In the troposphere, the temperature typically decreases with increasing altitude. However, sometimes a temperature inversion can exist. A temperature inversion is an increase in atmospheric temperature with altitude. It is called an inversion because the normal relationship between temperature and altitude is opposite, or inverted. In a temperature inversion, warmer air sits on top of a layer of colder air.

How does this happen? An example of a temperature inversion in the troposphere occurs on cold, clear, calm winter nights. Under these conditions, when the Earth's surface stops radiating heat at night, the lower layer of the troposphere is no longer getting warmed. This causes the lower layer of air to become cooler than the layers above it, so that temperature increases with altitude.

Temperature inversions can create low clouds and fog. It can also worsen air pollution over a city. Air rises only if it is warmer than the surrounding air. Therefore, in a temperature inversion, the warm air above the cooler layer acts as a lid, trapping pollution.