In meteorology, an inversion is a deviation from the normal change of an
atmospheric property with altitude. It almost always refers to a "temperature
inversion", i.e. an increase in temperature with height, or to the layer
within which such an increase occurs. An inversion can lead to pollution such
as smog being trapped close to the ground, with possible adverse effects on
health. An inversion can also suppress convection by acting as a "cap". If this
cap is broken for any of several reasons, convection of any moisture
present can then erupt into violent thunderstorms. Temperature inversion can
notoriously result in freezing rain in cold climates.
Normal atmospheric conditions Usually, within the lower atmosphere the
air near the surface of the Earth is warmer than the air above it, largely
because the atmosphere is heated from below as solar radiation warms the
Earth's surface, which in turn then warms the layer of the atmosphere
directly above it, e.g., by thermals. Causes
Given enough pressure, the normal vertical temperature gradient is
inverted such that the air is colder near the surface of the Earth. This can
occur when, for example, a warmer, less-dense air mass moves over a cooler,
denser air mass. This type of inversion occurs in the vicinity of warm fronts,
and also in areas of oceanic upwelling such as along the California coast in
the United States. With sufficient humidity in the cooler layer, fog is
typically present below the inversion cap. An inversion is also produced
whenever radiation from the surface of the earth exceeds the amount of
radiation received from the sun, which commonly occurs at night, or during the
winter when the angle of the sun is very low in the sky. This effect is virtually
confined to land regions as the ocean retains heat far longer. In the polar
regions during winter, inversions are nearly always present over land.
A warmer air mass moving over a cooler one can "shut off" any convection which
may be present in the cooler air mass. This is known as a capping inversion.
However, if this cap is broken, either by extreme convection overcoming the
cap, or by the lifting effect of a front or a mountain range, the sudden release
of bottled-up convective energy – like the bursting of a balloon – can result
in severe thunderstorms. Such capping inversions typically precede the
development of tornadoes in the Midwestern United States. In this
instance, the "cooler" layer is actually quite warm, but is still denser and
usually cooler than the lower part of the inversion layer capping it.
Subsidence inversion An inversion can develop aloft as a
result of air gradually sinking over a wide area and being warmed by adiabatic
compression, usually associated with subtropical high-pressure areas. A
stable marine layer may then develop over the ocean as a result. As this
layer moves over progressively warmer waters, however, turbulence within the
marine layer can gradually lift the inversion layer to higher altitudes, and
eventually even pierce it, producing thunderstorms, and under the right
circumstances, tropical cyclones. The accumulated smog and dust under the
inversion quickly taints the sky reddish, easily seen on sunny days.
Consequences Temperature inversion stops atmospheric
convection from happening in the affected area and can lead to the air
becoming stiller and murky from the collection of dust and pollutants that
are no longer able to be lifted from the surface. This can become a problem in
cities where many pollutants exist. Inversion effects occur frequently in
big cities such as: but also in smaller cities like:
These cities are closely surrounded by hills and mountains, or on plains which
are surrounded by mountain chains, which makes an inversion trap the air in the
city. During a severe inversion, trapped air pollutants form a brownish haze that
can cause respiratory problems. The Great Smog of 1952 in London, England,
is one of the most serious examples of such an inversion. It was blamed for an
estimated 11,000 to 12,000 deaths. Sometimes the inversion layer is at a
high enough altitude that cumulus clouds can condense but can only spread out
under the inversion layer. This decreases the amount of sunlight
reaching the ground and prevents new thermals from forming. As the clouds
disperse, sunny weather replaces cloudiness in a cycle that can occur
more than once a day. As the temperature of air increases, the
index of refraction of air decreases, a side effect of hotter air being less
dense. Normally this results in distant objects being shortened vertically, an
effect that is easy to see at sunset where the sun is visible as an oval. In
an inversion, the normal pattern is reversed, and distant objects are
instead stretched out or appear to be above the horizon, leading to the
phenomenon known as a Fata Morgana or mirage.
= Electromagnetic radiation = Very high frequency radio waves can be
refracted by inversions, making it possible to hear FM radio or watch VHF
low-band television broadcasts from long distances on foggy nights. The signal,
which would normally be refracted up and away from the ground-based antenna, is
instead refracted down towards the earth by the temperature-inversion boundary
layer. This phenomenon is called tropospheric ducting. Along coast lines
during Autumn and Spring, due to multiple stations being simultaneously
present because of reduced propagation losses, many FM radio stations are
plagued by severe signal degradation causing them to sound scrambled.
Inversions can magnify the so-called "green flash": a phenomenon occurring at
sunrise or sunset, usually visible for a few seconds, in which the sun's green
light is isolated due to dispersion – the shorter wavelength is refracted
most, so it is the first or last light from the upper rim of the solar disc to
be seen. = Sound =
When an inversion layer is present, if a sound or explosion occurs at ground
level, the sound wave is reflected from the warmer upper layer and returns
towards the ground. The sound, therefore, travels much farther than
normal. This is noticeable in areas around airports, where the sound of
aircraft taking off and landing often can be heard at greater distances around
dawn than at other times of day, and inversion thunder which is significantly
louder and travels further than when it is produced by lightning strikes under
normal conditions. = Shock waves =
The shock wave from an explosion can be reflected by an inversion layer in much
the same way as it bounces off the ground in an air-burst and can cause
additional damage as a result. This phenomenon killed three people in the
Soviet RDS-37 nuclear test when a building collapsed.
See also Index of meteorology articles
References