A trough is an elongated region of relatively low
atmospheric pressure, often associated with fronts.
Unlike fronts, there is not a universal symbol for a trough on a weather chart. In the United States, a trough may be marked as a dashed line. If it is not marked, troughs may still be identified as an extension of isobars away from a low pressure center. The weather charts in some countries or regions mark troughs by a line. In Hong Kong [ [http://www.hko.gov.hk/cgi-bin/hko/dwm_e.pl The Hong Kong Observatory, Weather Map at 08 HKT ] ] or 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 [ [http://www.smg.gov.mo/dm/wxChart/e_chartindex.php Weather Chart ] ] or Australia [ [http://www.bom.gov.au/cgi-bin/nmoc/latest_MSLP.pl?IDCODE=IDY00050 Latest NMOC MSLP Analysis Chart ] ] , it is a dotted line.
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.
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
A "lee trough" is one and the same thing as a "dynamic trough". According to the AMS Glossary, it is "A pressure trough formed on the lee side of a mountain range in situations where the wind is blowing with a substantial component across the mountain ridge; often seen on United States weather maps east of the Rocky Mountains, and sometimes east of the Appalachians, where it is less pronounced.", and can be formed either as a result of the adiabatic compression of sinking air on the "lee" side of a mountain range, or through cyclogenesis resulting from "the horizontal convergence associated with vertical stretching of air columns passing over the ridge and descending the lee slope." cite web
title = American Meteorological Society Glossary - Lee trough
publisher = [http://www.allenpress.com Allen Press Inc.]
date = 2000-06
url = http://amsglossary.allenpress.com/glossary/search?id=lee-trough1
accessdate = 2006-10-30 ] cite web
title = What is a Lee-side Trough (Low)?
author = Jeff Haby
publisher = [http://www.theweatherprediction.com/ TheWeatherPrediction.com]
url = http://www.theweatherprediction.com/habyhints/129/
accessdate = 2006-10-30 ]
inverted troughis oriented opposite to the usual orientation of mid-latitude troughs. The most common type of inverted trough is the tropical wave.
A thermal trough is a trough formed by intense heating, usually in an enclosed valley such as California's Central Valley. This trough in some ways resembles and functions like a monsoon, drawing in marine air which however, in the case of California, is too stable to produce thunderstorms on the Pacific side. The thermal trough here may however interact with the Southwest Monsoon and draw thunderstorms up along the east side of the Sierra Nevada.
Surface weather analysis
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