Frost, like snow, is the result of deposition of water vapor in saturated air. If solid surfaces in contact with the air are chilled below the deposition point (see frost point), then spicules of ice grow out from the solid surface. The size of the crystals is a matter of time and the amount of water vapor available.
Frost is often observed around cracks in wooden sidewalks due to the moist air escaping from the ground below. Other objects which frost develops on are those with low specific heat and high thermal emissivity, such as blackened metals; hence the accumulation of frost on the heads of rusty nails. The apparently erratic occurrence of frost in adjacent localities is due partly to differences of altitude, the lower areas becoming colder. It is also affected by differences in absorbtivity and specific heat of the ground which in the absence of wind greatly influences the temperature attained by the superincumbent air. It should be understood that vegetation is not damaged by frost itself, but by cold air; the appearance of frost merely indicates that the temperature has dropped below the freezing point. The formation of white frost on the indoor surface of window panes indicates low relative humidity of the indoor air, otherwise water would first condense in small drops and then freeze into clear ice.