Hence, some of the water vapor in the surrounding air condenses on the outside of the cold glass. When referring to cold protection, dew point is one of the better ways to describe the humidity or amount of water vapor in the air. The dew point is important on freeze nights because water vapor in the air can slow the rate of temperature fall. With a relatively high dew point on a cool night, radiant heat losses from a grove are reduced, and the temperature may be expected to fall slowly. But if the dew point is quite low, the temperature may be expected to fall rapidly.
Water vapor absorbs infrared radiation.
Water droplets or fog are an even more effective radiation absorber than water vapor. Hence, fog can reduce the rate of temperature drop on a frost night. The reason for this is that when dew condenses or ice forms, heat is given off. This heat release during condensation slows the rate at which the air temperature drops. If dew forms, water vapor is condensed from the air, and the humidity or dew point of that air is lowered.