Normally temperature decreases with elevation (the normal lapse rate). In cities suffering from smog, this may be reversed. A temperature inversion is a condition when the air

A temperature inversion occurs when the air near the ground is cooler than the warmer air lying above it.

The idea that ground-level air is simply lighter does not define the inverted condition.

Air near the ground being hotter than the air above describes the normal lapse rate, not an inversion.

The warm layer aloft acts like a lid, trapping pollutants and smog near the surface.

This reversal of the usual cooling-with-height pattern worsens urban air quality.

Winter mornings in cities like Delhi often see such inversions intensifying smog.