Temperature Inversion

When the air is cooler at the ground and warmer the higher you go, then a temperature inversion is in place over an area of the Earth's surface. The air is warmest at ground level most of the time, since sunlight heats the earth which in turn heats the air just above it. There is also a lot of insulation for the air at this level, since the rest of the atmosphere is between it and space, and the heat of the sun is not easily radiated upward from ground level through that ocean of air. The air gets progressively cooler as it is higher above the ground and less atmospheric layers insulate it from the cold of space.

When a temperature inversion occurs, however, the air is actually cooler at the ground than in the air above it. The most noticeable effect of an inversion like this is that pollutants such as wood smoke, vehicle exhaust, and so forth become trapped at ground level, creating dirty air and sometimes dangerous concentrations of pollutants. Some areas are subject to temperature inversions many times a year thanks to their geography, while others seldom see an inversion.

How a surface temperature inversion works

Surface temperature inversions require that the air at the ground become cooler than the air immediately above it -- a situation that can only result from special circumstances. The usual way an inversion forms is by cooling of the surface air overnight during the winter season, when nights are long and solar heat is lessened by the angle of the sun's rays through the atmosphere. This process happens more easily in valleys, as can be seen in the case of Los Angeles.

The temperature difference needs to be strong in order for an inversion to occur -- a degree or two is not enough. However, if a layer of colder air does indeed form at ground level under a layer of warmer air, there will be an inversion which can last anywhere from several hours to several days, depending on whether the cold air layer is quickly warmed by the sun. The layer of warmer air prevents ground-level air from rising -- since cold air naturally sinks and warm air naturally rises. Thus, since the cold air is denser than the warm air and can't flow upwards, it remains trapped -- and smoke, smog, and exhaust can all build up into a visible cloud beneath the warm layer.

Seen from a distance, the boundary of the inversion can be seen sharply if there is enough smoke in the air, as the picture of Lochcarron, Scotland in this article shows.

Capping inversions

Another type of inversion, the capping inversion, happens much higher in the atmosphere than surface inversions do. This inversion occurs above a normal temperature gradient -- warmer at the ground, cooler aloft, and then warmer again, forming a kind of temperature 'sandwich.' Capping inversions don't have the same air-stagnating effects as surface inversions, but may lead to fog and prevent higher cloud formation. However, the capping inversion can also hold lower-level instability in until it reaches an intensity where it bursts through the inversion layer. Since the instability must become intense in order to break the capping inversion, the inversion can 'hold in' the instability until it is so strong that severe weather is sure to ensue when it 'breaks loose.'