A low-pressure area, low area or low is a region where the air 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 atmosphere. 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.
Introduction
Low-pressure area
Low-pressure area | Introduction
It forms where the air pressure is lower than that of surrounding locations A low-pressure area, low area or low is a region where the air 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 atmosphere. The first area is on the east side of upper troughs and a second area occurs ahead of embedded shortwave troughs They form under areas of wind divergence that occur in the upper levels of the atmosphere.
we would expect what with low pressure in the northern hemisphere
user
234 significant extratropical cyclones form each winter
assistant
how does a low pressure area form?
user
Cyclogenesis is the development and strengthening of cyclonic circulations
assistant
what does the first word mean?
user
It  is the development and strengthening of cyclonic circulations in the atmosphere results in the formation of low pressure area
assistant
how is this associated with the weather?
user
Sorry. I cannot find the answer based on the context.
assistant
what are some of its related terms?
user
It is the opposite of cyclolysis
assistant
which scaled are used to measure it?
user
The macroscale, the synoptic scale, the mesoscale, and the microscale
assistant
what does the first one do?
user
It deals with systems with global size, such as the Madden–Julian oscillation
assistant
who invented the second one?
user
Sorry. I cannot find the answer based on the context.
assistant
what is the function of the third one?
user
It deals with  the study of weather systems smaller than synoptic scale systems but larger than microscale and storm-scale cumulus systems
assistant
is the fourth one the opposite of the first one?
user
Yes, it is smallest of the meteorological scales
assistant
what results does the second one give?
user
It  covers a portion of a continent, such as extratropical cyclones, with dimensions of across
assistant
where do low pressure areas form?
user
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.
Low-pressure area | Introduction
A low-pressure area, low area or low is a region where the air 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 atmosphere. 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.
Low-pressure area | Introduction
A low-pressure area, or "low", is a region where the atmospheric pressure at sea level is below that of surrounding locations. Low-pressure systems form under areas of wind divergence that occur in upper levels of the troposphere. 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: 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.
Pressure system | Low-pressure system
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 occur over the ocean areas poleward of the main polar front in both the Northern and Southern Hemispheres. They are part of the larger class of mesoscale 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 .
Low-pressure area | Formation
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 create pressure gradients that 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-defined circulation in the tropics it is termed a tropical cyclone.
Pressure system | Low-pressure system
Tropical cyclones 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 of at least are needed down to a depth of at least ; 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; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop.
Low-pressure area | Formation
Low amounts of wind shear 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. Mesocyclones form as warm core cyclones over land, and can lead to tornado formation. Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear. 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.
Low-pressure area | Formation
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. A low-pressure area is commonly associated with inclement weather, while a high-pressure area is associated with light winds and fair skies. 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 area | Introduction
Cyclogenesis is the development and strengthening of cyclonic circulations, or low-pressure areas, within the atmosphere. Cyclogenesis is the opposite of , and has an anticyclonic (high-pressure system) equivalent which deals with the formation of high-pressure areas—anticyclogenesis. Cyclogenesis is an umbrella term for several different processes, all of which result in the development of some sort of cyclone. Meteorologists use the term "cyclone" 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, mesocyclones, and polar lows lie within the smaller mesoscale.
Low-pressure area | Formation
This, combined with the rising of the hot air, results in a low-pressure area called a thermal low. Monsoon circulations are caused by thermal lows which form over large areas of land and their strength is driven by how land heats more quickly than the surrounding nearby ocean. This generates a steady wind blowing toward the land, bringing the moist near-surface air over the oceans with it. Similar rainfall 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 to expansion in lower pressure, which in turn produces condensation.
Low-pressure area | Formation
Subtropical cyclones are of intermediate size. Cyclogenesis can occur at various scales, from the microscale to the synoptic scale. Larger-scale troughs, 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 cyclones 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 lowers surface pressure.
Low-pressure area | Formation
Extratropical cyclones form along linear bands of temperature/dewpoint gradient with significant vertical wind shear, and are thus classified as baroclinic cyclones. Initially, cyclogenesis, or low pressure formation, occurs along frontal zones near a favorable quadrant of a maximum in the upper level jetstream known as a jet streak. The favorable quadrants are usually at the right rear and left front quadrants, where divergence ensues. The divergence causes air to rush out from the top of the air column. As mass in the column is reduced, atmospheric pressure at surface level (the weight of the air column) is reduced. The lowered pressure strengthens the cyclone (a low pressure system).
Extratropical cyclone | Cyclogenesis
Tropical cyclones can form during any month of the year globally, but can occur in either the northern hemisphere or the southern hemisphere during November.
Pressure system | Low-pressure system
High-pressure areas form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as convergence, near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere, and about half the atmospheric pressure at the surface. High-pressure systems are alternatively referred to as anticyclones. On English-language weather maps, high-pressure centers are identified by the letter H in English, within the isobar with the highest pressure value.
High-pressure area | Formation
Large polar cyclones help determine the steering of systems moving through the mid-latitudes, south of the Arctic and north of the Antarctic. The Arctic oscillation provides an index used to gauge the magnitude of this effect in the Northern Hemisphere. Extratropical cyclones 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 shows that between the 30th and 70th parallels there are an average of 37 cyclones in existence during any 6-hour period. A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter.
Low-pressure area | Mid-latitudes and subtropics
Kona lows, most extratropical cyclones, and tropical upper tropospheric cyclones are cold core lows. In the eastern half of the north Pacific ocean and north Indian ocean, the formation of a weak circulation underneath a mid to upper-tropospheric low which has cut off from the main belt of the westerlies during the cold season (winter) is called a subtropical cyclone. In the case of the north Indian ocean, the formation of this type of vortex leads to the onset of monsoon rains during the wet season. East coast lows form near and east of where a cold core low interacts with a sea surface temperature (SST) gradient along the east coast of continents, such as Asia, North America, southern Africa, and Australia within a pre-existing area of high pressure.
Cold-core low | Importance to cyclones within the subtropics and mid-latitudes
In Europe, particularly in the United Kingdom and in the Netherlands, recurring extratropical low-pressure weather systems are typically known as depressions. These tend to bring wet weather throughout the year. "Thermal lows" also occur during the summer over continental areas across the subtropics - such as the Sonoran Desert, the Mexican plateau, the Sahara, South America, and Southeast Asia. The lows are most commonly located over the Tibetan plateau and in the lee of the Rocky mountains.
Low-pressure area | Mid-latitudes and subtropics
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 resemble sea and land breezes, terms usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale - also stronger and seasonal.
Low-pressure area | Formation
On November 12, the National Hurricane Center (NHC) first noted the potential for a low pressure area to develop in the southwestern Caribbean Sea, assessing a low probability for tropical cyclone formation within five days. After a convectively coupled Kelvin wave and several nearby tropical waves interacted with each other in the aforementioned area, atmospheric pressure in that region began falling on November 14, resulting in a broad low pressure area forming north of Colombia on November 15. The low moved slowly and erratically, with a disorganized structure and light winds, disrupted by unfavorable upper-level conditions. On November 19, the convection increased and became better organized, amid marginally favorable conditions.
Hurricane Otto | Meteorological history
Low-pressure systems, also known as cyclones, are located in minima in the pressure field. Rotation is inward at the surface and counterclockwise in the northern hemisphere as opposed to inward and clockwise in the southern hemisphere due to the Coriolis force. Weather is normally unsettled in the vicinity of a cyclone, with increased cloudiness, increased winds, increased temperatures, and upward motion in the atmosphere, which leads to an increased chance of precipitation. Polar lows can form over relatively mild ocean waters when cold air sweeps in from the ice cap. The relatively warmer water leads to upward convection, causing a low to form, and precipitation usually in the form of snow.
Surface weather analysis | Low pressure
Elongated areas of low pressure form at the monsoon trough or intertropical convergence zone as part of the Hadley cell 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 in East Asia during August and 20th parallel 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 continents help create pressure gradients which drive monsoon circulations.
Low-pressure area | Monsoon trough
A tropical cyclone differs from a hurricane or typhoon based only on geographic location. Note that a tropical cyclone is fundamentally different from a mid-latitude cyclone. A hurricane is a storm that occurs in the Atlantic Ocean and northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a tropical cyclone occurs in the south Pacific or Indian Ocean. Friction with land slows down the wind flowing into low-pressure systems and causes wind to flow more inward, or flowing more ageostrophically, toward their centers. Tornadoes are often too small, and of too short duration, to be influenced by the Coriolis force, but may be so-influenced when arising from a low-pressure system.
Low-pressure area | Associated weather
In terms of climatology, high pressure forms at the horse latitudes, or torrid zone, between the latitudes of 20 and 40 degrees from the equator, as a result of air that has been uplifted at the equator. As the hot air rises it cools, losing moisture; it is then transported poleward where it descends, creating the high-pressure area. This is part of the Hadley cell circulation and is known as the subtropical ridge or subtropical high, and is strongest in the summer. The subtropical ridge is a warm core high-pressure system, meaning it strengthens with height. Many of the world's deserts are caused by these climatological high-pressure systems.
High-pressure area | In climatology
This, combined with the rising of the hot air, results in the formation of a low pressure area. Elevated areas can enhance the strength of the thermal low as they warm more quickly than the atmosphere which surrounds them at the same altitude. Over the water, instability lows form during the winter when the air overlying the land is colder than the warmer water body. Thermal lows tend to have weak circulations, and can extend to in height. Thermal lows over the western and southern portions of North America, northern Africa, and southeast Asia are strong enough to lead to summer monsoon conditions.
Thermal low | Introduction
At high altitudes, outward-spreading air rotates in the opposite direction. Cyclones rarely form along the equator due to the weak Coriolis effect present in this region.
Coriolis force | Flow around a low-pressure area
The initial low pressure area tracked eastward from the southern Great Plains on February 2. Gradually intensifying, it was situated over eastern Kentucky by 0000 UTC on February 4. As the system entered West Virginia, a secondary low began to develop over South Carolina. The new low quickly deepened, and absorbed the primary low over West Virginia into its expanding and strengthening circulation. It continued to rapidly mature as it moved northward from eastern Maryland, and it hugged the coast as it moved northeastward towards southern New England. The cyclone reached a strength of 980 millibars while located south of Long Island.
February 1995 nor'easter | Synoptic history
Within the United States, these systems have been observed to form in the Mississippi river valley, Southeast, Florida, and Great Plains.
Wake low | Formation
Near this center, the pressure gradient force (from the pressure at the center of the cyclone compared to the pressure outside the cyclone) and the Coriolis force must be in an approximate balance for the cyclone to avoid collapsing in on itself as a result of the difference in pressure. The central pressure of the cyclone will lower with increasing maturity, while outside of the cyclone, the sea-level pressure is about average. In most extratropical cyclones, the part of the cold front ahead of the cyclone will develop into a warm front, giving the frontal zone (as drawn on surface weather maps) a wave-like shape.
Extratropical cyclone | Surface pressure and wind distribution
Tropical cyclones typically form from large, disorganized areas of disturbed weather in tropical regions. As more thunderstorms form and gather, the storm develops rainbands which start rotating around a common center. As the storm gains strength, a ring of stronger convection forms at a certain distance from the rotational center of the developing storm. Since stronger thunderstorms and heavier rain mark areas of stronger updrafts, the barometric pressure at the surface begins to drop, and air begins to build up in the upper levels of the cyclone. This results in the formation of an upper level anticyclone, or an area of high atmospheric pressure above the central dense overcast.
Eye (cyclone) | Formation and detection
Tropical cyclones generally need to form more than or poleward of the 5th parallel north and 5th parallel south, allowing the Coriolis effect 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 is possible. Nearly one-third of the world's tropical cyclones form within the western Pacific Ocean, making it the most active tropical cyclone basin on Earth.
Low-pressure area | Tropical cyclone
High-pressure systems form due to downward motion through the troposphere, the atmospheric layer where weather occurs. Preferred areas within a synoptic flow pattern in higher levels of the troposphere are beneath the western side of troughs. On weather maps, these areas show converging winds (isotachs), also known as confluence, or converging height lines near or above the level of non-divergence, which is near the 500 hPa pressure surface about midway up through the troposphere. On weather maps, high-pressure centers are associated with the letter H placed within the isobar with the highest pressure value. On constant pressure upper level charts, it is located within the highest height line contour.
Anticyclogenesis | Process
Whether it be a depression in the intertropical convergence zone (ITCZ), a tropical wave, a broad surface front, or an outflow boundary, a low level feature with sufficient vorticity and convergence is required to begin tropical cyclogenesis. Even with perfect upper level conditions and the required atmospheric instability, the lack of a surface focus will prevent the development of organized convection and a surface low. Tropical cyclones can form when smaller circulations within the Intertropical Convergence Zone merge.
Tropical cyclogenesis | Low level disturbance
Wind is initially accelerated from areas of high pressure to areas of low pressure. This is due to density (or temperature and moisture) differences between two air masses. Since stronger high-pressure systems contain cooler or drier air, the air mass is denser 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. Thus, stronger areas of low pressure are associated with stronger winds. The Coriolis force caused by the Earth's rotation is what gives winds around low-pressure areas (such as in hurricanes, cyclones, and typhoons) their counter-clockwise (anticlockwise) 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).
Low-pressure area | Associated weather
Most tropical cyclones form in a worldwide band of thunderstorm activity near the equator, referred to as the Intertropical Front (ITF), the Intertropical Convergence Zone (ITCZ), or the monsoon trough. Another important source of atmospheric instability is found in tropical waves, which contribute to the development of about 85% of intense tropical cyclones in the Atlantic Ocean and become most of the tropical cyclones in the Eastern Pacific. The majority forms between 10 and 30 degrees of latitude away of the equator, and 87% forms no farther away than 20 degrees north or south. Because the Coriolis effect initiates and maintains their rotation, tropical cyclones rarely form or move within 5 degrees of the equator, where the effect is weakest.
Tropical cyclone | Locations
Thermal lows, or heat lows, are non-frontal low-pressure areas that occur over the continents in the subtropics during the warm season, as the result of intense heating when compared to their surrounding environments. Thermal lows occur near the Sonoran Desert, on the Mexican plateau, in California's Great Central Valley, the Sahara, over north-west Argentina in South America, over the Kimberley region of north-west Australia, the Iberian peninsula, and the Tibetan plateau. Over land, intense, rapid solar heating of the land surface results in heating of the lowest layers of the atmosphere via reradiated energy in the infrared spectrum. The resulting hotter air is less dense than surrounding cooler air.
Thermal low | Introduction
Tropical cyclones and winter storms are intense varieties of low pressure. Over land, thermal lows are indicative of hot weather during the summer.
Surface weather analysis | Low pressure
On 26 January, a low-pressure system formed to the east of the Solomon Islands, centred within a very long low-pressure trough stretching from Western Australia to the central South Pacific Ocean. The low-pressure system tracked generally towards the west over the following days, moving into the Australian region from the South Pacific basin on 27 January. On 3 February, a few days after the low-pressure trough had evolved into a monsoon trough, the BOM noted that the low-pressure system had developed into a tropical low. At the time, the atmospheric environment was assessed as being unfavourable for tropical cyclogenesis; however, the BOM indicated that conditions were likely to become more conducive over the following week.
2019–20 Australian region cyclone season | Tropical Low Uesi
As part of the Hadley cell, surface air flows toward the equator while the flow aloft is towards the poles. A low-pressure area of calm, light variable winds near the equator is known as the doldrums, near-equatorial trough, intertropical front, or the Intertropical Convergence Zone. When located within a monsoon region, this zone of low pressure and wind convergence is also known as the monsoon trough. Around 30° in both hemispheres, air begins to descend toward the surface in subtropical high-pressure belts known as subtropical ridges. The subsident (sinking) air is relatively dry because as it descends, the temperature increases, but the moisture content remains constant, which lowers the relative humidity of the air mass.
Trade winds | Cause
During this time, convection became displaced to the northwest while the circulation center was left exposed. The former cyclone gradually became more monsoonal in nature, with convection developing along a convergence line rather than in curved banding features. The poorly organised system became difficult to follow, though a small central dense overcast feature along the south side of its center allowed forecasters to discern its location. Despite moving back over water on 5 February and environmental conditions favoring redevelopment, convection failed to form around the low and instead, continued to flare along the convergence zone to the north. The remnant system then became nearly stationary just offshore for two days before adopting a slow north-northeasterly track.
Tropical Low Fletcher | Meteorological history
An area of low pressure soon formed within the system just off the coast of South Carolina. Generally favorable environmental conditions fostered further organization of the low, though convection remained minimal for several days. During the latter half of June 30, a United States Air Force weather reconnaissance mission into the system confirmed the presence of a well-organized circulation; however, displacement of showers and thunderstorms from its center delayed its classification. Subsequent organization of convection into a banding feature along the system's southern flank, as depicted by radar imagery, prompted the NHC to designate the low as Tropical Depression One at 00:00 UTC on July 1.
Hurricane Arthur | Meteorological history
An example of a convergence zone is the Intertropical Convergence Zone (ITCZ), a low pressure area which girdles the Earth at the Equator. Another example is the South Pacific convergence zone that extends from the western Pacific Ocean toward French Polynesia. The Intertropical Convergence Zone is the result of the Northeasterly trade winds and Southwesterly trade winds converging in an area of high latent heat and low pressure. As the two trade winds converge, the cool, dry air collects moisture from the warm ocean and rises, contributing to cloud formation and precipitation. The low pressure area that is created by the movement of the trade winds, acts as a vacuum, drawing in the cooler, dry air from high pressure areas (divergence zones), creating a convection cell commonly known as the Hadley Cell.
Convergence zone | Large scale
The term "cyclone" applies to numerous types of low pressure areas, one of which is the extratropical cyclone. The descriptor "extratropical" signifies that this type of cyclone generally occurs outside the tropics and in the middle latitudes of Earth between 30° and 60° latitude. They are termed "mid-latitude cyclones" if they form within those latitudes, or post-tropical cyclones if a tropical cyclone has intruded into the mid latitudes. Weather forecasters and the general public often describe them simply as "depressions" or "lows". Terms like frontal cyclone, frontal depression, frontal low, extratropical low, non-tropical low and hybrid low are often used as well.
Extratropical cyclone | Terminology
Another way to state this is to say that regions of upper air divergence are conducive to lower level convergence, cyclone formation, and positive vertical velocity. Therefore, identifying regions of upper air divergence is an important step in forecasting the formation of a surface low-pressure area.
Troposphere | Divergence and convergence
Extratropical cyclones form anywhere within the extratropical regions of the Earth (usually between 30° and 60° latitude from the equator), either through cyclogenesis or extratropical transition. A study of extratropical cyclones in the Southern Hemisphere shows that between the 30th and 70th parallels, there are an average of 37 cyclones in existence during any 6-hour period. A separate study in the Northern Hemisphere suggests that approximately 234 significant extratropical cyclones form each winter.
Extratropical cyclone | Formation
Eventually, the cyclone stacks with the upper level disturbance, becoming isolated within the cold sector, and begins to weaken as it becomes far removed from the original temperature discontinuity along the cold and warm fronts. At this point, it becomes a cold-core low. The frontal boundary becomes weaker and surrounds the equatorward portion of the cyclone, waiting for the next upper level disturbance to form a new low pressure area.
Norwegian cyclone model | Evolution
Waterspouts can also form from mesocyclones, but more often develop from environments of high instability and low vertical wind shear. Cyclolysis is the opposite of cyclogenesis, and is the high-pressure system equivalent, which deals with the formation of high-pressure areas—Anticyclogenesis.
Cyclone | Formation
The northwestern Mediterranean and the Gulf of Genoa in particular, are not only a transition area for passing cyclones, but are frequently areas of cyclogenesis. Low pressure areas move into or are formed as a result of North Atlantic air entering the Mediterranean Sea between the Alps and the Massif Central, via the Rhone Valley, or via the Carcassonne gap between the Pyrenees and Massif Central. This cold and moist air enters the Mediterranean basin, and is deflected by the high mountains of northwest Corsica, which divert the air mass to the northeast, triggering cool and wet Libeccio winds in response into the Ligurian Sea, which in turn hit the western Apennines located in the immediate vicinity of the sea.
Genoa low | Cyclogenesis
On constant pressure upper level charts, it is located within the highest height line contour.
High-pressure area | Formation
Forms of MCS that develop within the tropics use either the Intertropical Convergence Zone (ITCZ) or monsoon troughs as a focus for their development, generally within the warm season between spring and fall. One exception is that of lake-effect snow bands, which form due to cold air moving across relatively warm bodies of water, and occurs from fall through spring. Polar lows are a second special class of MCS which form at high latitudes during the cold season. Once the parent MCS dies, later thunderstorm development can occur in connection with its remnant mesoscale convective vortex (MCV). Mesoscale convective systems are important to the United States rainfall climatology over the Great Plains since they bring the region about half of their annual warm season rainfall.
Mesoscale convective system | Introduction
Tropical cyclones are areas of relatively low pressure in the troposphere, with the largest pressure perturbations occurring at low altitudes near the surface. On Earth, the pressures recorded at the centers of tropical cyclones are among the lowest ever observed at sea level. The environment near the center of tropical cyclones is warmer than the surroundings at all altitudes, thus they are characterized as "warm core" systems.
Tropical cyclone | Physical structure
In the case of the north Indian Ocean, the formation of this type of vortex leads to the onset of monsoon rains during the wet season. In the southern hemisphere, subtropical cyclones are regularly observed across southern portions of the Mozambique Channel. Most subtropical cyclones form when a deep cold-core extratropical cyclone drops down into the subtropics. The system becomes blocked by a high latitude ridge, and eventually sheds its frontal boundaries as its source of cool and dry air from the high latitudes diverts away from the system. Temperature differences between the 500 hPa pressure level and the sea surface temperatures initially exceed the dry adiabatic lapse rate, which causes an initial round of thunderstorms to form at a distance east of the center.
Subtropical cyclone | Formation
The cyclone will also distort in shape, becoming less symmetric with time. During extratropical transition, the cyclone begins to tilt back into the colder airmass with height, and the cyclone's primary energy source converts from the release of latent heat from condensation (from thunderstorms near the center) to baroclinic processes. The low pressure system eventually loses its warm core and becomes a cold-core system. The peak time of subtropical cyclogenesis (the midpoint of this transition) in the North Atlantic is in the months of September and October, when the difference between the temperature of the air aloft and the sea surface temperature is the greatest, leading to the greatest potential for instability.
Extratropical cyclone | Extratropical transition
Cold cyclones are stronger aloft than at the Earth's surface, or stronger in areas of the troposphere with lower pressures, per the thermal wind relationship and the hypsometric equation. The hypsometric equation dictates that colder atmospheres have less room between pressure surfaces, which corresponds to the concept of lower atmospheric thickness, and the thermal wind relation indicates that in this situation, the wind increases with height. It also means that an isolated pool of cold air aloft is associated with the feature. Because of its cold center, the center shows an area of thickness minimum. Since the temperature at any given radius is similar and parallel to the orientation of the pressure gradient, and therefore the wind, it is barotropic in nature.
Cold-core low | Characteristics
On a non-rotating planet, fluid would flow along the straightest possible line, quickly eliminating pressure gradients. The geostrophic balance is thus very different from the case of "inertial motions" (see below), which explains why mid-latitude cyclones are larger by an order of magnitude than inertial circle flow would be. This pattern of deflection, and the direction of movement, is called Buys-Ballot's law. In the atmosphere, the pattern of flow is called a cyclone. In the Northern Hemisphere the direction of movement around a low-pressure area is anticlockwise. In the Southern Hemisphere, the direction of movement is clockwise because the rotational dynamics is a mirror image there.
Coriolis force | Flow around a low-pressure area
A cyclone forms around a low-pressure area. Anticyclone, the term for the kind of weather around a high-pressure area, was coined in 1877 by Francis Galton to indicate an area whose winds revolved in the opposite direction of a cyclone. In British English, the opposite direction of clockwise is referred to as anticlockwise, making the label "anticyclones" a logical extension.
High-pressure area | Wind circulation in the northern and southern hemispheres
Thermal lows which develop near sea level can build in height during the warm season, or summer, to the elevation of the 700 hPa pressure surface, which lies near above sea level. Heat lows normally are stationary and have a weak cyclonic circulation. As they are strongest at the surface and warm near their center, and weaker aloft where the air is more stable, the thermal low is considered warm core. The strongest versions of these features globally are over Arabia, the northern portion of the Indian subcontinent, Arizona, Mexican plateau, northwest Argentina, southwestern Spain, Australia, and northern Africa. The formation of the heat low over northern Africa leads to a low-level westerly jet stream from June into October.
Thermal low | Formation
On 20 November 2013, an area of low pressure, accompanied by persistent deep convection, developed over the southern Indian Ocean, roughly 1,465 km (910 mi) north of RAAF Learmonth in Western Australia. At the mid-levels, the circulation appeared more broad in nature, with notable rainbands wrapping into the low. Environmental conditions favored tropical cyclogenesis, with moderate wind shear, high sea surface temperatures, and prominent outflow that extended into the mid-latitude westerlies. By this time, the Bureau of Meteorology (BOM) noted this system as a tropical low just outside their area of responsibility. Owing to increasing organization over the following day, the Joint Typhoon Warning Center (JTWC) issued a Tropical Cyclone Formation Alert for the system late on 21 November.
Cyclone Alessia | Meteorological history
On 20 December, an area of low pressure, associated with developing convection, formed about 1000 km (560 mi) east of Darwin in the Gulf of Carpentaria. The next day, the Bureau of Meteorology in Darwin designated the system as a tropical low. Banding features developed along the southern portion of the system and further development was anticipated as the low was located underneath an anticyclone. Around 5:26 a.m. local time on 23 December, the low made landfall near Port Roper, preventing further development of the storm. About 36 hours after landfall, the Australian Bureau of Meteorology issued their final advisory on the system as it dissipated over land.
2008–09 Australian region cyclone season | Tropical Low 04U
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. Over elevated surfaces, heating of the ground exceeds the heating of the surrounding air at the same altitude above sea level, which creates an associated heat low over the terrain and enhances any thermal lows which would have otherwise existed. During the cold season, (winter), warm water bodies such as the Great Lakes can induce an instability low.
Thermal low | Formation
This potential energy sink creates a kinetic energy source which injects a final burst of energy into the storm's motions. After this process occurs, the growth period of the cyclone, or cyclogenesis, ends, and the low begins to spin down (fill) as more air is converging into the bottom of the cyclone than is being removed out the top since upper-level divergence has decreased. Occasionally, cyclogenesis will re-occur with occluded cyclones. When this happens a new low center will form on the triple-point (the point where the cold front, warm front, and occluded front meet). During triple-point cyclogenesis, the occluded parent low will fill as the secondary low deepens into the main weathermaker.
Cyclogenesis | Maturity
On 12 January, a weak tropical low formed over the Arafura Sea. The system was last noted the following day, having a pressure no lower than 1004 mb (hPa; 29.65 inHg). The United States Navy continued monitoring the system until it was last noted on 16 January.
2012–13 Australian region cyclone season | Tropical Low 06U
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.
Low-pressure area | Monsoon trough
These thermal lows can be developed or created around Spain, Portugal, France etc., during the summer season because of the intense heat. Thermal low pressure can be located around the world, particularly in the summer or in tropical regions.
Spanish plume | Introduction
Binary interaction is seen between nearby extratropical cyclones when within of each other, with significant acceleration occurring when the low-pressure areas are within of one another. Interactions between their circulations at the 500 hPa level (18,000 feet above sea level) behave more predictably than their surface circulations. This most often results in a merging of the two low-pressure systems into a single extratropical cyclone, or can less commonly result in a mere change of direction of one or both of the cyclones. The precise results of such interactions depend on factors such as the size of the two cyclones, their distance from each other, and the prevailing atmospheric conditions around them.
Fujiwhara effect | Extratropical cyclones
During 13 May 2020, an area of low pressure developed over the Southeastern Bay of Bengal about to the southeast of Visakhapatnam in the Indian state of Andhra Pradesh. The area of low pressure was located within a favorable environment for further development with good equator-ward outflow, exceptionally warm sea surface temperatures between 32-34°C recorded in the basin, and low vertical wind shear. Over the next couple of days, the system became more marked as it gradually consolidated further, with bands of deep atmospheric convection wrapping into the system's low-level center. During 16 May, the India Meteorological Department (IMD) reported that the area of low pressure had developed into a depression and designated it as BOB 01 while it was located about to the south of Paradip in the Indian state of Odisha.
Cyclone Amphan | Meteorological history
The lowered pressure acts to draw in air, creating convergence in the low-level wind field. Low-level convergence and upper-level divergence imply upward motion within the column, making cyclones tend to be cloudy. As the cyclone strengthens, the cold front sweeps towards the equator and moves around the back of the cyclone. Meanwhile, its associated warm front progresses more slowly, as the cooler air ahead of the system is denser, and therefore more difficult to dislodge. Later, the cyclones occlude as the poleward portion of the cold front overtakes a section of the warm front, forcing a tongue, or trowal, of warm air aloft.
Extratropical cyclone | Cyclogenesis
The low became well defined, interacting with the adjacent upper vortex to produce a wide convective band—a concentrated region in which strong showers and thunderstorms can form—over the northeastern Caribbean. Over the next day, the system continued to acquire a prominent subtropical structure: it developed a markedly larger wind radius than typically seen in pure tropical cyclones. Satellite observations also revealed that although the lower wind circulation was distinct, it had become intertwined with the cyclonic vortex aloft to its southwest, confirming the hybrid nature of the system. The National Hurricane Center (NHC) thus classified the system as a subtropical depression at 06:00 UTC on October 6, when it was 265 mi (425 km) north-northwest of San Juan, Puerto Rico.
Hurricane Otto (2010) | Meteorological history
Mesoscale convective systems, which can evolve into tropical cyclones, form along areas such as tropical waves or easterly waves which progress westward along monsoon troughs and the Intertropical Convergence Zone in regions of ample low level moisture, convergent surface winds, and divergent winds aloft. This typically occurs north of the equator from Africa across the Atlantic and eastern Pacific oceans, as well as across the northwest and southwest Pacific oceans, from Australia eastward into Oceania, the Indian ocean, Indonesia, and from southeast Brazil into the southern Atlantic ocean. It is also noted on occasion in the southeast Pacific ocean mild to cool ENSO years, outside of El Niño.
Mesoscale convective system | Tropics
The low pressure area that would eventually develop into Cyclone Paula was first identified at 1800 UTC on February 25, embedded within a monsoon trough south of Espiritu Santo, Vanuatu. At the time, the system was positioned under an upper-level outflow center and located in an area of warm sea surface temperatures (SSTs) of about 30 °C (86 °F). Early the next day, a sharp increase in oceanic heat content triggered a rapid burst of convection in the northern and eastern quadrants of the system. Associated rainbands also began to increasingly curve toward the circulation center of the low pressure area.
Cyclone Paula | Meteorological history
Wake lows form due to adiabatic warming in the wake of mature squall lines at the back edge of their rain shields, where evaporative cooling is unable to offset warming due to atmospheric subsidence, or downward motion. They can be caused by gravity waves which duct through boundary layers which are deep and cold to the north of a weather front. As with mesoscale high-pressure areas behind a squall line, when new thunderstorm development stops along the squall line, the wake low will weaken as well. Clearing conditions will accompany the wake low, due to the descending warm air mass associated with the feature.
Wake low | Formation
The windfield of an extratropical cyclone constricts with distance in relation to surface level pressure, with the lowest pressure being found near the center, and the highest winds typically just on the cold/poleward side of warm fronts, occlusions, and cold fronts, where the pressure gradient force is highest. The area poleward and west of the cold and warm fronts connected to extratropical cyclones is known as the cold sector, while the area equatorward and east of its associated cold and warm fronts is known as the warm sector. The wind flow around an extratropical cyclone is counterclockwise in the northern hemisphere, and clockwise in the southern hemisphere, due to the Coriolis effect (this manner of rotation is generally referred to as "cyclonic").
Extratropical cyclone | Surface pressure and wind distribution
Within the Southern Hemisphere tropical cyclones generally form on a regular basis between the African coast and the middle of the South Pacific. Tropical and Subtropical Cyclones have also been noted occurring in the Southern Atlantic Ocean at times. For various reasons including where tropical cyclones form, there are several different ways to split the area between the American and African coasts. For instance the World Meteorological Organization define three different basins for the tracking and warning of tropical cyclones. These are the South-West Indian Ocean between the African Coast and 90°E, the Australian region between 90°E and 160°E and the South Pacific between 160°E and 120°W.
Tropical cyclone basins | Southern Hemisphere
The type that forms during the warm season over land has been noted across North America, Europe, and Asia, with a maximum in activity noted during the late afternoon and evening hours. Forms of MCS that develop in the tropics are found in use either the Intertropical Convergence Zone or monsoon troughs, generally within the warm season between spring and fall. More intense systems form over land than over water. One exception is that of lake-effect snow bands, which form due to cold air moving across relatively warm bodies of water, and occurs from fall through spring. Polar lows are a second special class of MCS.
Thunderstorm | Mesoscale convective systems
On 9 April, satellite scatterometer data indicated the presence of a broad low pressure circulation in the central Indian Ocean, near the western edge of the Australian region. The following day, the BOM reported that the system had developed into a weak tropical low, centred within a low pressure trough. The system remained mostly stationary for the next several days as low vertical wind shear, warm sea surface temperatures and good divergence in the upper troposphere allowed convection to develop over the tropical low. The low-level circulation began to consolidate into a more circular structure by 12 April, centred approximately 515 km (320 mi) to the west-northwest of the Cocos Islands.
2019–20 Australian region cyclone season | Tropical Low Jeruto
Tropical cyclones are typically between in diameter. The strong rotating winds of a tropical cyclone are a result of the conservation of angular momentum imparted by the Earth's rotation as air flows inwards toward the axis of rotation. As a result, they rarely form within 5° of the equator. Tropical cyclones are almost unknown in the South Atlantic due to a consistently strong wind shear and a weak Intertropical Convergence Zone. Conversely, the African easterly jet and areas of atmospheric instability give rise to cyclones in the Atlantic Ocean and Caribbean Sea, while cyclones near Australia owe their genesis to the Asian monsoon and Western Pacific Warm Pool.
Tropical cyclone | Introduction
A wave along a frontal boundary, in the form of a broad area of low pressure, develops as an upper level disturbance moves towards that portion of the boundary. Precipitation will begin to form ahead of the surface low, within the cold sector of the cyclone poleward of the warm front. As the low deepens, both the cold and warm fronts around the low become better defined. As the low matures, it couples with the upper level disturbance moving into the cyclone's cold sector. The cold front catches up to the westward portion of the warm front, forming an occluded front.
Norwegian cyclone model | Evolution
Under specific circumstances, upper level cold lows can break off from the base of the Tropical Upper Tropospheric Trough (TUTT), which is located mid-ocean in the Northern Hemisphere during the summer months. These upper tropospheric cyclonic vortices, also known as TUTT cells or TUTT lows, usually move slowly from east-northeast to west-southwest, and their bases generally do not extend below in altitude. A weak inverted surface trough within the trade wind is generally found underneath them, and they may also be associated with broad areas of high-level clouds. Downward development results in an increase of cumulus clouds and the appearance of a surface vortex.
Cyclone | TUTT cell
A surface low can have a variety of causes for forming. Topography can force a surface low to form when an existing baroclinic wave moves over a mountain barrier; this is known as "lee cyclogenesis" since the low forms on the leeward side of the mountains. Mesoscale convective systems can spawn surface lows which are initially warm core. The disturbance can grow into a wave-like formation along the front and the low will be positioned at the crest. Around the low, flow will become cyclonic, by definition. This rotational flow will push polar air equator-ward west of the low via its trailing cold front, and warmer air will push poleward low via the warm front.
Cyclogenesis | Modes of development
While an initial warm core system, such as an organized thunderstorm complex, is necessary for the formation of a tropical cyclone, a large flux of energy is needed to lower atmospheric pressure more than a few millibars (0.10 inch of mercury). Inflow of warmth and moisture from the underlying ocean surface is critical for tropical cyclone strengthening. A significant amount of the inflow in the cyclone is in the lowest of the atmosphere.
Inflow (meteorology) | Tropical cyclones
Since they form from extratropical cyclones, which have colder temperatures aloft than normally found in the tropics, the sea surface temperatures required is around 23 degrees Celsius (73 °F) for their formation, which is three degrees Celsius (5 °F) lower than for tropical cyclones. This means that subtropical cyclones are more likely to form outside the traditional bounds of the hurricane season. Although subtropical storms rarely have hurricane-force winds, they may become tropical in nature as their cores warm.
Cyclone | Subtropical
Usually the cold front will move at a quicker pace than the warm front and “catch up” with it due to the slow erosion of higher density airmass located out ahead of the cyclone and the higher density airmass sweeping in behind the cyclone, usually resulting in a narrowing warm sector. At this point an occluded front forms where the warm air mass is pushed upwards into a trough of warm air aloft, which is also known as a trowal (a trough of warm air aloft). All developing low pressure areas share one important aspect, that of upward vertical motion within the troposphere.
Cyclogenesis | Modes of development
A second type of subtropical cyclone is a mesoscale low originating in or near a frontolyzing zone of horizontal wind shear, also known as a dying frontal zone, with radius of maximum sustained winds generally less than . The entire circulation may initially have a diameter of less than . These generally short-lived systems may be either cold core or warm core, and in 1972 this type of subtropical cyclone was referred to as a "neutercane".
Subtropical cyclone | Mesoscale low
However, deep convection associated with the system remained disorganized, despite surface and air data indicating an organizing surface circulation. A low pressure area formed on September 22 near Curaçao in the south-central Caribbean Sea; at that point, the NHC assessed a high chance of tropical cyclone development. The thunderstorm activity remained disorganized, but on September 23 the convection increased and became better organized, and curved bands developed throughout the day. Late that day, a Hurricane Hunters flight into the system confirmed the development of a well-defined circulation, and the NHC initiated advisories on Tropical Depression Fifteen about 485 mi (755 km) east of Puerto Cabezas, Nicaragua.
Tropical Storm Matthew (2010) | Meteorological history
Extratropical cyclones slant back into colder air masses and strengthen with height, sometimes exceeding 30,000 feet (approximately 9 km) in depth. Above the surface of the earth, the air temperature near the center of the cyclone is increasingly colder than the surrounding environment. These characteristics are the direct opposite of those found in their counterparts, tropical cyclones; thus, they are sometimes called "cold-core lows". Various charts can be examined to check the characteristics of a cold-core system with height, such as the chart, which is at about altitude. Cyclone phase diagrams are used to tell whether a cyclone is tropical, subtropical, or extratropical.
Extratropical cyclone | Vertical structure
A minimum distance of 500 km (300 mi) from the equator is normally needed for tropical cyclogenesis. Whether it be a depression in the Intertropical Convergence Zone (ITCZ) or monsoon trough, a broad surface front, or an outflow boundary, a low level feature with sufficient vorticity and convergence is required to begin tropical cyclogenesis. About 85 to 90 percent of Pacific typhoons form within the monsoon trough. Even with perfect upper-level conditions and the required atmospheric instability, the lack of a surface focus will prevent the development of organized convection and a surface low. Vertical wind shear of less than 10 m/s (20 kn, 33 ft/s) between the ocean surface and the tropopause is required for tropical cyclone development.
Typhoon | Genesis
Extratropical cyclones are areas of low pressure which exist at the boundary of different air masses. Almost all storms found at mid-latitudes are extratropical in nature, including classic North American nor'easters and European windstorms. The most severe of these can have a clear "eye" at the site of lowest barometric pressure, though it is usually surrounded by lower, non-convective clouds and is found near the back end of the storm.
Eye (cyclone) | Extratropical cyclones
In regions of high updraft, there are more hydrometeors formed. Graupel is found mostly in regions of weak updrafts. Particle size tends to decrease with increasing altitude because at lower altitudes the larger particles collide and aggregate with the smaller particles. Because updrafts are important for cloud microphysics, it is also necessary to consider how convection parameterization schemes may influence microphysics. Small errors in the parameterization of the particle size distribution can have large impacts on the calculation of the terminal velocity. The composition, size, and number concentration of particles varies dramatically in stratiform and convective regions. The particle fall speed derived from observations of tropical cyclones varies significantly from those derived from midlatitude systems.
Hurricane dynamics and cloud microphysics | Hydrometeor formation and shapes
The direction of wind flow around an atmospheric high-pressure area and a low-pressure area, as seen from above, depends on the hemisphere. High-pressure systems rotate clockwise in the northern Hemisphere; low-pressure systems rotate clockwise in the southern hemisphere. The scientific terms in English used to describe the weather systems generated by highs and lows were introduced in the mid-1800s, mostly by the British. The scientific theories which explain the general phenomena originated about two centuries earlier. The term cyclone was coined by Henry Piddington of the British East India Company to describe the devastating storm of December 1789 in Coringa, India.
High-pressure area | Wind circulation in the northern and southern hemispheres
Nor'easters develop in response to the sharp contrast in the warm Gulf Stream ocean current coming up from the tropical Atlantic and the cold air masses coming down from Canada. When the very cold and dry air rushes southward and meets up with the warm Gulf stream current, which is often near even in mid-winter, intense low pressure develops. In the upper atmosphere, the strong winds of the jet stream remove and replace rising air from the Atlantic more rapidly than the Atlantic air is replaced at lower levels; this and the Coriolis force help develop a strong storm. The storm tracks northeast along the East Coast, normally from North Carolina to Long Island, then moves toward the area east of Cape Cod.
Nor'easter | Formation
On December 28, the ITCZ spurred an increase in convection in the northeastern portion of the basin. An elongated low pressure area developed by 31 December, accompanied by intermittent thunderstorms. The system slowly consolidated, and the circulation center was exposed but well-defined. Despite moderate wind shear, the convection organized into rainbands on the system's southern periphery. On 2 January, the MFR classified the system as Tropical Disturbance 03, and later that day upgraded it to a tropical depression. However, persistent shear caused the convection to diminish on 3 January, which exposed the circulation. As a result, the MFR discontinued advisories on the system that day.
2010–11 South-West Indian Ocean cyclone season | Tropical Depression 03
Tropical cyclones are known to form even when normal conditions are not met. For example, cooler air temperatures at a higher altitude (e.g., at the 500 hPa level, or 5.9 km) can lead to tropical cyclogenesis at lower water temperatures, as a certain lapse rate is required to force the atmosphere to be unstable enough for convection. In a moist atmosphere, this lapse rate is 6.5 °C/km, while in an atmosphere with less than 100% relative humidity, the required lapse rate is 9.8 °C/km. At the 500 hPa level, the air temperature averages −7 °C (18 °F) within the tropics, but air in the tropics is normally dry at this height, giving the air room to wet-bulb, or cool as it moistens, to a more favorable temperature that can then support convection.
Sea surface temperature | Tropical cyclones
There are a number of structural characteristics common to all cyclones. A cyclone is a low-pressure area. A cyclone's center (often known in a mature tropical cyclone as the eye), is the area of lowest atmospheric pressure in the region. Near the center, the pressure gradient force (from the pressure in the center of the cyclone compared to the pressure outside the cyclone) and the force from the Coriolis effect must be in an approximate balance, or the cyclone would collapse on itself as a result of the difference in pressure. Because of the Coriolis effect, the wind flow around a large cyclone is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Cyclone | Structure
In the Horse latitudes, generally in the region between 30 and 35 degrees of north and south latitude, there are more or less permanent highs. This is the subsiding part of Hadley cells. Indeed, near the equator, where the Coriolis force is quite weak, a direct circulation of air is established. In the lower levels of the atmosphere, the difference in temperature between the equator and the northerly warmer regions gives rise to the intertropical convergence zone where the warmer air rises because of convergence and relatively lower density. Subsequently, this air spreads north and south and cools at higher altitude and descends further north and south.
South Atlantic High | Formation
Low pressure also creates surface winds deriving from high pressure zones. Various symbols are used not just for frontal zones and other surface boundaries on weather maps, but also to depict the present weather at various locations on the weather map. In addition, areas of precipitation help determine the frontal type and location.
Weather front | Surface weather analysis
In response to relaxing shear aloft, convection redeveloped around the newly formed center of circulation. It subsequently acquired a sufficiently organized structure and post-analysis found that a tropical depression formed at around 1200 UTC on October 8. Upon developing, the depression was designated as "Sixteen-E" about 120 mi (195 km) south-southwest of San Salvador, El Salvador, and operationally, advisories were initiated at 2100 UTC — 9 hours after post-analytic estimates of formation. Despite being in an area with favorable conditions, lack of inner core organization proved difficult for rapid deepening to occur, and initially, model guidance did not forecast any significant strengthening.
Tropical Storm Odile (2008) | Meteorological history
This suggests a preference for formation of east coast cyclones between extreme events of the Southern Oscillation Index.
Australian east coast low | Pattern
The northern end of the squall line is commonly referred to as the cyclonic end, with the southern side rotating anticyclonically. Because of the coriolis force, the northern end may evolve further, creating a "comma shaped" mesolow, or may continue in a squall-like pattern. A wake low is another kind of mesoscale low-pressure area to the rear of a squall line near the back edge of the stratiform rain area. Due to the subsiding warm air associated with the system's formation, clearing skies are associated with the wake low. Severe weather, in the form of high winds, can be generated by the wake low when the pressure difference between the mesohigh preceding it and the wake low is intense enough.
Squall | Mesolow
Along its southerly periphery, the clockwise circulation often impels easterly waves, and tropical cyclones that develop from them, across the ocean towards landmasses in the western portion of ocean basins during the hurricane season. The highest barometric pressure ever recorded on Earth was measured in Tosontsengel,_Zavkhan, Mongolia on 19 December 2001.
High-pressure area | In climatology
A wake low, or wake depression, is a mesoscale low-pressure area which trails the mesoscale high following a squall line. Due to the subsiding warm air associated with the system's formation, clearing skies are associated with the wake low. Once difficult to detect in surface weather observations due to their broad spacing, the formation of mesoscale weather station networks, or mesonets, has increased their detection. Severe weather, in the form of high winds, can be generated by the wake low when the pressure difference between the mesohigh preceding it and the wake low is intense enough. When the squall line is in the process of decay, heat bursts can be generated near the wake low.
Wake low | Introduction
A non-tropical low pressure area developed about 240 miles (390 km) south-southwest of Bermuda on April 18 through the interaction of an upper-level trough and a surface frontal trough. The surface trough, which extended from the gale center to Hispaniola, brought a plume of moisture northward from the Caribbean Sea into the circulation, which caused heavy rainfall in Puerto Rico. The non-tropical low tracked generally northward, with a ridge to its east and west, and on April 19 the system began producing sporadic convection near its center; early that day, satellite imagery indicated the presence of a tight inner core of winds.
Tropical Storm Ana (2003) | Meteorological history