Temperature inversion
A layer of warm air sitting on top of cooler air near the ground. It acts like a lid, trapping smoke, fog and pollution low down.
Normally the air gets colder the higher you go, so warm air near the ground rises and carries smoke and dust up and away. In a temperature inversion the pattern flips: a layer of warmer air sits above cooler air near the surface. Because the cool air underneath is denser, it cannot rise through the warm layer, and the warm layer acts like a lid.
You have probably seen one. On a still, clear winter morning, smoke from chimneys spreads out flat instead of rising, and mist lies in the valleys. Inversions usually form in the early evening, when the ground cools quickly, and often last until a few hours after sunrise. Clear skies and winds below about 3 mph favour them.
Inversions matter because they trap whatever is below them: fog, car exhaust, wildfire smoke. Some of the worst smog episodes in history happened under strong inversions.
A common misconception is that an inversion means it is warm. In fact the air at ground level is often bitterly cold; it is the air a little higher up that is warmer than it should be.