Low pressure area 475199 220084582 2008-06-18T05:35:55Z Lou-86 6585997 [[WP:UNDO|Undid]] revision 220081362 by [[Special:Contributions/65.93.166.224|65.93.166.224]] ([[User talk:65.93.166.224|talk]]) [[Image:Low pressure symbol.svg|thumb|The letter [[L]] reprents the center of a Low pressure area.]] [[Image:Low pressure system over Iceland.jpg|thumb|right|A large low-pressure system swirls off the southwestern coast of Iceland.]] A '''low pressure area''', or "'''low'''", is a region where the [[atmospheric pressure]] is lower in relation to the surrounding area. [[Tropical storm]]s, [[mid-latitude cyclone|extratropical cyclone]]s, subpolar cyclones, and subarctic cyclones are called '''low-pressure cells'''. Lows are frequently associated with stronger [[wind]]s and [[atmospheric]] lift. This lift will generally produce [[cloud cover]] through [[adiabatic lapse rate|adiabatic]] cooling, once the air becomes saturated as it rises. Thus, low pressure typically brings cloudy or [[overcast]] skies, which may minimize [[Diurnal temperature variation|diurnal temperature extremes]] in both [[summer]] and [[winter]]. Since the clouds reflect [[sunlight]], incoming [[shortwave radiation|shortwave]] [[solar radiation]] is less which causes lower [[temperature]]s during the day. At night, the absorptive effect of clouds on outgoing [[longwave radiation]], such as heat energy from the surface, allows for warmer diurnal low temperatures in all seasons. In Europe (especially in the UK), reoccurring low pressure weather systems are typically known as depressions. These tend to bring wet weather throughout the year. ==Climatology== [[Climatology|Climatologically]], low pressure forms at the [[Intertropical convergence zone]] (ITCZ), as part of the [[Hadley cell]] circulation. Many of the world's [[rainforests]] are associated with these climatological low pressure systems. [[weather front|Frontal]] lows are [[temperate zone]] phenomena and develop along the [[atmospheric circulation|polar front]] as a result of the interaction between cold and warm surface [[air mass]]es. ''Thermal lows'' also form over areas such as [[Death Valley]] as the result of intense ground heating; they are much smaller in geographic extent than either convegence lows or frontal lows. Surface low pressure systems will tend to be smaller in area and have stronger surface winds than a given high pressure system because of the addition of surface friction to the [[pressure gradient force]], [[centrifugal force]] and [[coriolis effect]] that drive the circulation ==Weather== Low pressure area is commonly associated with bad weather, while [[high pressure area]] is associated with plenty of sunlight or good weather. On the land or on the sea surface, after getting heat from the sunlight, water evaporation becomes more intense, and a formation of a localized low pressure area can be expected. A [[rainstorm]] or a [[tropical cyclone]] (if on the sea) can well be formed in such conditions. Wind intensity can be approximately measured by the atmospheric pressure difference between two relatively nearby locations. ==Thermal low== In [[desert]]s, lack of ground and plant moisture that would normally provide [[evaporative cooling]] can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air. This, combined with the rising of the hot air, results in an isolated low pressure area called a '''thermal low'''. ==See also== *[[Cyclone]] *[[High pressure area]] *[[Surface weather analysis]] *[[Weather map]] [[Category:Midlatitude weather]] [[Category:Atmospheric dynamics]] [[Category:Vortices]] [[ca:Depressió atmosfèrica]] [[da:Højtryk og lavtryk]] [[de:Tiefdruckgebiet]] [[es:Borrasca]] [[eu:Behe presio gune]] [[fr:Dépression (météorologie)]] [[hr:Ciklona]] [[is:Lægð (veðurfræði)]] [[he:שקע ברומטרי]] [[nl:Lagedrukgebied]] [[ja:低気圧]] [[no:Lavtrykk]] [[nn:Lågtrykk]] [[pl:Niż baryczny]] [[ru:Депрессия (метеорология)]] [[simple:Low pressure area]] [[fi:Matalapaine]] [[sv:Lågtryck]] [[tr:Alçak basınç]] [[zh:低氣壓]]