Normal lapse rate: The atmosphere is heated mainly by terrestrial radiation. Hence, the lower layer of the air in contact with the earth's surface is the warmest and temperature gradually decreases with the increasing height. On the average there is a fall of 1°C per 165 metres at ascent. This is known as the normal lapse rate of temperature.

Inversion of temperature: Inversion of temperature means the upsetting of the general rule of vertical distribution of temperature, that is temperature decreases with the increasing height. In this phenomenon, the temperature increases with the increasing altitude. Hence, the colder layer of the air is located near the surface of the earth and warmer layer of the air high above.

Inversion of temperature occurs in a calm, cold and clear winter night because the earth's surface cools more rapidly in such night and the layer immediately overlying the earth's surface also gets cooled to a greater degree than the higher layer. Hence, the temperature instead of decreasing, increases with the increasing height. This type of inversion of temperature is termed as surface inversion of temperature.

On clear and calm nights, it is generally experienced in valleys when the radiation has caused cooling and the cold air has shrunk down. At the same time, on the mountain slopes the air is warmer.

Favourable conditions for inversion of temperature: Following conditions favour the development of surface inversion of temperature:

1. Long nights.

2. Clear skies.

3. Snow covered surface.

4. Slight air movement.

5. Flattish surfaces with a few topographic depressions.