A thin layer of ice that has condensed out the air is called frost. Frost is a deposit of ice crystals that forms on surfaces when the temperature of the air is below freezing point and the humidity is high. Frost can form on any surface, but it is most commonly seen on metal, glass, and plastic surfaces. Frost is formed when water vapor in the air condenses into liquid water droplets. These droplets then freeze and form ice crystals. Frost can also form when snow or hail melts and then refreezes.