Discover

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.

Contents

Types of trough

Lee trough

Inverted trough

Thermal trough

References

See also

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]

Inverted trough

An inverted trough is oriented opposite to the usual orientation of mid-latitude troughs. The most common type of inverted trough is the tropical wave.

Thermal trough

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.

References

1. http://www.hko.gov.hk/cgi-bin/hko/dwm_c.pl
2. http://www.met.gov.fj/aifs_prods/0640.gif
3. http://www.hko.gov.hk/wxinfo/currwx/flw_description/image/trough2.png
4. http://www.hko.gov.hk/wxinfo/currwx/flw_description/image/trough3.png
5. http://www.smg.gov.mo/dm/wxChart/e_chartindex.php
6. American Meteorological Society Glossary - Lee trough
7. What is a Lee-side Trough (Low)? Jeff Haby

See also

★ Geopotential height

★ Ridge (meteorology)

★ Surface weather analysis