Atmospheric instability is a condition where theEarth's atmosphere is considered to beunstable and as a result localweather is highly variable through distance and time.[clarification needed][1] Atmospheric instability encourages vertical motion, which is directly correlated to different types of weather systems and their severity. For example, under unstable conditions, a liftedparcel of air will find cooler and denser surrounding air, making the parcel prone to further ascent, in a positive feedback loop.
Inmeteorology, instability can be described by various indices such as theBulk 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, involvetemperature changes through thetroposphere with height, orlapse rate.
Effects of atmospheric instability in moist atmospheres includethunderstorm development, which over warm oceans can lead totropical cyclogenesis, andturbulence. In dry atmospheres, inferiormirages,dust devils, steam devils, andfire whirls can form. Stable atmospheres can be associated withdrizzle,fog, increasedair pollution, a lack of turbulence, andundular bore formation.
There are two primary forms of atmospheric instability.[2]Underconvective instability, thermal mixing throughconvection in the form of rising warm air leads to the development ofclouds and possiblyprecipitation orconvective storms.Dynamic instability is produced through the horizontal movement of air and the physical forces it is subjected to such as theCoriolis force andpressure gradient force; resulting dynamic lifting and mixing produces cloud, precipitation and storms often on asynoptic scale.
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 9.8 °C (17.6 °F) 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 6 °C (11 °F) per kilometer ascent indicates stability, while greater changes indicate instability. In the range between 6 °C (11 °F) and 9.8 °C (17.6 °F) temperature decrease per kilometer ascent, the term conditionally unstable is used.
The lifted index (LI), usually expressed inkelvins, is the temperature difference between the temperature of the environment Te(p) and an air parcel liftedadiabatically Tp(p) at a given pressure height in the troposphere, usually 500hPa (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, andsevere weather is anticipated with values below −6.[4]
| 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 hPadew point – 700 hPa dew point depression
Convective available potential energy (CAPE),[6] sometimes, simply, available potential energy (APE), is the amount ofenergy a parcel of air would have if lifted a certain distance vertically through the atmosphere. CAPE is effectively the positivebuoyancy 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, expressedB-; the opposite ofconvective 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 asnegative 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.
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 whetherconvection is free or forced. High values indicate unstable and/or weakly shearedenvironments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 10 to 45 suggest environmental conditions favorable forsupercell development.
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 considerdiurnal radiative changes or moisture below 850 hPa.[8]
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 amarine 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 acold front oroutflow 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 agravity wave. Although the undular bore waves appear as bands of clouds across the sky, they aretransverse 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 ormedium 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]
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 thestratosphere), 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 verticalwind shear and significant low level spin (or vorticity), such thunderstorm activity can grow in coverage and develop into atropical 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 inferiormirages.[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 intensewildfires are called fire whirls, which can spread a fire beyond its previous bounds.[16] A steam devil is arotatingupdraft that involvessteam orsmoke. They can form from smoke issuing from apower plantsmokestack.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]
{{cite web}}: CS1 maint: multiple names: authors list (link){{cite book}}:|journal= ignored (help)