Water vapor in clouds absorbs infrared radiation and radiates the heat back to the earth's surface. Air also cools as heat radiates from it into space. For example, air cools more quickly on clear nights than on cloudy nights. Clouds trap heat and keep it from radiating into space. On clear nights, the infrared that the earth radiates can escape into space.
Clouds play an important part in the water cycle, and therefore the climate of a region.
The earth's water is continuously changing forms through evaporation and condensation. These changes help to move water between the surface and the atmosphere. This continuous movement of water is called the water cycle. The sun provides the energy needed to evaporate water. Global winds then carry the water vapor through the troposphere.