Inversion of temperature is a meteorological condition in which a layer of warm air is situated above a layer of cooler air at the same height.
This situation affects stability in the atmosphere and restricts the vertical mixing of atmospheric constituents. Inversions occur when warm air is underlain by cool air at the same level, with warmer layers above.
An inversion acts as a cap that can trap warmer, less dense air beneath it when the topography forces underlying air to move upward, rather than away from an area. Under these conditions, pollution does not disperse well and one is more apt to see smog, haze, and photochemical smog.
Inversion is usually of short duration but quite common nonetheless. A long winter night with clear skies and still air is ideal situation for inversion.
Being heavy and dense, the cold air acts almost like water and moves down the slope to pile up deeply in pockets and valley bottoms with warm air above. This is called air drainage. It protects plants from frost damages.
