Hard Frost

in meteorology, a solid white covering of minute ice crystals formed by the sublimation of water vapor on cold surfaces exposed to the wind. Hard frost is seen on the windward side of stone walls, columns, rocks and other massive objects having high heat capacity and rather good thermal conductivity. It forms in sub-zero (°C) air temperatures but only after the temperature has been rising and the objects mentioned above, which have been greatly cooled in the preceding period, have not yet taken on the temperature of the warmer surrounding air. The thickness of hard frost does not exceed several millimeters. Hard frost is to be distinguished from glaze ice.