Low-pressure area

This depiction of the Hadley cell shows the process which sustains low pressure areas. Diverging winds aloft allow for lower pressure and convergence at the Earth's surface, which leads to upward motion.

A low pressure area, or "low", is a region where the atmospheric pressure at sea level is lower in relation to surrounding locations. Low pressure systems form under areas wind divergence which occur in upper levels of the troposphere. Within the field of atmospheric dynamics, these areas of wind divergence aloft are either on the east side of upper troughs which form half of a Rossby wave within the Westerlies (a trough with large wavelength which extends through the troposphere), or 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. Low pressure systems which form due to localized heating caused by greater sunshine over deserts and other land masses are called thermal lows. 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. Low pressure areas can also form due to organized thunderstorm activity over warm water. When such a low acquires a well-defined circulation it is termed a tropical cyclone. The name for the process concerning the formation of low pressure areas is known as cyclogenesis.

Atmospheric lift will also generally produce cloud cover through adiabatic cooling, once the air becomes saturated as it rises. Thus, low pressure areas typically bring cloudy skies, which act to minimize diurnal temperature extremes. Since clouds reflect sunlight, incoming shortwave solar radiation is less 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 which are 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 United Kingdom), recurring low pressure weather systems are typically known as depressions.

Formation

A large low-pressure system swirls off the southwestern coast of Iceland.

Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere (a low pressure area).^[1] Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone. It can occur at various scales, from the microscale to the synoptic scale. Larger scale troughs, which are also called Rossby waves, are synoptic in scale.^[2] Shortwave troughs embedded within the flow around larger scale troughs are smaller in scale, or mesoscale in nature.^[3] Both Rossby waves and shortwaves embedded within the flow around Rossby waves migrate equatorward of the polar cyclone. All share one important aspect, that of upward vertical motion within the troposphere. Such upward motions decrease the mass of local atmospheric columns of air, which lower surface pressure.^[4]

Extratropical cyclones form as waves along weather fronts due to a passing by shortwave aloft or upper level jet streak before occluding later in their life cycle as cold core cyclones.^{[5][6][7] [8]} Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm core with well-defined circulations.^[9] Mesocyclones form as warm core cyclones over land, and can lead to tornado formation.^[10] Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear.^[11] In deserts, lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air. This, combined with the rising of the hot air, results in a low pressure area called a thermal low.^[12] Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high pressure system) equivalent which deals with the formation of high pressure areas—Anticyclogenesis.^[13]

Climatology

The large polar cyclone helps determine the steering of systems moving through the mid-latitudes, south of the Arctic. An index which is used to gage the magnitude of this effect is the Arctic oscillation.^[14] Extratropical cyclones tend to form east of climatological trough positions aloft near the east coast of continents, or west side of oceans.^[15] In Europe, particularly the United Kingdom, recurring extratropical low pressure weather systems are typically known as depressions.^[16] These tend to bring wet weather throughout the year. Thermal lows also occur over continental areas across the subtropics during the summer such as the Sonoran Desert, the Mexican plateau, Sahara, South America, and Southeast Asia.^[12] The lows are most commonly located over the Tibetan plateau and in the lee of the Rocky mountains.^[15] Elongated areas of low pressure form at the monsoon trough or Intertropical convergence zone as part of the Hadley cell circulation.^[17] Many of the world's rainforests are associated with these climatological low pressure systems.^[18]

Associated weather

Schematic representation of flow around a low-pressure area in the Northern hemisphere. The pressure-gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows

Wind flows from areas of high pressure to areas of low pressure.^[19] This is due to density differences between the two air masses. Since stronger high pressure systems contain cooler or drier air, the air mass is more dense and flows towards areas that are warm or moist, which are in the vicinity of low pressure areas in advance of their associated cold fronts. The stronger the pressure difference, or pressure gradient, between a high pressure system and a low pressure system, the stronger the wind.^[20] Thus, stronger areas of low pressure are associated with stronger winds.

The coriolis force caused by the Earth's rotation is what gives winds within low pressure systems their counterclockwise circulation in the northern hemisphere (as the wind moves inward and is deflected right from the center of high pressure) and clockwise circulation in the southern hemisphere (as the wind moves inward and is deflected left from the center of high pressure). Friction with land slows down the wind flowing into high pressure systems and causes wind to flow more inward, or flowing more ageostrophically, toward their centers.^[20] A low pressure area is commonly associated with inclement weather,^[21] while high pressure area is associated with light winds and fair skies.^[22]

See also

• Surface weather analysis
• Weather map

References