A trough is an elongated (extended) region of relatively low atmospheric pressure, often associated with fronts. 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.
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 or bold line. In the UK, Hong Kong and Fiji, it is represented by a bold line extended from a low pressure center or between two low pressure centers; in Macau and Australia, it is a dashed line. If they are not marked, troughs may still be identified as an extension of isobars away from a low pressure center.
A trough is not in a general term a boundary but an elongated area of lower air pressure. There are changes in wind direction across a trough but there is no change in air mass. While not specifically a surface boundary, troughs reflect the change in atmospheric conditions in the upper atmosphere. As such, troughs can be areas where showers and thunderstorms can form.
If a trough forms in the mid-latitudes, a temperature difference at some distance between the two sides of the trough usually exists and the trough might become a weather front at some point. However such a weather front is usually less convective than a trough in the tropics or subtropics (such as a tropical wave). Inversely, sometimes collapsed frontal systems will degenerate into troughs.
Sometimes the region between two high pressure centers may also 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.
A trough is the result of the movements of the air in the atmosphere. In regions where there is upward movement near the ground and diverge at altitude, there is a loss of mass. The pressure becomes lower at this point.
At upper level of the atmosphere, this occurs when there is a meeting of a mass of cold air and another hot along a thin ribbon called a frontal baroclinic zone. We then have the creation of a jet stream that plunges the cold air towards the equator and hot air towards the poles, creating a ripple in the circulation that is called a Rossby wave. These undulations give the hollows and peaks of altitude. In general, absolute vorticity advection is positive between these two features, but closer to the ridge, whereas it is negative just behind a trough.
At the surface, lifting air under positive vorticity advection is reflected by the formation of depressions and troughs. There will therefore be a slope between the barometric high altitude and that on the ground, this slope going towards the mass of cold air at high altitude (generally towards the Pole).
Troughs have an orientation relative to the poles which is rarely North-South. In the Northern Hemisphere, positively tilted troughs will extend from the lowest pressure northeast to southwest while negatively tilted troughs have a northwest to southeast orientation. In the Southern Hemisphere, the positive tilt will be southeast to northwest and the negative one southwest to northeast.
A trough will usually begin with a positive tilt as cold air moves toward the Equator. The trough will become neutral (North-South) and then negatively tilted as the energy carried by the cold air races east though the atmospheric circulation and distorts its shape. The positive tilt is thus the building phase of the trough and the negative tilt is the dissipation of its energy. Therefore, the clouds and precipitation will develop in the positive phase and the most severe weather will be in the negative phase.
In addition to standard troughs, some may be described further with a qualifying term indicating a specific or a set of characteristics.
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 toward the east.
A lee trough, also known as a dynamic trough, 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." It 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."
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, upper level troughs and ridges often alternate 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.
Blocks in meteorology are large-scale patterns in the atmospheric pressure field that are nearly stationary, effectively “blocking” or redirecting migratory cyclones. They are also known as blocking highs or blocking anticyclones. These blocks can remain in place for several days or even weeks, causing the areas affected by them to have the same kind of weather for an extended period of time (e.g. precipitation for some areas, clear skies for others). In the Northern Hemisphere, extended blocking occurs most frequently in the spring over the eastern Pacific and Atlantic Oceans.Low-pressure area
A low-pressure area, low, depression or cyclone is a region on the topographic map where the atmospheric pressure is lower than that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in the upper levels of the troposphere. The formation process of a low-pressure area is known as cyclogenesis. Within the field of meteorology, atmospheric divergence aloft occurs in two areas. The first area is on the east side of upper troughs, which form half of a Rossby wave within the Westerlies (a trough with large wavelength that extends through the troposphere). A second area of wind divergence aloft occurs ahead of embedded shortwave troughs, which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity.
Thermal lows form due to localized heating caused by greater sunshine over deserts and other land masses. Since localized areas of warm air are less dense than their surroundings, this warmer air rises, which lowers atmospheric pressure near that portion of the Earth's surface. Large-scale thermal lows over continents help drive monsoon circulations. Low-pressure areas can also form due to organized thunderstorm activity over warm water. When this occurs over the tropics in concert with the Intertropical Convergence Zone, it is known as a monsoon trough. Monsoon troughs reach their northerly extent in August and their southerly extent in February. When a convective low acquires a well-hot circulation in the tropics it is termed a tropical cyclone. Tropical cyclones can form during any month of the year globally, but can occur in either the northern or southern hemisphere during December.
Atmospheric lift will also generally produce cloud cover through adiabatic cooling once the air becomes saturated as it rises, although the low-pressure area typically brings cloudy skies, which act to minimize diurnal temperature extremes. Since clouds reflect sunlight, incoming shortwave solar radiation decreases, which causes lower temperatures during the day. At night the absorptive effect of clouds on outgoing longwave radiation, such as heat energy from the surface, allows for warmer diurnal low temperatures in all seasons. The stronger the area of low pressure, the stronger the winds experienced in its vicinity. Globally, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky mountains. In Europe (particularly in the British Isles and Netherlands), recurring low-pressure weather systems are typically known as "depressions".Weather front
A weather front is a boundary separating two masses of air of different densities, and is the principal cause of meteorological phenomena outside the tropics. In surface weather analyses, fronts are depicted using various colored triangles and half-circles, depending on the type of front. The air masses separated by a front usually differ in temperature and humidity.
Cold fronts may feature narrow bands of thunderstorms and severe weather, and may on occasion be preceded by squall lines or dry lines. Warm fronts are usually preceded by stratiform precipitation and fog. The weather usually clears quickly after a front's passage. Some fronts produce no precipitation and little cloudiness, although there is invariably a wind shift.Cold fronts and occluded fronts generally move from west to east, while warm fronts move poleward. Because of the greater density of air in their wake, cold fronts and cold occlusions move faster than warm fronts and warm occlusions. Mountains and warm bodies of water can slow the movement of fronts. When a front becomes stationary—and the density contrast across the frontal boundary vanishes—the front can degenerate into a line which separates regions of differing wind velocity, known as a shearline. This is most common over the open ocean.