The temperature of the environment is warm when the air gas molecules get warm. The accumulated heat from the sun, mainly by radiation, on the day times are emitted back from the earth at nights.

The earth’s surface radiate energy in the form of Infrared radiation at night. This will attribute to the heating of air molecules in the environment. However, the heat from the radiation is usually gone into space; But if the clouds are present in the sky, they will trap the infrared radiations and prevent the heat from leaving the environment. As clouds are a form of water vapors, they have a tendency to absorb more heat compared to air molecules. This heat will be emitted back to the earth surface from the clouds, which causes the temperature to rise further. While in the case of clear sky, there is nothing to prevent radiation from escaping from the environment.