Trough (meteorology)
A trough is an elongated region of relatively low atmospheric pressure, often associated with fronts.
In United States, a trough may be marked as a brown 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[1] or Fiji[2], it is represented by a bold line extended from a low pressure center [3] or between two low pressure centers [4]; in Macau[5], it is a dotted line.
Sometimes, a trough may be a region between two high pressure centers and there is not a low pressure center. In this case, it is still represented by a line between the two high pressure centers.
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.
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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
Lee trough
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."[6][7]
References
- ^ http://www.hko.gov.hk/cgi-bin/hko/dwm_c.pl
- ^ http://www.met.gov.fj/aifs_prods/0640.gif
- ^ http://www.hko.gov.hk/wxinfo/currwx/flw_description/image/trough2.png
- ^ http://www.hko.gov.hk/wxinfo/currwx/flw_description/image/trough3.png
- ^ http://www.smg.gov.mo/dm/wxChart/e_chartindex.php
- ^ American Meteorological Society Glossary - Lee trough. Allen Press Inc. (2000-06). Retrieved on 2006-10-30.
- ^ Jeff Haby. What is a Lee-side Trough (Low)?. TheWeatherPrediction.com. Retrieved on 2006-10-30.
