Frost
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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 elevation, the lower areas becoming colder on calm nights. 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. 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 droplets and then freeze into clear ice.
Because cold air is more dense that warm air, and forms close to the ground, we find in calm weather that the cold air will pool at ground level. This is known as Surface Temperature Inversion, and explains why frost is more common and extensive in low-lying areas such as valleys and hollows. Areas where frost forms due to cold air trapped against the ground or against a solid barrier such as a wall are known as frost pockets.
Although the reported temperature may be higher than the freezing point of water, frost may still form due to the surface temperature inversion -- where the temperature at ground level is colder, and reaches the frost point.
Vegetation will not necessarily be damaged on a night where the air cools leaves below their freezing point. In the absence of a site nucleating the formation of ice crystals, the leaves remain in a supercooled liquid state, safely reaching temperatures of -4 °C to -12 °C. However, once frost forms, the leaf cells may be damaged by sharp ice crystals. Certain bacteria are known to be particularly effective at triggering frost formation, raising the nucleation temperature to about -1 °C. In the absence of these ice-nucleating bacteria, frost damage can be greatly reduced.