Low pressure area - AbsoluteAstronomy.com

A low-pressure area, or "low", is a region where the 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...

at sea level

Sea level

Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

is below that of surrounding locations. Low-pressure systems form under areas of wind divergence which occur in upper levels of the troposphere

Troposphere

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

. The formation process of a low-pressure area is known as cyclogenesis. Within the field of atmospheric dynamics, areas of wind divergence aloft occur in two areas. The first area is on the east side of upper troughs which form half of a Rossby wave

Rossby wave

Atmospheric Rossby waves are giant meanders in high-altitude winds that are a major influence on weather.They are not to be confused with oceanic Rossby waves, which move along the thermocline: that is, the boundary between the warm upper layer of the ocean and the cold deeper part of the...

within 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...

(a trough with large wavelength

Wavelength

In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

which extends through the troposphere). A second area of wind divergence aloft occurs ahead of embedded shortwave troughs

Shortwave (meteorology)

A shortwave or shortwave trough is an embedded kink in the trough / ridge pattern. This is the opposite of longwaves, which are responsible for synoptic scale systems, although shortwaves may be contained within or found ahead of longwaves and range from the mesocale to the synoptic scale...

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 low

Thermal low

Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics such as near the Sonoran Desert, the Mexican plateau, Sahara, South America over northwest Argentina, Australia, the Iberian peninsula, and Tibetan plateau during the warm season as the...

s 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

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's surface. Large-scale thermal lows over continent

Continent

A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

s help drive monsoon

Monsoon

Monsoon is traditionally defined as a seasonal reversing wind accompanied by corresponding changes in precipitation, but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with the asymmetric heating of land and sea...

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

Intertropical Convergence Zone

The Intertropical Convergence Zone , known by sailors as The Doldrums, is the area encircling the earth near the equator where winds originating in the northern and southern hemispheres come together....

, it is known as a monsoon trough

Monsoon trough

The monsoon trough is that portion of the Intertropical Convergence Zone which extends into or through a monsoon circulation, as depicted by a line on a weather map showing the locations of minimum sea level pressure, and as such, is a convergence zone between the wind patterns of the southern and...

. Monsoon troughs reach their northerly extent in August and their southerly extent in February. When a convective low acquires a well-defined circulation in the tropics it is termed 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...

. Tropical cyclones can form during any month of the year globally, but can occur in either the northern and southern hemisphere during November.

Atmospheric lift will also generally produce cloud cover

Cloud cover

Cloud cover refers to the fraction of the sky obscured by clouds when observed from a particular location...

through adiabatic cooling once the air becomes saturated as it rises, although the low-pressure area typically bring cloudy skies, which act to minimize diurnal temperature extremes

Diurnal temperature variation

Diurnal temperature variation is a meteorological term that relates to the variation in temperature that occurs from the highs of the day to the cool of nights.-Temperature lag:Temperature lag is an important factor in diurnal temperature variation...

. Since clouds reflect sunlight

Sunlight

Sunlight, in the broad sense, is the total frequency spectrum of electromagnetic radiation given off by the Sun. On Earth, sunlight is filtered through the Earth's atmosphere, and solar radiation is obvious as daylight when the Sun is above the horizon.When the direct solar radiation is not blocked...

, incoming shortwave

Shortwave radiation

Shortwave radiation is a term used to describe radiant energy with wavelengths in the visible , near-ultraviolet , and near-infrared spectra....

solar radiation is less which causes lower temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

s 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 wind

Wind

Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...

s which are experienced in its vicinity. Globally, low-pressure systems are most frequently located over the Tibetan Plateau

Tibetan Plateau

The Tibetan Plateau , also known as the Qinghai–Tibetan Plateau is a vast, elevated plateau in Central Asia covering most of the Tibet Autonomous Region and Qinghai, in addition to smaller portions of western Sichuan, southwestern Gansu, and northern Yunnan in Western China and Ladakh in...

and in the lee of the Rocky mountains

Rocky Mountains

The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...

. In Europe (particularly in the United Kingdom

United Kingdom

The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

), recurring low-pressure weather systems are typically known as depressions.

Formation

Cyclogenesis is the development strengthening of cyclonic circulations, or low-pressure areas, within the atmosphere. Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high pressure system) equivalent which deals with the formation of high pressure area

High pressure area

A high-pressure area is a region where the atmospheric pressure at the surface of the planet is greater than its surrounding environment. Winds within high-pressure areas flow outward due to the higher density air near their center and friction with land...

s—Anticyclogenesis

Anticyclogenesis

Anticyclogenesis is the development or strengthening of anticyclonic circulation in the atmosphere. It is the opposite of anticyclolysis, and has a cyclonic equivalent—cyclogenesis. Anticyclones are alternatively referred to as high pressure systems. High pressure areas form due to downward...

. Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone

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...

. The term cyclone is used where circular pressure systems flow in the direction of the Earth's rotation, which normally coincides with areas of low pressure. The largest low-pressure systems are cold-core polar cyclones and extratropical cyclones which lie on the synoptic scale. Warm-core cyclones such as tropical cyclones, mesocyclone

Mesocyclone

A mesocyclone is a vortex of air, approximately 2 to 10 miles in diameter , within a convective storm....

s, and polar low

Polar low

A polar low is a small-scale, long-lived atmospheric low pressure system that is found over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. The systems usually have a horizontal length scale of less than and exist for no more than a couple of days. ...

s lie within the smaller mesoscale

Mesoscale meteorology

Mesoscale meteorology is the study of weather systems smaller than synoptic scale systems but larger than microscale and storm-scale cumulus systems. Horizontal dimensions generally range from around 5 kilometers to several hundred kilometers...

. Subtropical cyclones are of intermediate size. Cyclogenesis 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. Shortwave troughs embedded within the flow around larger scale troughs are smaller in scale, or mesoscale in nature. Both Rossby waves and shortwaves embedded within the flow around Rossby waves migrate equatorward of the polar cyclone

Polar cyclone

Polar cyclones are low-pressure areas which strengthen in the winter and weaken in the summer...

s located in both the Northern and Southern hemispheres. 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.

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.
Polar lows are small-scale, short-lived atmospheric low-pressure systems that are found over the ocean areas poleward of the main polar front

Polar front

In meteorology, the polar front is the boundary between the polar cell and the Ferrel cell in each hemisphere. At this boundary a sharp gradient in temperature occurs between these two air masses, each at very different temperatures....

in both the Northern and Southern Hemispheres. They are part of the larger class of mesoscale

Mesoscale meteorology

weather systems. Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and gas and oil platforms. They are vigorous systems that have near-surface winds of at least 17 metres per second (38 mph).

Tropical cyclone

Tropical cyclone

s form due to latent heat driven by significant thunderstorm activity, and are warm core with well-defined circulations. Certain criteria need to be met for their formation. In most situations, water temperatures

Sea surface temperature

Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...

of at least 26.5 °C (79.7 °F) are needed down to a depth of at least 50 m (164 ft); waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms. Another factor is rapid cooling with height, which allows the release of the heat of condensation that powers a tropical cyclone. High humidity is needed, especially in the lower-to-mid troposphere

Troposphere

The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....

; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop. Low amounts of wind shear

Wind shear

Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...

are needed, as high shear is disruptive to the storm's circulation. Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place. Mesocyclone

Mesocyclone

A mesocyclone is a vortex of air, approximately 2 to 10 miles in diameter , within a convective storm....

s form as warm core cyclones over land, and can lead to tornado formation. Waterspout

Waterspout

A waterspout is an intense columnar vortex that occurs over a body of water and is connected to a cumuliform cloud. In the common form, it is a non-supercell tornado over water. While it is often weaker than most of its land counterparts, stronger versions spawned by mesocyclones do occur...

s can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear

Wind shear

Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...

In desert

Desert

A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...

s, 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. Monsoon

Monsoon

circulations are caused by thermal lows which form over large areas of land and their strength is driven by how land heats quicker than the surrounding nearby ocean. This creates a steady wind blowing toward the land, bringing the moist near-surface air over the oceans with it. Similar rainfall

Precipitation (meteorology)

In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

is caused by the moist ocean air being lifted upwards by mountains, surface heating, convergence at the surface, divergence aloft, or from storm-produced outflows at the surface. However the lifting occurs, the air cools due expansion in lower pressure, which in turn produces condensation

Condensation

Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of vaporization. When the transition happens from the gaseous phase into the solid phase directly, the change is called deposition....

. In winter, the land cools off quickly, but the ocean keeps the heat longer due to its higher specific heat. The hot air over the ocean rises, creating a low-pressure area and a breeze from land to ocean while a large area of drying high pressure is formed over the land, increased by wintertime cooling. Monsoons are similar to sea and land breezes, a term usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale, stronger and seasonal.

Mid-latitudes and subtropics

Large polar cyclones help determine the steering of systems moving through the mid-latitudes, south of the Arctic and north of the Antarctic

Antarctic

The Antarctic is the region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica and the ice shelves, waters and island territories in the Southern Ocean situated south of the Antarctic Convergence...

. An index which is used to gage the magnitude of this effect in the Northern Hemisphere is the Arctic oscillation

Arctic oscillation

The Arctic oscillation or Northern Annular Mode/Northern Hemisphere Annular Mode is an index of the dominant pattern of non-seasonal sea-level pressure variations north of 20N latitude, and it is characterized by pressure anomalies of one sign in the Arctic with the opposite anomalies centered...

. Extratropical cyclone

Extratropical cyclone

Extratropical cyclones, sometimes called mid-latitude cyclones or wave cyclones, are a group of cyclones defined as synoptic scale low pressure weather systems that occur in the middle latitudes of the Earth having neither tropical nor polar characteristics, and are connected with fronts and...

s tend to form east of climatological trough positions aloft near the east coast of continents, or west side of oceans. A study of extratropical cyclones in the Southern Hemisphere

Southern Hemisphere

The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

shows that between the 30th

30th parallel south

The 30th parallel south is a circle of latitude that is 30 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Australasia, the Pacific Ocean and South America....

and 70th parallels

70th parallel south

The 70th parallel south is a circle of latitude that is 70 degrees south of the Earth's equatorial plane in the Antarctic. The parallel passes through the Southern Ocean and Antarctica.Mariners regard the waters in the 70° south as "Godless"....

, there are an average of 37 cyclones in existence during any 6-hour period. A separate study in the Northern Hemisphere

Northern Hemisphere

The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

suggests that approximately 234 significant extratropical cyclones form each winter. In Europe, particularly the United Kingdom, recurring extratropical low-pressure weather systems are typically known as depressions. 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

Sonoran Desert

The Sonoran Desert is a North American desert which straddles part of the United States-Mexico border and covers large parts of the U.S. states of Arizona and California and the northwest Mexican states of Sonora, Baja California, and Baja California Sur. It is one of the largest and hottest...

, the Mexican plateau

Mexican Plateau

The Central Mexican Plateau, also known as the Mexican Altiplano or Altiplanicie Mexicana, is a large arid-to-semiarid plateau that occupies much of northern and central Mexico...

, Sahara

Sahara

The Sahara is the world's second largest desert, after Antarctica. At over , it covers most of Northern Africa, making it almost as large as Europe or the United States. The Sahara stretches from the Red Sea, including parts of the Mediterranean coasts, to the outskirts of the Atlantic Ocean...

, South America

South America

South America is a continent situated in the Western Hemisphere, mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere. The continent is also considered a subcontinent of the Americas. It is bordered on the west by the Pacific Ocean and on the north and east...

, and Southeast Asia. The lows are most commonly located over the Tibetan plateau and in the lee of the Rocky mountains.

Monsoon trough

Elongated areas of low pressure form at the monsoon trough

Monsoon trough

or Intertropical convergence zone

Intertropical Convergence Zone

as part of the Hadley cell

Hadley cell

The Hadley cell, named after George Hadley, is a circulation pattern that dominates the tropical atmosphere, with rising motion near the equator, poleward flow 10–15 kilometers above the surface, descending motion in the subtropics, and equatorward flow near the surface...

circulation. Monsoon troughing in the western Pacific reaches its zenith in latitude during the late summer when the wintertime surface ridge in the opposite hemisphere is the strongest. It can reach as far as the 40th parallel

40th parallel north

The 40th parallel north is a circle of latitude that is 40 degrees north of the Earth's equatorial plane. It crosses Europe, the Mediterranean Sea, Asia, the Pacific Ocean, North America, and the Atlantic Ocean....

in East Asia during August and 20th parallel

20th parallel south

The 20th parallel south is a circle of latitude that is 20 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Australasia, the Pacific Ocean and South America....

in Australia during February. Its poleward progression is accelerated by the onset of the summer monsoon which is characterized by the development of lower air pressure over the warmest part of the various continents. The large-scale thermal lows over continent

Continent

s help create pressure gradients which drive monsoon

Monsoon

circulations. In the southern hemisphere, the monsoon trough associated with the Australian monsoon reaches its most southerly latitude in February, oriented along a west-northwest/east-southeast axis. Many of the world's rainforests are associated with these climatological low-pressure systems.

Tropical cyclone

Tropical cyclones generally need to form more than 555 km (344.9 mi) or poleward of the 5th parallel north

5th parallel north

The 5th parallel north is a circle of latitude that is 5 degrees north of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Southeast Asia, the Pacific Ocean and South America....

and 5th parallel south

5th parallel south

The 5th parallel south is a circle of latitude that is 5 degrees south of the Earth's equatorial plane. It crosses the Atlantic Ocean, Africa, the Indian Ocean, Southeast Asia, Australasia, the Pacific Ocean and South America....

, allowing the Coriolis effect

Coriolis effect

In physics, the Coriolis effect is a deflection of moving objects when they are viewed in a rotating reference frame. In a reference frame with clockwise rotation, the deflection is to the left of the motion of the object; in one with counter-clockwise rotation, the deflection is to the right...

to deflect winds blowing towards the low-pressure center and creating a circulation. Worldwide, tropical cyclone activity peaks in late summer, when the difference between temperatures aloft and sea surface temperatures is the greatest. However, each particular basin has its own seasonal patterns. On a worldwide scale, May is the least active month while September is the most active month. November is the only month that activity in all the tropical cyclone basins possible. Nearly one-third of the world's tropical cyclones form within the western Pacific ocean, making it the most active tropical cyclone basin

Tropical cyclone basins

Traditionally, areas of tropical cyclone formation are divided into seven basins. These include the north Atlantic Ocean, the eastern and western parts of the northern Pacific Ocean, the southwestern Pacific, the southwestern and southeastern Indian Oceans, and the northern Indian Ocean. The...

on Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

Associated weather

Wind

is initially accelerated from areas of high pressure to areas of low pressure. This is due to density

Density

The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...

(or temperature and moisture) differences between two air mass

Air mass

In meteorology, an air mass is a volume of air defined by its temperature and water vapor content. Air masses cover many hundreds or thousands of square miles, and adopt the characteristics of the surface below them. They are classified according to latitude and their continental or maritime...

es. 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 front

Cold front

A cold front is defined as the leading edge of a cooler mass of air, replacing a warmer mass of air.-Development of cold front:The cooler and denser air wedges under the less-dense warmer air, lifting it...

s. The stronger the pressure difference, or pressure gradient

Pressure gradient

In atmospheric sciences , the pressure gradient is a physical quantity that describes in which direction and at what rate the pressure changes the most rapidly around a particular location. The pressure gradient is a dimensional quantity expressed in units of pressure per unit length...

, between a high pressure system and a low-pressure system, the stronger the wind. Thus, stronger areas of low pressure are associated with stronger winds.

The Coriolis force caused by the Earth

Earth

Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

'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 low-pressure systems and causes wind to flow more inward, or flowing more ageostrophically

Geostrophic wind

The geostrophic wind is the theoretical wind that would result from an exact balance between the Coriolis effect and the pressure gradient force. This condition is called geostrophic balance. The geostrophic wind is directed parallel to isobars . This balance seldom holds exactly in nature...

, toward their centers. A low-pressure area is commonly associated with inclement weather, while high pressure area

High pressure area

is associated with light winds and fair skies.

Formation

Mid-latitudes and subtropics

Monsoon trough

Tropical cyclone

Associated weather

See also