A low pressure area, or "low", is a region where the atmospheric pressure Atmospheric pressure is the force per unit area exerted against a surface by the weight of air above that surface in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low pressure areas have less atmospheric mass above at sea level Mean sea level is a measure of the average height of the ocean's surface (such as the halfway point between the mean high tide and the mean low tide); used as a standard in reckoning land elevation. MSL also plays an extremely important role in aviation, where standard sea level pressure is used as the measurement datum of altitude at flight is below that of surrounding locations. Low pressure systems form under areas of wind divergence which occur in upper levels of the troposphere The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% 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 Cyclogenesis is the development or strengthening of cyclonic circulation in the atmosphere . 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. Extratropical cyclones form as waves along weather. Within the field of atmospheric dynamics Meteorology is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and short term forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century. The nineteenth century saw breakthroughs occur after observing networks developed, 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 waves are giant meanders in high-altitude winds that are a major influence on weather. Their emergence is due to shear in rotating fluids, so that the Coriolis force changes along the sheared coordinate. In planetary atmospheres, they are due to the variation in the Coriolis effect with latitude. The waves were first identified in the Earth' within the Westerlies The Westerlies, anti-trades, or Prevailing Westerlies, are the prevailing winds in the middle latitudes between 35 and 65 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 cyclones in this general manner. Tropical cyclones (a trough with large 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 characteristic of both traveling waves and which extends through the troposphere). A second area of wind divergence aloft occurs ahead of embedded shortwave troughs 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 the synoptic scale. Shortwaves are most frequently caused by either a cold which are of smaller wavelength. Diverging winds aloft ahead of these troughs cause atmospheric An atmosphere is a layer of gases that may surround a material body of sufficient mass,and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low. Some planets consist mainly of various gases, but only their outer layer is their lift within the troposphere below, which lowers surface pressures as upward motion partially counteracts the force of gravity Gravitation, or gravity, is one of the four fundamental interactions of nature , in which objects with mass attract one another. In everyday life, gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped. Gravitation causes dispersed matter to coalesce, thus accounting for.

Thermal lows Thermal 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 result of intense heating when compared to their surrounding 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 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. It is sometimes referred to as the World, the Blue Planet,[note 6] or by its Latin name, Terra.[note 7]'s surface. Large-scale thermal lows over continents A continent is one of several 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 help drive monsoon Monsoon is traditionally defined as a seasonal reversing wind accompanied by seasonal 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. Usually, the term monsoon is used to refer to the rainy phase of a seasonally-changing circulations. Low pressure areas can also form due to organized thunderstorm activity over warm water. When such an occurrence occurs over the tropics in concert with the Intertropical Convergence Zone The ITCZ was originally identified from the 1920s to the 1940s as the "Intertropical Front" , but after the recognition in the 1940s and 1950s of the significance of wind field convergence in tropical weather production, the term "ITCZ" was then applied, it is known as a 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 northern hemispheres. Westerly monsoon winds lie. 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 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 feed on heat released when moist air rises, resulting in condensation of water vapor contained in the moist air. They are fueled by a different heat mechanism than other cyclonic. 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 refers to the fraction of the sky obscured by clouds when observed from a particular location through adiabatic The lapse rate is defined as the rate of decrease with height for an atmospheric variable. The variable involved is temperature unless specified otherwise. The terminology arises from the word lapse in the sense of a decrease or decline; thus, the lapse rate is the rate of decrease with height and not simply the rate of change. While most often cooling, once the air becomes saturated as it rises. Thus, low pressure areas typically bring cloudy skies, which act to minimize diurnal temperature extremes 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. Since clouds reflect 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, incoming shortwave Shortwave radiation is a term used to describe the radiant energy in the visible (VIS), near-ultraviolet (UV), and near-infrared (NIR) wavelengths. The wavelength range is not always exactly defined, as there is no standard cut-off for the NIR. Shortwave radiation may be as broadly defined as between 0.1 and 5.0 micrometers) or as narrow as 0.2 to solar radiation 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 is less which causes lower temperatures Historically, two equivalent concepts of temperature have developed, the thermodynamic description and a microscopic explanation based on statistical physics. Since thermodynamics deals entirely with macroscopic measurements, the thermodynamic definition of temperature, first stated by Lord Kelvin, is stated entirely in empirical, measurable during the day. At night, the absorptive effect of clouds on outgoing longwave radiation Outgoing Longwave Radiation is the energy leaving the earth as infrared radiation at low energy. Earth's radiation balance is very closely achieved since the OLR very nearly equals the Shortwave Absorbed Radiation received at high energy from the sun. Thus, the First Law of Thermodynamics (energy conservation) is satisfied and the Earth's average, 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 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. Winds are commonly classified by their spatial which are experienced in its vicinity. Globally, low pressure systems are most frequently located over the Tibetan Plateau The Tibetan Plateau , also known as the Qinghai-Tibetan Plateau (Chinese: 青藏高原; Pinyin: Qingzang Gaoyuan) is a vast, elevated plateau in Central Asia covering most of the Tibet Autonomous Region and Qinghai Province in China and Ladakh in India-controlled Kashmir. It occupies an area of around 1,000 by 2,500 kilometers, and has an average and in the lee of the Rocky mountains The Rocky Mountains are a major mountain range in western North America. The North American Rocky Mountains stretch more than 3,000 miles (4,830 km) from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States. The range's highest peak is Mount Elbert located in Colorado at 14,440 feet (4,401. In Europe (particularly in the United Kingdom), recurring low pressure weather systems are typically known as depressions.

Formation

Cyclogenesis is the development or strengthening of cyclonic circulations, or low pressure areas, within the atmosphere.^[1] Cyclogenesis is the opposite of cyclolysis, and has an anticyclonic (high pressure system) equivalent which deals with the formation of high pressure areas 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. Due to the coriolis force, winds flow clockwise around high-pressure systems in the northern—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 motion through the troposphere, the atmospheric layer.^[2] Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of 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 counter clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale cyclonic circulations are centered on areas of. The term cyclone is used where circular wind systems flow in the direction of the Earth's rotation,^[3][4], which normally coincides with areas of low pressure.^[5][6] The largest low-pressure systems are cold-core polar cyclones and extratropical cyclones which lie on the synoptic scale The synoptic scale in meteorology is a horizontal length scale of the order of 1000 kilometres (about 620 miles) or more. This corresponds to a horizontal scale typical of mid-latitude depressions. Most high and low pressure areas seen on weather maps such as surface weather analyses are synoptic-scale systems, driven by the location of rossby. Warm-core cyclones such as tropical cyclones, mesocyclones A mesocyclone is a vortex of air, approximately 2 to 10 km in diameter , within a convective storm. That is, it is air that rises and rotates around a vertical axis, usually in the same direction as low pressure systems in a given hemisphere. They are most often cyclonic, that is, associated with a localized low-pressure region within a severe, and polar lows lie within the smaller mesoscale 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. Examples of mesoscale weather systems are sea breezes, squall lines, and mesoscale convective complexes. Subtropical cyclones are of intermediate size.^[7][8] which is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. 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.^[9] Shortwave troughs embedded within the flow around larger scale troughs are smaller in scale, or mesoscale in nature.^[10] Both Rossby waves and shortwaves embedded within the flow around Rossby waves migrate equatorward of the polar cyclones Polar cyclones are low-pressure areas which strengthen in the winter and weaken in the summer. A polar cyclone is a low pressure weather system usually spanning 1,000–2,000 kilometers in which the air is circulating in a counter-clockwise fashion (in the northern hemisphere). The reason for the rotation is the same as any other cyclone, the located in both the Northern and Southern hemispheres.^[11] 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.^[12]

Extratropical cyclones 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 horizontal gradients in temperature and dew point form as waves along weather fronts 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. The air masses separated by a front usually differ in temperature and humidity. Cold due to a passing by shortwave aloft or upper level jet streak before occluding later in their life cycle as cold core cyclones.^{[13][14][15] [16]} Polar lows are small-scale, short-lived atmospheric low pressure systems that are found over the ocean areas poleward of the main 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 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. Examples of mesoscale weather systems are sea breezes, squall lines, and mesoscale convective complexes 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).^[17]

Tropical cyclones 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 feed on heat released when moist air rises, resulting in condensation of water vapor contained in the moist air. They are fueled by a different heat mechanism than other cyclonic form due to latent heat driven by significant thunderstorm activity, and are warm core with well-defined circulations.^[18] Certain criteria need to be met for their formation. In most situations, water temperatures In practical terms, the exact meaning of surface varies according to the measurement method used. A satellite infrared radiometer indirectly measures the temperature of a very thin layer of about 10 micrometres thick of the ocean which leads to the phrase skin temperature (because infrared radiation is emitted from this layer). A microwave of at least 26.5 °C (79.7 °F) are needed down to a depth of at least 50 m (160 ft);^[19] waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms.^[20] Another factor is rapid cooling with height, which allows the release of the heat of condensation The enthalpy of vaporization, , also known as the heat of vaporization or heat of evaporation, is the energy required to transform a given quantity of a substance into a gas that powers a tropical cyclone.^[19] High humidity is needed, especially in the lower-to-mid troposphere The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 75% 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.^[19] Low amounts of 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. Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across fronts and near the coast, and vertical shear typically near the surface, are needed, as high shear is disruptive to the storm's circulation.^[19] Lastly, a formative tropical cyclone needs a pre-existing system of disturbed weather, although without a circulation no cyclonic development will take place.^[19] Mesocyclones A mesocyclone is a vortex of air, approximately 2 to 10 km in diameter , within a convective storm. That is, it is air that rises and rotates around a vertical axis, usually in the same direction as low pressure systems in a given hemisphere. They are most often cyclonic, that is, associated with a localized low-pressure region within a severe form as warm core cyclones over land, and can lead to tornado formation.^[21] Waterspouts 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. Waterspouts do not suck up water; the water seen in the can also form from mesocyclones, but more often develop from environments of high instability and low vertical 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. Wind shear can be broken down into vertical and horizontal components, with horizontal wind shear seen across fronts and near the coast, and vertical shear typically near the surface,.^[22]

In deserts A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Deserts are defined as areas with an average annual precipitation of less than 250 millimetres per year, or as areas where more water is lost by evapotranspiration than falls as precipitation. In the Köppen, 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.^[23] 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.^[24] Similar rainfall is caused by the moist ocean air being lifted upwards by mountains,^[25] surface heating,^[26] convergence at the surface,^[27] divergence aloft, or from storm-produced outflows at the surface.^[28] However the lifting occurs, the air cools due expansion in lower pressure, which in turn produces condensation. 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.^[24] 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.^[29]

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