The change of phase of the water is an important energy source for storms and atmospheric circulation patterns. The change of phase of water also affects the buoyancy of air, which is important in cloud and precipitation formation. Water vapor is the source of all clouds and precipitation. The potential for cloud formation and dissipation depends on the amount of water vapor in the atmosphere. The amount of water in the atmosphere is an important factor that determines the rate of evaporation.
Evaporation is important to many forms of plant and animal life.
Human comfort is a function of temperature, humidity, and wind speed. Water vapor is a principle absorber of shortwave and longwave radiant energy. It is the most important greenhouse gas. Reports of current weather conditions often include the dew point temperature and the relative humidity . These are just two of several ways to express the amount of water vapor in the atmosphere.