Trough (meteorology)
Encyclopedia
A trough is an elongated region of relatively low atmospheric pressure
Atmospheric pressure
Atmospheric pressure is the force per unit area exerted into a surface by the weight of air above that surface in the atmosphere of Earth . In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point...

, often associated with front
Weather front
A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front...

s.

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 or Fiji, it is represented by a bold line extended from a low pressure center or between two low pressure centers; in Macau or Australia, it is a dotted line. If it is not marked, troughs may still be identified as an extension of isobars away from a low pressure center
Pressure system
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...

.

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
Weather front
A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front...

. A weather front is usually less convective than a trough in the tropics or subtropics (such as a tropical wave
Tropical wave
Tropical waves, easterly waves, or tropical easterly waves, also known as African easterly waves in the Atlantic region, are a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which move from east to west across the tropics causing areas of...

). Sometimes, collapsed frontal systems will degenerate into troughs.

Convective cells may develop in the vicinity of troughs and give birth to a tropical cyclone
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...

. 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
Westerlies
The Westerlies, anti-trades, or Prevailing Westerlies, are the prevailing winds in the middle latitudes between 30 and 60 degrees latitude, blowing from the high pressure area in the horse latitudes towards the poles. These prevailing winds blow from the west to the east, and steer extratropical...

, troughs and ridges
Ridge (meteorology)
A ridge is an elongated region of relatively high atmospheric pressure, the opposite of a trough....

 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 wave
Tropical wave
Tropical waves, easterly waves, or tropical easterly waves, also known as African easterly waves in the Atlantic region, are a type of atmospheric trough, an elongated area of relatively low air pressure, oriented north to south, which move from east to west across the tropics causing areas of...

s are a type of trough in easterly currents, a cyclonic
Cyclone
In meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth. This is usually characterized by inward spiraling winds that rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale...

 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

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
Adiabatic process
In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero. Such a process can occur if the container of the system has thermally-insulated walls or the process happens in an extremely short time,...

 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."

Inverted trough

An inverted trough is an atmospheric trough which is oriented opposite to most troughs of the mid-latitudes. Most (but not all) inverted troughs are tropical waves (also commonly called easterly waves).

Most troughs of low pressure in the mid-latitudes of the Northern Hemisphere are characterized by decreasing atmospheric pressure from south to north while inverted troughs are characterized by decreasing pressure from north to south. The situation is opposite in the Southern Hemisphere. Inverted troughs in both hemispheres move to the west from the east, while mid-latitude troughs generally move with the westerlies
Westerlies
The Westerlies, anti-trades, or Prevailing Westerlies, are the prevailing winds in the middle latitudes between 30 and 60 degrees latitude, blowing from the high pressure area in the horse latitudes towards the poles. These prevailing winds blow from the west to the east, and steer extratropical...

 toward the east.

All 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.

See also

  • Geopotential height
    Geopotential height
    Geopotential height is a vertical coordinate referenced to Earth's mean sea level — an adjustment to geometric height using the variation of gravity with latitude and elevation. Thus it can be considered a "gravity-adjusted height"...

  • Ridge (meteorology)
    Ridge (meteorology)
    A ridge is an elongated region of relatively high atmospheric pressure, the opposite of a trough....

  • Surface weather analysis
    Surface weather analysis
    Surface weather analysis is a special type of weather map that provides a view of weather elements over a geographical area at a specified time based on information from ground-based weather stations...

  • Weather front
    Weather front
    A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena. In surface weather analyses, fronts are depicted using various colored lines and symbols, depending on the type of front...

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