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A trough is an elongated region of relatively low atmospheric pressure, often associated with fronts.〔F. J. Monkhouse. ''A Dictionary of Geography''. London: Edward Arnold (Publishers) Ltd., 1978〕 Unlike fronts, there is not a universal symbol for a trough on a weather chart. The weather charts in some countries or regions mark troughs by a line. In the United States, a trough may be marked as a dashed line. In the UK, Hong Kong〔(The Hong Kong Observatory, Weather Map at 08 HKT )〕 and Fiji,〔http://www.met.gov.fj/aifs_prods/0640.gif〕 it is represented by a bold line extended from a low pressure center 〔http://www.hko.gov.hk/wxinfo/currwx/flw_description/image/trough2.png〕 or between two low pressure centers;〔http://www.hko.gov.hk/wxinfo/currwx/flw_description/image/trough3.png〕 in Macau〔(Weather Chart )〕 and Australia,〔(Latest NMOC MSLP Analysis Chart )〕 it is a dotted line. If they are not marked, troughs may still be identified as an extension of isobars away from a low pressure center. Sometimes the region between two high pressure centers may assume the character of a trough when there is a detectable wind shift noted at the surface. In the absence of a wind shift, the region is designated a col, akin to a geographic saddle between two mountain peaks. If a trough forms in the mid-latitudes, a temperature difference between two sides of the trough usually exists in the form of a weather front. A weather front is usually less convective than a trough in the tropics or subtropics (such as a tropical wave). Sometimes collapsed frontal systems will degenerate into troughs. Convective cells may develop in the vicinity of troughs and give birth to a tropical cyclone. Some tropical or subtropical regions such as the Philippines or south China are greatly affected by convection cells along a trough. In the mid-latitude westerlies, troughs and ridges often alternate, especially when upper-level winds are in a high-amplitude pattern. For a trough in the westerlies, the region just west of the trough axis is typically an area of convergent winds and descending air - and hence high pressure - while the region just east of the trough axis is an area of fast, divergent winds and low pressure. Tropical waves are a type of trough in easterly currents, a cyclonic northward deflection of the trade winds. Troughs may be at the surface, or aloft, or both under various conditions. Most troughs bring clouds, showers, and a wind shift, particularly following the passage of the trough. This results from convergence or "squeezing" which forces lifting of moist air behind the trough line. ==Types of trough== In addition to standard troughs, some may be described further with a qualifying term indicating a specific or a set of characteristics. 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「Trough (meteorology)」の詳細全文を読む スポンサード リンク
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