Inversion (meteorology)

Temperature inversion in Bratislava

Casual view from old part of city, same Bridge

A temperature inversion is a meteorological phenomenon where air temperature increases with height.

Normal atmospheric conditions

Usually, within the lower atmosphere (the troposphere) the air near the surface of the Earth is warmer than the air above it, largely because the atmosphere is heated from below by solar radiation absorbed at the surface.

Hot air, however, rises. This is convection in which the warmer air rises up, to be replaced with cooler air which is then heated. It is this process that leads to cloud building, thermals, and other convection related atmospheric behaviour.

How inversions occur

Sometimes the gradient is inverted, so that the air gets colder nearer the surface of the Earth: this is a temperature inversion. It can be created by the movement of air masses of different temperature moving over each other. A warm air mass moving over a colder one can "shut off" the convection effects, keeping the cooler air mass trapped below. (see capping inversion). It commonly occurs at night: when solar heating ceases, the surface cools by radiation, and cools the immediately overlying atmosphere, (see Surface temperature inversion). Over most of Antarctica, there is a near-permanent inversion.

Consequences of an inversion

With the disruption of normal convection, a number of phenomena are associated with a temperature inversion. Because the higher air is warmer, air that is rising in thermals is prevented by the inversion layer from rising any higher. In this case the moisture in the thermals often cannot reach the height at which it would condense to form cumulus clouds and so the sky is cloudless. The reduction in thermic activity creates a general "stillness" of the air, and the air becomes murky because dust and pollutants are no longer lifted from the surface.

Sometimes the inversion layer is higher so that the cumulus clouds can condense but then they spread out under the inversion layer. This cuts out sunlight to the ground and prevents new thermals from forming. A period of cloudiness is followed by sunny weather as the clouds disperse. This cycle can occur more than once in a day.

The index of refraction of air decreases as the air temperature increases, 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 "squished" into an orb). In an inversion the normal pattern is reversed, and distant objects are instead stretched out or appear to be above the horizon. This leads to the interesting optical effects of Fata Morgana or mirage.

Similarly, radio (being part of the electromagnetic spectrum, like light) can be redirected by such inversions. This is why it is not uncommon to hear radio (and sometimes TV) broadcasts from seemingly impossible distances on foggy nights. The signal, still more than powerful enough to receive even at hundreds or thousands of miles or kilometres, would normally be refracted up and away from the ground-based antenna, but is now refracted back down instead. This phenomenon is called tropospheric ducting, or a tropo duct.