyclonic storm that happens on Earth where water powers them. Another theory is that warmer gases rise in a column of cold air, creating a vortex as is the case of other storms that include
Anne's Spot Anne's Spot refers to a reddish-colored anticyclonic oval in Saturn's atmosphere, observed in 1980 and 1981 at 55°S by the '' Voyager'' space probes.
It was probably also observed in 2004 at about 53°S by the Cassini orbiter Cassini may refer to: ...
on
Saturn and the
Great Dark Spot
The Great Dark Spot (also known as GDS-89, for Great Dark Spot, 1989) was one of a series of dark spots on Neptune similar in appearance to Jupiter's Great Red Spot. In 1989, GDS-89 was the first Great Dark Spot on Neptune to be observed by NASA' ...
on Neptune. Anticyclones had also been detected near the poles of Venus.
See also
*
Anticyclonic storm
*
Anticyclonic tornado
*
Atmosphere of Earth
*
Atmospheric circulation
*
Atmospheric pressure
*
Great Smog of London
The Great Smog of London, or Great Smog of 1952, was a severe air pollution event that affected London, England, in December 1952. A period of unusually cold weather, combined with an anticyclone and windless conditions, collected airborne poll ...
*
Barometric ridge
*
Block (meteorology)
*
North American High
*
Ocean gyre
*
Pressure system
References
External links
Intertropical Convergence Zone photo- NASA Goddard Space Flight Center
{{Authority control
Meteorological phenomena
Tropical cyclone meteorology