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A pressure system is a region of the Earth's atmosphere where air pressure is a relative peak or lull in the sea level pressure distribution. The surface pressure at sea level varies minimally, with the lowest value measured and the highest recorded . High- and low-pressure systems evolve due to interactions of temperature differentials in the atmosphere, temperature differences between the atmosphere and water within oceans and lakes, the influence of upper-level disturbances, as well as the amount of solar heating or radiational cooling an area receives. Pressure systems cause the day-to-day weather experienced locally. Low-pressure systems are associated with clouds and precipitation that minimize temperature changes through the day, whereas high-pressure systems normally associated with dry weather and mostly clear skies with larger diurnal temperature changes due to greater radiation at night and greater sunshine during the day. Pressure systems are analyzed by those in the field of meteorology within surface weather maps. == Low-pressure system == A low-pressure area, or "low", is a region where the atmospheric pressure at sea level is below that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis.〔Arctic Climatologyal Snow and Ice Data Center. Retrieved on 2009-02-21.〕 Within the field of atmospheric dynamics, areas of wind divergence aloft occur in two areas: # 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) # ahead of embedded shortwave troughs, which have smaller wavelengths 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 create pressure gradients that drive monsoon circulations. Low-pressure areas can also form due to organized thunderstorm activity over warm water. When such an occurrence 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.〔Australian Broadcasting Corporation (2000). (Monsoon. ) Retrieved on 2008-05-03.〕〔U. S. Navy. (1.2 Pacific Ocean Surface Streamline Pattern. ) Retrieved on 2006-11-26.〕 When a convective low acquires a well-defined 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 hemisphere or the southern hemisphere during November. Atmospheric lift caused by low-level wind convergence into the surface low brings clouds and potentially precipitation. The low-pressure area's cloudy skies act to minimize diurnal temperature extremes. Since clouds reflect sunlight, incoming shortwave solar radiation is less, 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.〔JetStream (2008). (Origin of Wind. ) National Weather Service Southern Region Headquarters. Retrieved on 2009-02-16.〕 Around the world, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky mountains.〔L. de la Torre, Nieto R., Noguerol M., Añel J.A., Gimeno L. (2008). (A climatology based on reanalysis of baroclinic developmental regions in the extratropical northern hemisphere. ) Ann New York Academy of Science;vol. 1146: pp. 235-255. Retrieved on 2009-03-02.〕 In Europe (in particular, in the United Kingdom), recurring low-pressure weather systems are typically known as depressions. The lowest recorded non-tornadic barometric pressure was , occurred in the Western Pacific during Typhoon Tip on 12 October 1979. 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「Pressure system」の詳細全文を読む スポンサード リンク
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