Atmospheric instability

Atmospheric instability is a condition where the Earth's atmosphere is generally considered to be unstable and as a result the weather is subjected to a high degree of variability through distance and time.[1] Atmospheric stability is a measure of the atmosphere's tendency to discourage or deter vertical motion, and vertical motion is directly correlated to different types of weather systems and their severity. In unstable conditions, a lifted thing, such as a parcel of air will be warmer than the surrounding air at altitude. Because it is warmer, it is less dense and is prone to further ascent.

In meteorology, instability can be described by various indices such as the Bulk Richardson Number, lifted index, K-index, convective available potential energy (CAPE), the Showalter, and the Vertical totals. These indices, as well as atmospheric instability itself, involve temperature changes through the troposphere with height, or lapse rate. Effects of atmospheric instability in moist atmospheres include thunderstorm development, which over warm oceans can lead to tropical cyclogenesis, and turbulence. In dry atmospheres, inferior mirages, dust devils, steam devils, and fire whirls can form. Stable atmospheres can be associated with drizzle, fog, increased air pollution, a lack of turbulence, and undular bore formation.

Iraqi Dust Devil
A dust devil in Ramadi, Iraq.


Anvil shaped cumulus panorama edit crop
Anvil shaped thundercloud in the mature stage over Swifts Creek, Victoria

There are two primary forms of atmospheric instability:[2]

Under convective instability thermal mixing through convection in the form of warm air rising leads to the development of clouds and possibly precipitation or convective storms. Dynamic instability is produced through the horizontal movement of air and the physical forces it is subjected to such as the Coriolis force and pressure gradient force. Dynamic lifting and mixing produces cloud, precipitation and storms often on a synoptic scale.

Cause of instability

Whether or not the atmosphere has stability depends partially on the moisture content. In a very dry troposphere, a temperature decrease with height less than 9.8C per kilometer ascent indicates stability, while greater changes indicate instability. This lapse rate is known as the dry adiabatic lapse rate.[3] In a completely moist troposphere, a temperature decrease with height less than 6C per kilometer ascent indicates stability, while greater changes indicate instability. In the range between 6C and 9.8C temperature decrease per kilometer ascent, the term conditionally unstable is used.

Indices used for its determination

Lifted Index

The lifted index (LI), usually expressed in kelvins, is the temperature difference between the temperature of the environment Te(p) and an air parcel lifted adiabatically Tp(p) at a given pressure height in the troposphere, usually 500 hPa (mb). When the value is positive, the atmosphere (at the respective height) is stable and when the value is negative, the atmosphere is unstable. Thunderstorms are expected with values below −2, and severe weather is anticipated with values below −6.[4]

K Index

K-index value Thunderstorm Probability
Less than 20 None
20 to 25 Isolated thunderstorms
26 to 30 Widely scattered thunderstorms
31 to 35 Scattered thunderstorms
Above 35 Numerous thunderstorms[5]

The K index is derived arithmetically: K-index = (850 hPa temperature – 500 hPa temperature) + 850 hPa dew point – 700 hPa dew point depression

  • The temperature difference between 850 hPa (5,000 feet (1,500 m) above sea level) and 500 hPa (18,000 feet (5,500 m) above sea level) is used to parameterize the vertical temperature lapse rate.
  • The 850 hPa dew point provides information on the moisture content of the lower atmosphere.
  • The vertical extent of the moist layer is represented by the difference of the 700 hPa temperature (10,000 feet (3,000 m) above sea level) and 700 hPa dew point.[4]


Conditions favorable for thunderstorm types and complexes

Convective available potential energy (CAPE),[6] sometimes, simply, available potential energy (APE), is the amount of energy a parcel of air would have if lifted a certain distance vertically through the atmosphere. CAPE is effectively the positive buoyancy of an air parcel and is an indicator of atmospheric instability, which makes it valuable in predicting severe weather. CIN, convective inhibition, is effectively negative buoyancy, expressed B-; the opposite of convective available potential energy (CAPE), which is expressed as B+ or simply B. As with CAPE, CIN is usually expressed in J/kg but may also be expressed as m2/s2, as the values are equivalent. In fact, CIN is sometimes referred to as negative buoyant energy (NBE).

It is a form of fluid instability found in thermally stratified atmospheres in which a colder fluid overlies a warmer one. When an air mass is unstable, the element of the air mass that is displaced upwards is accelerated by the pressure differential between the displaced air and the ambient air at the (higher) altitude to which it was displaced. This usually creates vertically developed clouds from convection, due to the rising motion, which can eventually lead to thunderstorms. It could also be created in other phenomenon, such as a cold front. Even if the air is cooler on the surface, there is still warmer air in the mid-levels, that can rise into the upper-levels. However, if there is not enough water vapor present, there is no ability for condensation, thus storms, clouds, and rain will not form.

Bulk Richardson Number

The Bulk Richardson Number (BRN) is a dimensionless number relating vertical stability and vertical wind shear (generally, stability divided by shear). It represents the ratio of thermally-produced turbulence and turbulence generated by vertical shear. Practically, its value determines whether convection is free or forced. High values indicate unstable and/or weakly sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 10 to 45 suggest environmental conditions favorable for supercell development..

Showalter index

The Showalter index is a dimensionless number computed by taking the temperature at the 850 hPa level which is then taken dry adiabatically up to saturation, then up to the 500 hPa level, which is then subtracted by the observed 500 hPa level temperature. If the value is negative, then the lower portion of the atmosphere is unstable, with thunderstorms expected when the value is below −3.[7] The application of the Showalter index is especially helpful when there is a cool, shallow air mass below 850 hPa that conceals the potential convective lifting. However, the index will underestimate the potential convective lifting if there are cool layers that extend above 850 hPa and it does not consider diurnal radiative changes or moisture below 850 hPa.[8]


Undular bore waves over Arabian Sea
Image of an undular bore wave

Stable atmosphere

Stable conditions, such as during a clear and calm night, will cause pollutants to become trapped near ground level.[9] Drizzle occurs within a moist air mass when it is stable. Air within a stable layer is not turbulent.[10] Conditions associated with a marine layer, a stable atmosphere common on the west side of continents near cold water currents, leads to overnight and morning fog.[11] Undular bores can form when a low level boundary such as a cold front or outflow boundary approaches a layer of cold, stable air. The approaching boundary will create a disturbance in the atmosphere producing a wave-like motion, known as a gravity wave. Although the undular bore waves appear as bands of clouds across the sky, they are transverse waves, and are propelled by the transfer of energy from an oncoming storm and are shaped by gravity. The ripple like appearance of this wave is described as the disturbance in the water when a pebble is dropped into a pond or when a moving boat creates waves in the surrounding water. The object displaces the water or medium the wave is travelling through and the medium moves in an upward motion. However, because of gravity, the water or medium is pulled back down and the repetition of this cycle creates the transverse wave motion.[12]

Unstable atmosphere

Hot road mirage
Mirage over a hot road, with the appearance of "fake water" on its surface

Within an unstable layer in the troposphere, the lifting of air parcels will occur, and continue for as long as the nearby atmosphere remains unstable. Once overturning through the depth of the troposphere occurs (with convection being capped by the relatively warmer, more stable layer of the stratosphere), deep convective currents lead to thunderstorm development when enough moisture is present. Over warm ocean waters and within a region of the troposphere with light vertical wind shear and significant low level spin (or vorticity), such thunderstorm activity can grow in coverage and develop into a tropical cyclone.[13] Over hot surfaces during warm days, unstable dry air can lead to significant refraction of the light within the air layer, which causes inferior mirages.[14]

When winds are light, dust devils can develop on dry days within a region of instability at ground level.[15] Small-scale, tornado-like circulations can occur over or near any intense surface heat source, which would have significant instability in its vicinity. Those that occur near intense wildfires are called fire whirls, which can spread a fire beyond its previous bounds.[16] A steam devil is a rotating updraft that involves steam or smoke. They can form from smoke issuing from a power plant smokestack. Hot springs and warm lakes are also suitable locations for a steam devil to form, when cold arctic air passes over the relatively warm water.[15]

See also


  1. ^ Stability of Air Archived February 9, 2008, at the Wayback Machine
  2. ^ Explanation of Atmospheric Stability/Instability - by Steve W. Woodruff Archived June 12, 2008, at the Wayback Machine
  3. ^ John E. Oliver (2005). Encyclopedia of world climatology. Springer. p. 449. ISBN 978-1-4020-3264-6.
  4. ^ a b Edward Aguado & James E. Burt (2007). Understanding weather and climate. Pearson Prentice Hall. pp. 416–418. ISBN 978-0-13-149696-5.
  5. ^ National Weather Service Forecast Office, Detroit, Michigan (2010-01-25). Gloassary: K. National Weather Service Central Region Headquarters. Retrieved on 2011-02-24
  6. ^ M. W. Moncrieff; M.J. Miller (1976). "The dynamics and simulation of tropical cumulonimbus and squall lines". Q. J. R. Meteorol. Soc. 120 (432): 373–94. Bibcode:1976QJRMS.102..373M. doi:10.1002/qj.49710243208. Archived from the original (abstract) on 2012-12-16.
  7. ^ Rattan K. Datta (1996). Advances in tropical meteorology: meteorology and national development: proceedings of the National Symposium TROPMET-93 organised by the Indian Meteorological Society at New Delhi from March 17–19, 1993 with the theme "meteorology and national development". Concept Publishing Company. p. 347. ISBN 978-81-7022-532-4.
  8. ^ "NOAA's National Weather Service - Glossary". NOAA.
  9. ^ Dennis A. Snow (2003-01-01). Plant Engineer's Reference Book. Butterworth-Heinemann. pp. 28/8–28/10. ISBN 978-0-7506-4452-5.
  10. ^ Phil Croucher (2004-03-01). Jar professional pilot studies. pp. 8–29. ISBN 978-0-9681928-2-5.
  11. ^ National Weather Service Office, Oxnard, California (2012). "Climate of Los Angeles". National Weather Service Western Region Headquarters. Retrieved 2012-02-16.CS1 maint: Multiple names: authors list (link)
  12. ^ Martin Setvak; Jochen Kerkmann; Alexander Jacob; HansPeter Roesli; Stefano Gallino & Daniel Lindsey (2007-03-19). "Outflow from convective storm, Mauritania and adjacent Atlantic Ocean (13 August 2006)" (PDF). Agenzia Regionale per la Protezione dell'Ambiente Ligure. Archived from the original (PDF) on 25 July 2011. Retrieved 2009-07-03.
  13. ^ Chris Landsea. "How do tropical cyclones form?". Frequently Asked Questions: Hurricanes, Typhoons and Tropical Cyclones. Atlantic Oceanographic and Meteorological Laboratory. Archived from the original on 2009-08-27. Retrieved 2006-07-25.
  14. ^ Michael Vollmer (March 2009). "Mirrors in the air: mirages in nature and in the laboratory". Physics Education. 44 (2): 167. Bibcode:2009PhyEd..44..165V. doi:10.1088/0031-9120/44/2/008.
  15. ^ a b David McWilliams Ludlum (1991-10-15). National Audubon Society field guide to North American weather. Random House Digital, Inc. pp. 520–523. ISBN 978-0-679-40851-2.
  16. ^ Stephen J. Pyne; Patricia L. Andrews & Richard D. Laven (1996-04-26). Introduction to wildland fire. Agricultural and Forest Meteorology. 86. John Wiley and Sons. p. 77. Bibcode:1997AgFM...86..140U. doi:10.1016/S0168-1923(97)00032-4. ISBN 978-0-471-54913-0.
2009 Krasnozavodsk tornado

The 2009 Krasnozavodsk tornado was an F3 tornado that occurred on Junе 3, 2009, in Krasnozavodsk near Sergiev Posad in the Moscow region at 22.15 MST. It was the first powerful tornado in the Moscow area since 1984, which damaged around 40 buildings, trees and cars, but without fatalities. By damage registered in photo and video materials, this tornado is categorised at F2 at its rise, and at F3 at maximum stage.

Altocumulus castellanus cloud

In meteorology, Altocumulus Castellanus (ACCAS) is a cloud type named for its tower-like projections that billow upwards from the base of the cloud. The base of the cloud can form as low as 2,000 metres (6,500 feet), or as high as 6,000 metres (20,000 feet). They are very similar to cumulus congestus clouds, but at a higher level and with the cloud heaps joined at the base.

Castellanus clouds are evidence of mid-atmospheric instability and a high mid-altitude lapse rate. They may be a harbinger of heavy showers and thunderstorms and, if surface-based convection can connect to the mid-tropospheric unstable layer, continued development of Castellanus clouds can produce cumulonimbus clouds.

Altocumulus castellanus clouds are typically accompanied by moderate turbulence as well as potential icing conditions. For these reasons, flight through these clouds is often best avoided by aircraft.

Atmospheric convection

Atmospheric convection is the result of a parcel-environment instability, or temperature difference layer in the atmosphere. Different lapse rates within dry and moist air masses lead to instability. Mixing of air during the day which expands the height of the planetary boundary layer leads to increased winds, cumulus cloud development, and decreased surface dew points. Moist convection leads to thunderstorm development, which is often responsible for severe weather throughout the world. Special threats from thunderstorms include hail, downbursts, and tornadoes.

Cinco Ribeiras

Cinco Ribeiras is a civil parish in the municipality of Angra do Heroísmo on the island of Terceira in the Portuguese archipelago of the Azores. The population in 2011 was 704, in an area of 12.80 km².

Cirrocumulus castellanus

Cirrocumulus castellanus is a type of cirrocumulus cloud. The name cirrocumulus castellanus is derived from Latin, meaning "of a castle". These clouds appear as round turrets that are rising from either a lowered line or sheet of clouds. Cirrocumulus castellanus is an indicator of atmospheric instability at the level of the cloud. The clouds form when condensation occurs in the base cloud, causing latent heating to occur. This causes air to rise from the base cloud, and if the air ascends into conditionally unstable air, cirrocumulus castellanus will form.

Cirrocumulus floccus

Cirrocumulus floccus is a type of cirrocumulus cloud. The name cirrocumulus floccus is derived from Latin, meaning "a lock of wool". Cirrocumulus floccus appears as small tufts of cloud with rounded heads, but ragged bottoms. The cloud can produce virga, precipitation that evaporates before reaching the ground. Like cirrocumulus castellanus, cirrocumulus floccus is an indicator of atmospheric instability at the level of the cloud. In fact, cirrocumulus floccus can form from cirrocumulus castellanus, being the evolutionary state after the base of the original cloud has dissipated.

Classifications of snow

Classifications of snow describe and categorize the attributes of snow-generating weather events, including the individual crystals both in the air and on the ground, and the deposited snow pack as it changes over time. Snow can be classified by describing the weather event that is producing it, the shape of its ice crystals or flakes, how it collects on the ground, and thereafter how it changes form and composition. Depending on the status of the snow in the air or on the ground, a different classification applies.

Snowfall arises from a variety of events that vary in intensity and cause, subject to classification by weather bureaus. Some snowstorms are part of a larger weather pattern. Other snowfall occurs from lake effects or atmospheric instability near mountains. Falling snow takes many different forms, depending on atmospheric conditions, especially vapor content and temperature, as it falls to the ground. Once on the ground, snow crystals metamorphose into different shapes, influenced by wind, freeze-thaw and sublimation. Snow on the ground forms a variety of shapes, formed by wind and thermal processes, all subject to formal classifications both by scientists and by ski resorts. Those who work and play in snowy landscapes have informal classifications, as well.

There is a long history of northern and alpine cultures describing snow in their different languages, including Inupiat, Russian and Finnish. However, the lore about the multiplicity of Eskimo words for snow originates from controversial scholarship on a topic that's difficult to define, because of the structures of the languages involved.

Convective available potential energy

In meteorology, convective available potential energy (commonly abbreviated as CAPE), is the amount of energy a given mass of air (called an air parcel) would have if lifted a certain distance vertically through the atmosphere. CAPE is effectively the positive buoyancy of an air parcel and is an indicator of atmospheric instability, which makes it very valuable in predicting severe weather. It is a form of fluid instability found in thermally stratified atmospheres in which a colder fluid overlies a warmer one. An air mass will rise if it is less dense than the surrounding air (its buoyant force is greater than its weight). This can create vertically developed clouds due to the rising motion, which could lead to thunderstorms. It could also be created by other phenomena, such as a cold front. Even if the air is cooler on the surface, there is still warmer air in the mid-levels, that can rise into the upper-levels. However, if there is not enough water vapor present, there is no ability for condensation, thus storms, clouds, and rain will not form.

Dynamic instability

Dynamic instability may refer to any of several scientific phenomena:

Aircraft dynamic modes, including aircraft dynamic instability

Atmospheric instability, in meteorology

Dynamic instability of microtubules, in biology

Firehose instability, in astrophysics

Flutter, in aeroelasticity, a branch of mechanics

Hydrodynamic instability, in fluid dynamics

Others in Category:Fluid dynamic instability

Eric Eady

Eric Thomas Eady (5 September 1915 – 26 March 1966) was a British meteorology researcher and author of the Eady Model of baroclinic instability, modelling baroclinic generation of weather systems.

Eady was born in Ealing and attended Ealing, Hammersmith and West London College. He earned a scholarship to Christ's College, Cambridge, where he received a BSc in mathematics in 1935. In 1937 he became a weather forecaster in the UK Meteorological Office. In 1946, he resigned from the office to started a PhD in mathematics at Imperial College London. His 1948 thesis was titled The theory of development in dynamical meteorology, which was an early work on atmospheric instability and the development of weather systems. Eady widened his interests to include oceanography in his later career.

In his later years, he became depressed by his career and isolated himself from his social circle. In 1966, he died at Royal Surrey County Hospital, age 50, after an overdose of sleeping pills.

Funnel cloud

A funnel cloud is a funnel-shaped cloud of condensed water droplets, associated with a rotating column of wind and extending from the base of a cloud (usually a cumulonimbus or towering cumulus cloud) but not reaching the ground or a water surface. A funnel cloud is usually visible as a cone-shaped or needle like protuberance from the main cloud base. Funnel clouds form most frequently in association with supercell thunderstorms.

If a funnel cloud touches the ground it becomes a tornado. Most tornadoes begin as funnel clouds, but many funnel clouds do not make ground contact and so do not become tornadoes. Also, a tornado does not necessarily need to have an associated condensation funnel. If strong cyclonic winds are occurring at the surface (and connected to a cloud base, regardless of condensation), then the feature is a tornado. Some tornadoes may appear only as a debris swirl, with no obvious funnel cloud extending below the rotating cloud base.

In cloud nomenclature, any funnel- or inverted-funnel-shaped cloud descending from cumulus or cumulonimbus clouds is technically described as an accessory feature called tuba. The terms tuba and funnel cloud are nearly but not exactly synonymous; a wall cloud, for example, is also a form of tuba.


In numerous fields of study, the component of instability within a system is generally characterized by some of the outputs or internal states growing without bounds. Not all systems that are not stable are unstable; systems can also be marginally stable or exhibit limit cycle behavior.

In structural engineering, a structure can become unstable when excessive load is applied. Beyond a certain threshold, structural deflections magnify stresses, which in turn increases deflections. This can take the form of buckling or crippling. The general field of study is called structural stability.

Atmospheric instability is a major component of all weather systems on Earth.

Skew-T log-P diagram

A skew-T log-P diagram is one of four thermodynamic diagrams commonly used in weather analysis and forecasting. In 1947, N. Herlofson proposed a modification to the emagram that allows straight, horizontal isobars and provides for a large angle between isotherms and dry adiabats, similar to that in the tephigram. It was thus more suitable for some of the newer analysis techniques being invented by the United States Air Force.

Such a diagram has pressure plotted on the vertical axis, with a logarithmic scale (thus the "log-P" part of the name), and the temperature plotted skewed, with isothermal lines at 45° to the plot (thus the "skew-T" part of the name). Plotting a hypothetical set of measurements with constant temperature for all altitudes would result in a line angled 45° to the right. In practice, since temperature usually drops with altitude, the graphs are usually mostly vertical (see examples linked to below).

The major use for skew-T log-P diagrams is the plotting of radiosonde soundings, which give a vertical profile of the temperature and dew point temperature throughout the troposphere and lower stratosphere. The isopleths on the diagram can then be used to simplify many tedious calculations involved, which were previously performed by hand or not at all. Many skew-T log-P diagrams also include a vertical representation of the wind speed and direction using wind barbs. Important atmospheric characteristics such as saturation, atmospheric instability, and wind shear are critical in severe weather forecasting, by which skew-T log-P diagrams allow quick visual analysis. The diagrams are widely used by glider pilots to forecast the strength of thermals and the height of the base of the associated cumulus clouds.

Static stability

Static stability is the ability of a robot to remain upright when at rest, or under acceleration and deceleration

Static stability may also refer to:

In aircraft or missiles:

Static margin — a concept used to characterize the static stability and controllability of aircraft and missiles.

Longitudinal static stability — the stability of an aircraft in the longitudinal, or pitching, plane during static (established) conditions.In meteorology:

Atmospheric instability#Stable atmosphereBuoyancy

Static stability (also called hydrostatic stability or vertical stability) — the ability of a fluid at rest to become turbulent or laminar due to the effects of buoyancy.In sailing:

Static stability — the angle of roll, or heel, achieved under constant wind conditions.

Tornado outbreak of May 15–17, 2013

The tornado outbreak of May 15–17, 2013 was a small but intense and deadly tornado outbreak that produced several damaging tornadoes in northern Texas, south-central Oklahoma, northern Louisiana, and northern Alabama. In mid-May 2013, an upper-level shortwave trough tracked across the Southern Plains of the United States. An associated low-pressure area and atmospheric instability resulted in the formation of tornadoes across northern Texas and Oklahoma on May 15. Afterwards the storm system weakened as it tracked westward, though six additional tornadoes were reported in Texas, Louisiana, and Alabama in the two days following May 15. Over a period of nearly two days, the storm system produced 26 tornadoes in four states. The strongest of these was an EF4 tornado which struck Hood County, Texas on May 15. However, on May 16 and May 17 no tornadoes were confirmed to have been stronger than EF1 intensity. In addition to tornadoes, large hail was reported, peaking at 4 in (10 cm) in diameter near Mineral Wells, Texas on May 15.

The EF4 tornado in Hood County, Texas, accounted for all six deaths caused by the severe storms, making it the first deadly tornado event in Texas since the 2007 Piedras Negras-Eagle Pass tornadoes. An additional 63 people were injured, many of which were due to the same EF4 tornado. A second tornado, rated EF3, was similarly damaging and impacted areas southwest of Cleburne, Texas, injuring seven. Damage across the four states due to the storm system reached roughly $272 million in damage.

Tropical cyclogenesis

Tropical cyclogenesis is the development and strengthening of a tropical cyclone in the atmosphere. The mechanisms through which tropical cyclogenesis occurs are distinctly different from those through which temperate cyclogenesis occurs. Tropical cyclogenesis involves the development of a warm-core cyclone, due to significant convection in a favorable atmospheric environment.Tropical cyclogenesis requires six main factors: sufficiently warm sea surface temperatures (at least 26.5 °C (79.7 °F)), atmospheric instability, high humidity in the lower to middle levels of the troposphere, enough Coriolis force to develop a low-pressure center, a pre-existing low-level focus or disturbance, and low vertical wind shear.Tropical cyclones tend to develop during the summer, but have been noted in nearly every month in most basins. Climate cycles such as ENSO and the Madden–Julian oscillation modulate the timing and frequency of tropical cyclone development. There is a limit on tropical cyclone intensity which is strongly related to the water temperatures along its path.An average of 86 tropical cyclones of tropical storm intensity form annually worldwide. Of those, 47 reach hurricane/typhoon strength, and 20 become intense tropical cyclones (at least Category 3 intensity on the Saffir–Simpson Hurricane Scale).

Tropical cyclone

A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain. Depending on its location and strength, a tropical cyclone is referred to by different names, including hurricane (), typhoon (), tropical storm, cyclonic storm, tropical depression, and simply cyclone. A hurricane is a tropical cyclone that occurs in the Atlantic Ocean and northeastern Pacific Ocean, and a typhoon occurs in the northwestern Pacific Ocean; in the south Pacific or Indian Ocean, comparable storms are referred to simply as "tropical cyclones" or "severe cyclonic storms"."Tropical" refers to the geographical origin of these systems, which form almost exclusively over tropical seas. "Cyclone" refers to their winds moving in a circle, whirling round their central clear eye, with their winds blowing counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. The opposite direction of circulation is due to the Coriolis effect. Tropical cyclones typically form over large bodies of relatively warm water. They derive their energy through the evaporation of water from the ocean surface, which ultimately recondenses into clouds and rain when moist air rises and cools to saturation. This energy source differs from that of mid-latitude cyclonic storms, such as nor'easters and European windstorms, which are fueled primarily by horizontal temperature contrasts. Tropical cyclones are typically between 100 and 2,000 km (62 and 1,243 mi) 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. Also, the African easterly jet and areas of atmospheric instability which give rise to cyclones in the Atlantic Ocean and Caribbean Sea, along with the Asian monsoon and Western Pacific Warm Pool, are features of the Northern Hemisphere and Australia.

Coastal regions are particularly vulnerable to the impact of a tropical cyclone, compared to inland regions. The primary energy source for these storms is warm ocean waters, therefore these forms are typically strongest when over or near water, and weaken quite rapidly over land. Coastal damage may be caused by strong winds and rain, high waves (due to winds), storm surges (due to wind and severe pressure changes), and the potential of spawning tornadoes. Tropical cyclones also draw in air from a large area—which can be a vast area for the most severe cyclones—and concentrate the precipitation of the water content in that air (made up from atmospheric moisture and moisture evaporated from water) into a much smaller area. This continual replacement of moisture-bearing air by new moisture-bearing air after its moisture has fallen as rain, which may cause extremely heavy rain and river flooding up to 40 kilometres (25 mi) from the coastline, far beyond the amount of water that the local atmosphere holds at any one time.

Though their effects on human populations are often devastating, tropical cyclones can relieve drought conditions. They also carry heat energy away from the tropics and transport it toward temperate latitudes, which may play an important role in modulating regional and global climate.


A typhoon is a mature tropical cyclone that develops between 180° and 100°E in the Northern Hemisphere. This region is referred to as the Northwestern Pacific Basin, and is the most active tropical cyclone basin on Earth, accounting for almost one-third of the world's annual tropical cyclones. For organizational purposes, the northern Pacific Ocean is divided into three regions: the eastern (North America to 140°W), central (140°W to 180°), and western (180° to 100°E). The Regional Specialized Meteorological Center (RSMC) for tropical cyclone forecasts is in Japan, with other tropical cyclone warning centers for the northwest Pacific in Hawaii (the Joint Typhoon Warning Center), the Philippines and Hong Kong. While the RSMC names each system, the main name list itself is coordinated among 18 countries that have territories threatened by typhoons each year A hurricane is a storm that occurs in the Atlantic Ocean or the northeastern Pacific Ocean, a typhoon occurs in the northwestern Pacific Ocean, and a tropical cyclone occurs in the South Pacific or the Indian Ocean.Within the northwestern Pacific, there are no official typhoon seasons as tropical cyclones form throughout the year. Like any tropical cyclone, there are a few main requirements for typhoon formation and development: (1) sufficiently warm sea surface temperatures, (2) atmospheric instability, (3) high humidity in the lower to middle levels of the troposphere, (4) enough Coriolis effect to develop a low pressure center, (5) a pre-existing low level focus or disturbance, and (6) a low vertical wind shear. While the majority of storms form between June and November, a few storms do occur between December and May (although tropical cyclone formation is at a minimum during that time). On average, the northwestern Pacific features the most numerous and intense tropical cyclones globally. Like other basins, they are steered by the subtropical ridge towards the west or northwest, with some systems recurving near and east of Japan. The Philippines receive the brunt of the landfalls, with China and Japan being impacted slightly less. Some of the deadliest typhoons in history have struck China. Southern China has the longest record of typhoon impacts for the region, with a thousand-year sample via documents within their archives. Taiwan has received the wettest known typhoon on record for the northwest Pacific tropical cyclone basins.

Meteorological data and variables

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