What Causes Frost?
Frost usually forms when a surface cools through loss of infrared radiation to a temperature which is colder than the dew point of the air next to the surface, and the temperature of that surface is below freezing (32 deg F, or 0 deg. C). The source of this moisture is water vapor contained in the air.
Frost is moisture from the air that has collected as tiny crystals of ice on leaves and blades of grass. Frost forms in much the same way as dew.
During the daytime, the sun warms the earth. As soon as the sun sets, the earth and the air surrounding it start to cool. When air cools, it begins to give up some of its moisture.
When the air temperature is above freezing, the moisture in the air changes into drops of water, which collect as dew on solid objects. But when the night is very cold, the moisture forms ice crystals, or frost, instead of dew.