An anticyclone is a weather phenomenon defined by the United States National Weather Service's
glossary as "a large-scale circulation of winds around a central region of high atmospheric
pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere".
Effects of surface-based anticyclones include clearing skies as well as cooler, drier air.
Fog can also form overnight within a region of higher pressure. Mid-tropospheric systems,
such as the subtropical ridge, 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 the subtropical ridge.
History Sir Francis Galton first discovered anticyclones
in the 1860s. Preferred areas within a synoptic flow pattern in higher levels of the troposphere
are beneath the western side of troughs, or dips in the Rossby wave pattern. High-pressure
systems are alternatively referred to as anticyclones. Their circulation is sometimes referred to
as cum sole. The subtropical ridge forms due to the Hadley cell circulation between the
equator and the subtropics of the Northern Hemisphere and Southern Hemisphere. 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, 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 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 between a high-pressure system and a low-pressure system, the stronger the
wind. The coriolis force caused by Earth's rotation gives winds within high-pressure
systems their clockwise circulation in the northern hemisphere and anticlockwise circulation
in the southern hemisphere. Friction with land slows down the wind flowing out of high-pressure
systems and causes wind to flow more outward from the center.
Effects Surface-based systems
High-pressure systems are frequently associated with light winds at the surface and subsidence
through the lower portion of the troposphere. Subsidence will generally dry out an air mass
by adiabatic heating. Thus, high pressure typically brings clear skies. Because no clouds
are present to reflect sunlight during the day, there is more incoming shortwave solar
radiation and temperatures rise. At night, the absence of clouds means that outgoing
longwave radiation is not absorbed, 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 ridge, leading to widespread
haze. If the low level relative humidity rises towards 100 percent overnight, fog can form.
Strong but vertically shallow high-pressure systems moving from higher latitudes to lower
latitudes in the northern hemisphere are associated with continental arctic air masses. 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 because they are warmed at the base and trap moisture as they move
over the warmer oceans. High pressures that build to the north and extend southwards often
bring clear weather because they are cooled at the base 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 lows can develop. However, warm and moist
air masses which move poleward from tropical sources are slower to modify than arctic air
masses. Mid-tropospheric systems
The circulation around mid-level ridges, and the subsidence at their center, act to steer
tropical cyclones around their periphery. Due to the subsidence within this type of
system, a cap can be set up which inhibits the development of free convection. This limits
thunderstorm activity near their center and traps low-level pollutants such as ozone 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.
Upper tropospheric systems The existence of an upper level ridge allows
upper level divergence 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 tropical cyclones strengthen these ridges,
a positive feedback loop develops between the convective tropical cyclone and the upper
level high, strengthening both systems. This loop stops once ocean temperatures under the
system cool sufficiently, under 26.5 °C, which forces the thunderstorm activity to
wane, which then weakens the upper level ridge. Importance to global monsoon regimes
When the subtropical ridge in the northwest Pacific is stronger than normal, it leads
to a wet monsoon season for Asia. The subtropical ridge position is linked to how far northward
monsoon moisture and thunderstorms extend into the United States. Typically, the subtropical
ridge across North America migrates far enough northward to begin monsoon conditions across
the Desert Southwest from July to September. When the subtropical ridge is farther north
than normal towards the Four Corners, monsoon thunderstorms can spread northward into Arizona.
When suppressed to the south, the atmosphere dries out across the Desert Southwest, causing
a break in the monsoon regime. in the southern hemisphere in winter, the intensification
of the mascaleni high brings moist air over the continent resulting into formation medium
clouds thus lowering the day temperatures. Depiction on weather maps
On weather maps, high-pressure centers are associated with the letter H in English, or
A in Spanish, within the isobar with the highest pressure value. On constant-pressure upper-level
charts, anticyclones are located within the highest height line contour.
Extraterrestrial versions On Jupiter, there are two examples of an extraterrestrial
anticyclonic storm; the Great Red Spot and the recently formed Oval BA. 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 on Saturn,
and the Great Dark Spot on Neptune. Anticyclones have been detected near the poles of Venus.
See also
Atmospheric circulation Atmospheric pressure
Block Earth's atmosphere
Gyre North American High
Pressure system Anticyclonic tornado
References
External links Intertropical Convergence Zone photo - NASA
Goddard Space Flight Center