A capping inversion is an elevated
inversion layer that caps a convective
planetary boundary layer.
The boundary layer is the part of the atmosphere which is closest to the ground. Normally, the sun heats the ground, which in turn heats the air just above it.
Thermals form when this warm air rises into the cold air (warm air is less
dense than cold air), a process called
convection
Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...
. A convective layer such as this has the potential for cloud formation, since
condensation
Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...
occurs as the warm air rises and cools. An inversion occurs when the normal temperature (warm air below, cold air above) profile is reversed, creating a
stable configuration of dense, cold air sitting below lighter, warm air. An elevated inversion layer is thus a region of warm air above a region of cold air, but higher in the atmosphere (generally not touching the surface).
A capping inversion occurs when there is a boundary layer with a normal temperature profile (warm air rising into cooler air) and the layer above that is an inversion layer (cooler air below warm air). Inversions can develop under
high pressure areas. Downward motion within these systems can cause warming aloft, producing a capping inversion.
Cloud formation from the lower layer is "capped" by the inversion layer.
Air stagnation may result from a capping inversion from diffusing from a region, increasing the concentration of pollutants and exacerbating poor
air quality
Air pollution is the presence of substances in the Atmosphere of Earth, air that are harmful to humans, other living beings or the environment. Pollutants can be Gas, gases like Ground-level ozone, ozone or nitrogen oxides or small particles li ...
.
[ ] If the capping inversion layer or "cap" is too strong it will prevent thunderstorms from developing. A strong cap can result in
foggy conditions.
However, if the air at the surface is
unstable
In dynamical systems instability means that some of the outputs or internal state (controls), states increase with time, without bounds. Not all systems that are not Stability theory, stable are unstable; systems can also be marginal stability ...
enough, strong updrafts can be forced through the capping inversion. This selective process of only allowing the strongest updrafts to form
thunderstorm
A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustics, acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorm ...
s often results in outbreaks of
severe weather
Severe weather is any dangerous meteorological phenomenon with the potential to cause damage, serious social disruption, or loss of human life. These vary depending on the latitude, altitude, topography, and atmospheric conditions. High ...
. The role of capping inversions in bolstering the intensity of severe weather was realized in conceptual models that were developed by atmospheric science researchers in the late 1960s and had been recognized as a characteristic of tornado-producing airmasses as early as 1954. In some severe weather events, this capping inversion can emerge when a warm and dry
mixed layer originating over a high
plateau
In geology and physical geography, a plateau (; ; : plateaus or plateaux), also called a high plain or a tableland, is an area of a highland consisting of flat terrain that is raised sharply above the surrounding area on at least one side. ...
moves over a cooler and moister airmass, forming an "elevated mixed layer" (EML).
[ ] ENLs can be identified in
radiosonde soundings by their steep temperature
lapse rates and an increase in
relative humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog t ...
from the bottom to the top of the layer. They can be found worldwide downwind of high terrain, such as over
South Asia
South Asia is the southern Subregion#Asia, subregion of Asia that is defined in both geographical and Ethnicity, ethnic-Culture, cultural terms. South Asia, with a population of 2.04 billion, contains a quarter (25%) of the world's populatio ...
, eastern Australia, east of the
Rocky Mountains
The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in great-circle distance, straight-line distance from the northernmost part of Western Can ...
in the central U.S. and northern
Mexico
Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...
, and east of the foothills of the
Andes
The Andes ( ), Andes Mountains or Andean Mountain Range (; ) are the List of longest mountain chains on Earth, longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range ...
.
[ ] The
Spanish plume weather pattern over western and central Europe, commonly associated with strong thunderstorm events, is also associated with an EML.
See also
*
Convective available potential energy
*
Convective inhibition
References
External links
Capping Inversion- AMS Glossary of Meteorology
Capping Inversion- National Science Digital Library
{{Meteorological variables
Boundary layer meteorology
Atmospheric thermodynamics
fr:Couche d'inversion#Inhibition de la convection