# Atmospheric instability > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Atmospheric_instability > Markdown URL: https://mediated.wiki/source/Atmospheric_instability.md > Source: https://en.wikipedia.org/wiki/Atmospheric_instability > Source revision: 1347512925 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) {{short description|Condition where the Earth's atmosphere is generally considered to be unstable}} [[File:Iraqi Dust Devil.jpg|thumb|right|A dust devil in [Ramadi](/source/Ramadi), [Iraq](/source/Iraq).]] '''Atmospheric instability''' is a condition where the [Earth's atmosphere](/source/Earth's_atmosphere) is considered to be [unstable](/source/unstable) and as a result local [weather](/source/weather) is highly variable through distance and time.{{clarify|date=August 2020}}[http://www.ace.mmu.ac.uk/eae/Weather/Older/Stability_of_Air.html Stability of Air] {{webarchive |url=https://web.archive.org/web/20080209030952/http://www.ace.mmu.ac.uk/eae/Weather/Older/Stability_of_Air.html |date=February 9, 2008 }} Atmospheric instability encourages vertical motion, which is directly correlated to different types of weather systems and their severity. For example, under unstable conditions, a lifted [parcel of air](/source/air_parcel) will find cooler and denser surrounding air, making the parcel prone to further ascent, in a positive feedback loop. In [meteorology](/source/meteorology), instability can be described by various indices such as the [Bulk Richardson Number](/source/Bulk_Richardson_Number), [lifted index](/source/lifted_index), [K-index](/source/K-index_(meteorology)), [convective available potential energy (CAPE)](/source/convective_available_potential_energy), the Showalter, and the Vertical totals. These indices, as well as atmospheric instability itself, involve [temperature](/source/temperature) changes through the [troposphere](/source/troposphere) with height, or [lapse rate](/source/lapse_rate). Effects of atmospheric instability in moist atmospheres include [thunderstorm](/source/thunderstorm) development, which over warm oceans can lead to [tropical cyclogenesis](/source/tropical_cyclogenesis), and [turbulence](/source/turbulence). In dry atmospheres, inferior [mirage](/source/mirage)s, [dust devil](/source/dust_devil)s, steam devils, and [fire whirl](/source/fire_whirl)s can form. Stable atmospheres can be associated with [drizzle](/source/drizzle), [fog](/source/fog), increased [air pollution](/source/air_pollution), a lack of turbulence, and [undular bore](/source/undular_bore) formation. ==Forms== [[File:Anvil shaped cumulus panorama edit crop.jpg|250px|thumb|right|Anvil shaped [thundercloud](/source/cumulonimbus) in the mature stage over [Swifts Creek, Victoria](/source/Swifts_Creek%2C_Victoria), Australia]] There are two primary forms of atmospheric instability.[http://www.piercecollege.edu/offices/weather/stability.html Explanation of Atmospheric Stability/Instability - by Steve W. Woodruff] {{webarchive |url=https://web.archive.org/web/20080612212855/http://www.piercecollege.edu/offices/weather/stability.html |date=June 12, 2008 }} Under ''[convective instability](/source/convective_instability)'', thermal mixing through [convection](/source/Atmospheric_convection) in the form of rising warm air leads to the development of [clouds](/source/clouds) and possibly [precipitation](/source/precipitation_(meteorology)) or [convective storms](/source/convective_storms). ''[Dynamic instability](/source/Dynamic_instability_(fluid_mechanics))'' is produced through the horizontal movement of air and the physical forces it is subjected to such as the [Coriolis force](/source/Coriolis_force) and [pressure gradient force](/source/pressure_gradient_force); resulting dynamic lifting and mixing produces cloud, precipitation and storms often on a [synoptic scale](/source/synoptic_scale_meteorology). == Cause of instability == {{See also|Lapse rate}} Whether or not the atmosphere has stability depends partially on the moisture content. In a very dry troposphere, an environmental lapse rate (the rate at which the temperature of the environment surrounding an air mass decreases with height) of less than {{convert|9.8|C-change}} per kilometer ascent indicates stability, while greater changes indicate instability. This lapse rate is known as the dry adiabatic lapse rate.{{cite book|url=https://books.google.com/books?id=-mwbAsxpRr0C&pg=PA448|page=449|author=John E. Oliver|title=Encyclopedia of world climatology|publisher=Springer|date=2005|isbn=978-1-4020-3264-6}} When the environmental lapse rate is less than the adiabatic lapse rate, an air parcel that undergoes adiabatic change of pressure will eventually be colder and thus denser than the surrounding air, causing it to descend to its initial pressure.{{cite book |last1=Hartmann |first1=Dennis L. |title=Global Physical Climatology |date=2016 |publisher=Elsevier |location=Amsterdam, Netherlands |isbn=978-0-12-328531-7 |pages=15-17 |edition=Second}} When the environmental lapse rate is greater than the dry adiabatic lapse rate, air parcels are warmer and less dense than the surrounding air, causing them to continue rising while being accelerated by buoyancy. In a completely moist troposphere, a temperature decrease with height less than {{convert|6|C-change}} per kilometer ascent indicates stability, while greater changes indicate instability. In the range between {{convert|6|C-change}} and {{convert|9.8|C-change}} temperature decrease per kilometer ascent, the term conditionally unstable is used, where the atmosphere is stable for dry air parcels, and unstable for moist saturated air parcels. ==Indices used for its determination== {{See also|Bulk Richardson Number|Convective available potential energy|K-index (meteorology)|Lifted index}} ===Lifted Index=== The lifted index (LI), usually expressed in [kelvin](/source/kelvin)s, is the temperature difference between the temperature of the environment Te(p) and an air parcel lifted [adiabatically](/source/Adiabatic_process) Tp(p) at a given pressure height in the troposphere, usually 500 [hPa](/source/Pascal_(unit)) ([mb](/source/Bar_(unit))). 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](/source/severe_weather) is anticipated with values below −6.{{cite book|pages=416–418|title=Understanding weather and climate|author1=Edward Aguado |author2=James E. Burt |name-list-style=amp |publisher=Pearson Prentice Hall|year=2007|isbn=978-0-13-149696-5}} ===K Index=== {| class="wikitable" style="float: right;" |- ! 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 thunderstormsNational Weather Service Forecast Office, Detroit, Michigan (2010-01-25). [http://submit.crh.noaa.gov/dtx/glossary/k.php Gloassary: K.] {{Webarchive|url=https://web.archive.org/web/20121130033201/http://submit.crh.noaa.gov/dtx/glossary/k.php |date=2012-11-30 }} National Weather Service Central Region Headquarters. Retrieved on 2011-02-24 |} The K index is derived arithmetically: K-index = (850 hPa temperature – 500 hPa temperature) + 850 hPa [dew point](/source/dew_point) – 700 hPa dew point depression * The temperature difference between 850 hPa ({{convert|5000|ft|m}} above sea level) and 500 hPa ({{convert|18000|ft|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 ({{convert|10000|ft|m}} above sea level) and 700 hPa dew point. ===CAPE and CIN=== right|thumb|250 px|Conditions favorable for thunderstorm types and complexes Convective available potential energy (CAPE),{{cite journal |author1=M. W. Moncrieff |author2=M.J. Miller | year = 1976 | title = The dynamics and simulation of tropical cumulonimbus and squall lines | journal = Q. J. R. Meteorol. Soc. | volume = 120 | pages = 373–94 | url = http://www3.interscience.wiley.com/cgi-bin/abstract/113524305/ | archive-url = https://archive.today/20121216160636/http://www3.interscience.wiley.com/cgi-bin/abstract/113524305/ | url-status = dead | archive-date = 2012-12-16 | doi = 10.1002/qj.49710243208 | format = abstract|bibcode = 1976QJRMS.102..373M | issue = 432 | url-access = subscription }} sometimes, simply, available potential energy (APE), is the amount of [energy](/source/energy) a parcel of air would have if lifted a certain distance vertically through the atmosphere. CAPE is effectively the positive [buoyancy](/source/buoyancy) of an air parcel and is an indicator of atmospheric instability, which makes it valuable in predicting severe weather. CIN, [convective inhibition](/source/convective_inhibition), is effectively negative buoyancy, expressed '''B-'''; the opposite of [convective available potential energy (CAPE)](/source/convective_available_potential_energy), 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](/source/Atmospheric_convection) is free or forced. High values indicate unstable and/or weakly sheared [environment](/source/natural_environment)s; 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](/source/supercell) development. ===Showalter index=== The Showalter index, developed by meteorologist {{Interlanguage link|Albert K. Showalter|fr|Albert K. Showalter}}, 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.{{cite book|url=https://books.google.com/books?id=Z0_Tc_pKXKEC&pg=PA347|title=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"|author=Rattan K. Datta|publisher=Concept Publishing Company|date=1996|isbn=978-81-7022-532-4|page=347}} 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](/source/Diurnal_cycle) radiative changes or moisture below 850 hPa.{{cite web |url=http://forecast.weather.gov/glossary.php?word=SHOWALTER%20INDEX |title=NOAA's National Weather Service - Glossary |publisher=NOAA}} == Effects == [[File:Ondes de pression au-dessus de la Mer d'Arabie.jpg|right|thumb|Image of an [undular bore](/source/undular_bore) wave]] {{See also|Dust devil|Mirage|Thunderstorm|Undular bore}} ===Stable atmosphere=== Stable conditions, such as during a clear and calm night, will cause pollutants to become trapped near ground level.{{cite book|url=https://books.google.com/books?id=KJOoQm3fbEoC&pg=SA28-PA8|pages=28/8–28/10|title=Plant Engineer's Reference Book|author=Dennis A. Snow|publisher=Butterworth-Heinemann|date=2003-01-01|isbn=978-0-7506-4452-5}} Drizzle occurs within a moist air mass when it is stable. Air within a stable layer is not turbulent.{{cite book|url=https://books.google.com/books?id=KY-MBUeQoZEC&pg=SA8-PA29|pages=8–29|title=Jar professional pilot studies|author=Phil Croucher|date=2004-03-01|publisher=Lulu.com|isbn=978-0-9681928-2-5}} Conditions associated with a [marine layer](/source/marine_layer), a stable atmosphere common on the west side of continents near cold water currents, leads to overnight and morning fog.{{cite web|url=http://www.wrh.noaa.gov/lox/climate/climate_intro.php|title=Climate of Los Angeles|author=National Weather Service Office, Oxnard, California|publisher=National Weather Service Western Region Headquarters|year=2012|access-date=2012-02-16}} [Undular bore](/source/Undular_bore)s can form when a low level boundary such as a [cold front](/source/cold_front) or [outflow boundary](/source/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](/source/gravity_wave). Although the undular bore waves appear as bands of clouds across the sky, they are [transverse waves](/source/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](/source/Transmission_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.{{cite web|url=http://www.arpal.org/Pubbl/paper/eumetsat-mauritania.pdf |title=Outflow from convective storm, Mauritania and adjacent Atlantic Ocean (13 August 2006) |author1=Martin Setvak |author2=Jochen Kerkmann |author3=Alexander Jacob |author4=HansPeter Roesli |author5=Stefano Gallino |author6=Daniel Lindsey |name-list-style=amp |access-date=2009-07-03 |date=2007-03-19 |publisher=Agenzia Regionale per la Protezione dell'Ambiente Ligure |url-status=dead |archive-url=https://web.archive.org/web/20110725013807/http://www.arpal.org/Pubbl/paper/eumetsat-mauritania.pdf |archive-date=25 July 2011 }} ===Unstable atmosphere=== thumb|right|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](/source/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](/source/wind_shear) and significant low level spin (or vorticity), such thunderstorm activity can grow in coverage and develop into a [tropical cyclone](/source/tropical_cyclone).{{cite web|author=Chris Landsea |author-link=Chris Landsea |publisher=[Atlantic Oceanographic and Meteorological Laboratory](/source/Atlantic_Oceanographic_and_Meteorological_Laboratory) |work=Frequently Asked Questions: Hurricanes, Typhoons and Tropical Cyclones |title=How do tropical cyclones form? |access-date=2006-07-25 |url=http://www.aoml.noaa.gov/hrd/tcfaq/A15.html |url-status=dead |archive-url=https://web.archive.org/web/20090827030639/http://www.aoml.noaa.gov/hrd/tcfaq/A15.html |archive-date=2009-08-27 }} Over hot surfaces during warm days, unstable dry air can lead to significant refraction of the light within the air layer, which causes inferior [mirage](/source/mirage)s.{{cite journal|date=March 2009|page=167|journal=Physics Education|volume=44|issue=2|author=Michael Vollmer|title=Mirrors in the air: mirages in nature and in the laboratory|doi=10.1088/0031-9120/44/2/008|bibcode = 2009PhyEd..44..165V |s2cid=121672201 }} When winds are light, dust devils can develop on dry days within a region of instability at ground level. 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 [wildfire](/source/wildfire)s are called fire whirls, which can spread a fire beyond its previous bounds.{{cite book|url=https://books.google.com/books?id=yT6bzpUyFIwC&pg=PA77|page=77|title=Introduction to wildland fire|journal=Agricultural and Forest Meteorology|volume=86|issue=1–2|author1=Stephen J. Pyne |author2=Patricia L. Andrews |author3=Richard D. Laven |name-list-style=amp |publisher=John Wiley and Sons|date=1996-04-26|isbn=978-0-471-54913-0|bibcode=1997AgFM...86..140U|doi=10.1016/S0168-1923(97)00032-4}} A steam devil is a [rotating](/source/rotating) [updraft](/source/updraft) that involves [steam](/source/steam) or [smoke](/source/smoke). They can form from smoke issuing from a [power plant](/source/power_plant) [smokestack](/source/smokestack). [Hot springs](/source/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.{{cite book|url=https://books.google.com/books?id=CFNT3Y63Jb4C&pg=PA521|pages=520–523|title=National Audubon Society field guide to North American weather|author=David McWilliams Ludlum|publisher=Random House Digital, Inc.|date=1991-10-15|isbn=978-0-679-40851-2}} == See also == * [Atmospheric thermodynamics](/source/Atmospheric_thermodynamics) * [Buoyancy](/source/Buoyancy) * [Stable and unstable stratification](/source/Stable_and_unstable_stratification) == References == {{Reflist|2}} {{Meteorological variables}} {{DEFAULTSORT:Atmospheric Instability}} Category:Atmospheric dynamics Category:Atmospheric thermodynamics --- Adapted from the Wikipedia article [Atmospheric instability](https://en.wikipedia.org/wiki/Atmospheric_instability) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Atmospheric_instability?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.