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A low-pressure area, low or depression, is a region where the atmospheric pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence which occur in the upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas. The first area is on the east side of upper troughs which form half of a Rossby wave within the Westerlies (a trough with large wavelength which extends through the troposphere). A second area of wind divergence aloft occurs ahead of embedded shortwave troughs which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity. Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses. Since localized areas of warm air are less dense than their surroundings, this warmer air rises which lowers atmospheric pressure near that portion of the Earth's surface. Large-scale thermal lows over continents help drive monsoon circulations. Low-pressure areas can also form due to organized thunderstorm activity over warm water. When this occurs over the tropics in concert with the Intertropical Convergence Zone, it is known as a monsoon trough. Monsoon troughs reach their northerly extent in August and their southerly extent in February. When a convective low acquires a well-hot circulation in the tropics it is termed a tropical cyclone. Tropical cyclones can form during any month of the year globally, but can occur in either the northern or southern hemisphere during November. Atmospheric lift will also generally produce cloud cover through adiabatic cooling once the air becomes saturated as it rises, although the low-pressure area typically brings cloudy skies, which act to minimize diurnal temperature extremes. Since clouds reflect sunlight, incoming shortwave solar radiation decreases, which causes lower temperatures 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. The stronger the area of low pressure, the stronger the winds experienced in its vicinity. Globally, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky mountains. In Europe (particularly in the United Kingdom), recurring low-pressure weather systems are typically known as "depressions". ==Formation== (詳細はcyclonic circulations, or low-pressure areas, within the atmosphere.〔Arctic Climatology and Meteorology (2009). (Cyclogenesis. ) National Snow and Ice Data Center. Retrieved on 2009-02-21.〕 Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high-pressure system) equivalent which deals with the formation of high-pressure areas—anticyclogenesis. Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone. The term cyclone is used where circular pressure systems flow in the direction of the Earth's rotation,〔 〕 which normally coincides with areas of low pressure.〔(【引用サイトリンク】 publisher= (University Corporation for Atmospheric Research ) )〕 The largest low-pressure systems are cold-core polar cyclones and extratropical cyclones which lie on the synoptic scale. Warm-core cyclones such as tropical cyclones, mesocyclones, and polar lows lie within the smaller mesoscale. Subtropical cyclones are of intermediate size. Cyclogenesis can occur at various scales, from the microscale to the synoptic scale. Larger scale troughs, which are also called Rossby waves, are synoptic in scale. Shortwave troughs embedded within the flow around larger scale troughs are smaller in scale, or mesoscale in nature. Both Rossby waves and shortwaves embedded within the flow around Rossby waves migrate equatorward of the polar cyclones located in both the Northern and Southern hemispheres. All share one important aspect, that of upward vertical motion within the troposphere. Such upward motions decrease the mass of local atmospheric columns of air, which lower surface pressure. Extratropical cyclones form as waves along weather fronts due to a passing by shortwave aloft or upper level jet streak before occluding later in their life cycle as cold core cyclones.〔Glossary of Meteorology (2009). (Short Wave. ) American Meteorological Society. Retrieved on 2009-03-02.〕〔Glossary of Meteorology (2009). (Upper-Level Trough. ) American Meteorological Society. Retrieved on 2009-03-02.〕〔Carlyle H. Wash, Stacey H. Heikkinen, Chi-Sann Liou, and Wendell A. Nuss (1989). (A Rapid Cyclogenesis Event during GALE IOP 9. ) Monthly Weather Review pp. 234–257. Retrieved on 2008-06-28.〕 Polar lows are small-scale, short-lived atmospheric low-pressure systems that are found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. They are part of the larger class of mesoscale weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms. They are vigorous systems that have near-surface winds of at least . Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm core with well-defined circulations. Certain criteria need to be met for their formation. In most situations, water temperatures of at least are needed down to a depth of at least ; waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms. Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone.〔 High humidity is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop.〔 Low amounts of wind shear are needed, as high shear is disruptive to the storm's circulation.〔 Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place.〔 Mesocyclones form as warm core cyclones over land, and can lead to tornado formation. Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.〔 〕 In deserts, 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 a low-pressure area called a thermal low.〔Glossary of Meteorology (2009). (Thermal Low. ) American Meteorological Society. Retrieved on 2009-03-02.〕 Monsoon circulations are caused by thermal lows which form over large areas of land and their strength is driven by how land heats quicker than the surrounding nearby ocean. This creates a steady wind blowing toward the land, bringing the moist near-surface air over the oceans with it.〔Dr. Louisa Watts (2009). ( What causes the west African monsoon? ) National Centre for Environmental Science. Retrieved on 2009-04-04.〕 Similar rainfall is caused by the moist ocean air being lifted upwards by mountains,〔Dr. Michael Pidwirny (2008). (CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes. ) Physical Geography. Retrieved on 2009-01-01.〕 surface heating,〔Bart van den Hurk and Eleanor Blyth (2008). (Global maps of Local Land-Atmosphere coupling. ) KNMI. Retrieved on 2009-01-02.〕 convergence at the surface,〔Robert Penrose Pearce (2002). (Meteorology at the Millennium. ) Academic Press, p. 66. ISBN 978-0-12-548035-2. Retrieved on 2009-01-02.〕 divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, the air cools due expansion in lower pressure, which in turn produces condensation. In winter, the land cools off quickly, but the ocean keeps the heat longer due to its higher specific heat. The hot air over the ocean rises, creating a low-pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling.〔 Monsoons are similar to sea and land breezes, a term usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale also stronger and seasonal. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Low-pressure area」の詳細全文を読む スポンサード リンク
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