![Iraqi Dust Devil](https://upload.wikimedia.org/wikipedia/commons/8/87/Iraqi_Dust_Devil.jpg)
Atmospheric instability is a condition where the
Earth's atmosphere
The atmosphere of Earth is the layer of gases, known collectively as air, retained by Earth's gravity that surrounds the planet and forms its planetary atmosphere. The atmosphere of Earth protects life on Earth by creating pressure allowing fo ...
is generally considered to be
unstable
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 mar ...
and as a result the
weather
Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover, cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmos ...
is subjected to a high degree of variability through distance and time. 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
Meteorology is a branch of the atmospheric sciences (which include atmospheric chemistry and physics) with a major focus on weather forecasting. The study of meteorology dates back millennia, though significant progress in meteorology did no ...
, instability can be described by various indices such as the
Bulk Richardson Number,
lifted index,
K-index
The ''K''-index quantifies disturbances in the horizontal component of earth's magnetic field with an integer in the range 0–9 with 1 being calm and 5 or more indicating a geomagnetic storm. It is derived from the maximum fluctuations ...
,
convective available potential energy (CAPE), the Showalter, and the Vertical totals. These indices, as well as atmospheric instability itself, involve
temperature
Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measurement, measured with a thermometer.
Thermometers are calibrated in various Conversion of units of temperature, temp ...
changes through the
troposphere
The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur. From ...
with height, or
lapse rate. Effects of atmospheric instability in moist atmospheres include
thunderstorm
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are some ...
development, which over warm oceans can lead to
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. Tr ...
, and
turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
. In dry atmospheres, inferior
mirages,
dust devil
A dust devil is a strong, well-formed, and relatively short-lived whirlwind. Its size ranges from small (half a metre wide and a few metres tall) to large (more than 10 m wide and more than 1 km tall). The primary vertical motion is u ...
s, steam devils, and
fire whirls can form. Stable atmospheres can be associated with
drizzle
Drizzle is a light precipitation consisting of liquid water drops smaller than those of rain – generally smaller than in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates from dri ...
,
fog, increased
air pollution
Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different typ ...
, a lack of turbulence, and
undular bore
In meteorology, an undular bore is a wave disturbance in the Earth's atmosphere and can be seen through unique cloud formations. They normally occur within an area of the atmosphere which is stable in the low levels after an outflow boundary or ...
formation.
Forms
![Anvil shaped cumulus panorama edit crop](https://upload.wikimedia.org/wikipedia/commons/9/98/Anvil_shaped_cumulus_panorama_edit_crop.jpg)
There are two primary forms of atmospheric instability:
*
Convective instability
In meteorology, convective instability or stability of an air mass refers to its ability to resist vertical motion. A ''stable'' atmosphere makes vertical movement difficult, and small vertical disturbances dampen out and disappear. In an ''uns ...
*
Dynamic instability (fluid mechanics)
In fluid dynamics, hydrodynamic stability is the field which analyses the stability and the onset of instability of fluid flows. The study of hydrodynamic stability aims to find out if a given flow is stable or unstable, and if so, how these inst ...
Under convective instability thermal mixing through
convection
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...
in the form of warm air rising leads to the development of
clouds and possibly
precipitation
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls under gravitational pull from clouds. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. ...
or
convective storms
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are someti ...
. 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
In fluid mechanics, the pressure-gradient force is the force that results when there is a difference in pressure across a surface. In general, a pressure is a force per unit area, across a surface. A difference in pressure across a surface t ...
. Dynamic lifting and mixing produces cloud, precipitation and storms often on a
synoptic scale
The synoptic scale in meteorology (also known as large scale or cyclonic scale) is a horizontal length scale of the order of 1000 kilometers (about 620 miles) or more. This corresponds to a horizontal scale typical of mid-latitude depressions (e. ...
.
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. 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
kelvin
The kelvin, symbol K, is the primary unit of temperature in the International System of Units (SI), used alongside its prefixed forms and the degree Celsius. It is named after the Belfast-born and University of Glasgow-based engineer and phy ...
s, 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 HPA may refer to:
Organizations
* Harry Potter Alliance, a charity
* Halifax Port Authority, Canada
* Hamburg Port Authority, Germany
* Hawaii Preparatory Academy, a school in Hawaii, US
* Health Protection Agency, UK
* Heerespersonalamt, the Ger ...
(
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
Severe weather is any dangerous meteorological phenomenon with the potential to cause damage, serious social disruption, or loss of human life. Types of severe weather phenomena vary, depending on the latitude, altitude, topography, and atmos ...
is anticipated with values below −6.
K Index
The K index is derived arithmetically: K-index = (850 hPa temperature – 500 hPa temperature) + 850 hPa
dew point
The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will cond ...
– 700 hPa dew point depression
* The temperature difference between 850 hPa ( above sea level) and 500 hPa ( 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 ( above sea level) and 700 hPa dew point.
CAPE and CIN
![CAPE vs SHEAR](https://upload.wikimedia.org/wikipedia/commons/f/f4/CAPE_vs_SHEAR.svg)
Convective available potential energy (CAPE), sometimes, simply, available potential energy (APE), is the amount of
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
a parcel of air would have if lifted a certain distance vertically through the atmosphere. CAPE is effectively the positive
buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...
of an air parcel and is an indicator of atmospheric instability, which makes it valuable in predicting severe weather. CIN,
convective inhibition
Convective inhibition (CIN or CINH) is a numerical measure in meteorology that indicates the amount of energy that will prevent an air parcel from rising from the surface to the level of free convection.
CIN is the amount of energy required to ov ...
, 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 m
2/s
2, 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
Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...
is free or forced. High values indicate unstable and/or weakly sheared
environment
Environment most often refers to:
__NOTOC__
* Natural environment, all living and non-living things occurring naturally
* Biophysical environment, the physical and biological factors along with their chemical interactions that affect an organism or ...
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
A supercell is a thunderstorm characterized by the presence of a mesocyclone: a deep, persistently rotating updraft. Due to this, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms ( ...
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. 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.
Effects
Stable atmosphere
Stable conditions, such as during a clear and calm night, will cause pollutants to become trapped near ground level. Drizzle occurs within a moist air mass when it is stable. Air within a stable layer is not turbulent. Conditions associated with a
marine layer
A marine layer is an air mass that develops over the surface of a large body of water, such as an ocean or large lake, in the presence of a temperature inversion. The inversion itself is usually initiated by the cooling effect of the water on th ...
, a stable atmosphere common on the west side of continents near cold water currents, leads to overnight and morning fog.
Undular bore
In meteorology, an undular bore is a wave disturbance in the Earth's atmosphere and can be seen through unique cloud formations. They normally occur within an area of the atmosphere which is stable in the low levels after an outflow boundary or ...
s can form when a low level boundary such as a
cold front
A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern ...
or
outflow boundary
An outflow boundary, also known as a gust front, is a storm-scale or mesoscale boundary separating thunderstorm-cooled air ( outflow) from the surrounding air; similar in effect to a cold front, with passage marked by a wind shift and usually ...
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
In physics, a transverse wave is a wave whose oscillations are perpendicular to the direction of the wave's advance. This is in contrast to a longitudinal wave which travels in the direction of its oscillations. Water waves are an example of t ...
, 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
Medium may refer to:
Science and technology
Aviation
*Medium bomber, a class of war plane
*Tecma Medium, a French hang glider design
Communication
* Media (communication), tools used to store and deliver information or data
* Medium of ...
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.
Unstable atmosphere
![Hot road mirage](https://upload.wikimedia.org/wikipedia/commons/7/7b/Hot_road_mirage.jpg)
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
Wind shear (or windshear), sometimes referred to as wind gradient, is a difference in wind speed and/or direction over a relatively short distance in the atmosphere. Atmospheric wind shear is normally described as either vertical or horizont ...
and significant low level spin (or vorticity), such thunderstorm activity can grow in coverage and develop into a
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 and squalls. Depen ...
.
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.
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
A wildfire, forest fire, bushfire, wildland fire or rural fire is an unplanned, uncontrolled and unpredictable fire in an area of combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identi ...
s are called fire whirls, which can spread a fire beyond its previous bounds.
A steam devil is a
rotating
Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...
updraft
In meteorology, an updraft is a small-scale current of rising air, often within a cloud.
Overview
Localized regions of warm or cool air will exhibit vertical movement. A mass of warm air will typically be less dense than the surrounding regi ...
that involves
steam or
smoke
Smoke is a suspension of airborne particulates and gases emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-produc ...
. They can form from smoke issuing from a
power plant
A power station, also referred to as a power plant and sometimes generating station or generating plant, is an industrial facility for the electricity generation, generation of electric power. Power stations are generally connected to an el ...
smokestack
A chimney is an architectural ventilation structure made of masonry, clay or metal that isolates hot toxic exhaust gases or smoke produced by a boiler, stove, furnace, incinerator, or fireplace from human living areas. Chimneys are typ ...
.
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.
See also
*
Atmospheric thermodynamics Atmospheric thermodynamics is the study of heat-to-work transformations (and their reverse) that take place in the earth's atmosphere and manifest as weather or climate. Atmospheric thermodynamics use the laws of classical thermodynamics, to descr ...
*
Buoyancy
Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...
*
Stable and unstable stratification
References
{{DEFAULTSORT:Atmospheric Instability
Atmospheric dynamics
Atmospheric thermodynamics