frost

Frost forms when objects at the earth's surface are colder than the air around them. Not only are the objects cold, they are cold enough to freeze the water vapor in the air that condenses, and then freezes, onto these surfaces. Sometimes this looks like a dusting of snow.

A hard frost refers to a frost that occurs when the surface temperature of the earth remains below freezing for several hours. In this case, the water vapor in the atmosphere freezes into larger crystals than the dusting, rime, or layer of frost that forms at slightly warmer temperatures.