Temperature inversion

Normally temperature falls with height (the normal lapse rate). Sometimes this is reversed and temperature rises with height: inversion of temperature. Inversions are usually short, but common.

When it happens:

• The ideal setting is a long winter night with clear skies and still air. The day's heat radiates away during the night, and by early morning the ground is cooler than the air above it.
• Over polar areas, inversion is normal all year round.
• It usually lasts a few hours, until the sun rises and warms the ground.

Effects: a surface inversion makes the lower air stable. Smoke and dust collect beneath the inversion layer and spread sideways, filling the lowest layer of air; dense morning fog is common, especially in winter.

Air drainage in hills and mountains: cold air formed at night is heavy and dense, so it flows downslope almost like water and piles up in pockets and valley bottoms, with warm air above. This also protects plants on the slopes from frost damage.

Example (new): fruit growers in hill valleys often plant orchards part-way up the slopes rather than on the valley floor, because on clear winter nights the coldest air collects at the bottom and the slopes stay a little warmer.