(Gaseous) water vapor that begins to change to tiny water droplets (a liquid state) or ice crystals when the air gets cold enough. This process begins at the dew point. Energy required to vaporize the water is released, about 585 cal/g of water at 20 degrees Celsius.
[Physical Review, v 38, 5303, 1988.] [Oceanography; Thurman, Harold; 513; 1994; Macmillan, New York.]
Source: Atmospheric Chemistry Glossary