A reversal of the normal pattern in the troposphere, where temperature, instead of falling with height, increases with height, so that a layer of warmer air sits above colder air near the surface.
- Normally temperature falls with altitude at the environmental lapse rate (roughly 6.5° Celsius per kilometre); inversion reverses this for a layer.
- Common causes: clear, calm, dry nights when the ground cools rapidly by radiation (radiation inversion), cold air sinking and pooling in valleys (valley inversion), and air subsiding under a high-pressure system.
- The warm-over-cold layering is very stable: it suppresses vertical mixing, traps smoke, dust, fog, and pollutants near the surface, and worsens air quality (as in Delhi's winter smog).
- In hill and valley regions, inversion can produce a "frost hollow," where cold air collects at the valley floor and damages crops, while slopes stay milder.
- Inversions usually break up after sunrise as the surface warms.
The definition (temperature rising with height), the calm-clear-night condition, and the link to fog, smog, and trapped pollution are standard climatology and environment items.
Normal lapse (temperature falls with height) versus inversion (temperature rises with height); inversion is most likely on calm, clear, dry winter nights, not windy or cloudy ones; valley floors are colder than slopes during inversion.
A layer where temperature rises with height instead of falling, trapping fog and pollutants; common on calm, clear winter nights.