Hard frost

Frost is the coating or deposit of ice that may form in humid air in cold conditions, usually overnight. In temperate climates it most commonly appears as fragile white crystals or frozen dew drops near the ground, but in cold climates it occurs in a greater variety of forms. Frost is composed of delicate branched patterns of ice crystals formed as the result of fractal process development.

The formation of frost is an indication that the air temperature has fallen below the freezing point of water, and plants that have evolved in warmer climates are known to suffer damage when the temperature falls low enough to freeze the water in the cells that make up the plant tissue. The tissue damage resulting from this process is known as "frost damage" and farmers in those regions where frost damage is known to affect their crops often invest in substantial means to protect their crops from such damage.

Frost forms when the temperature of a solid surface in the open cools to below the freezing point of water and for the most clearly crystalline forms of frost in particular, below the frost point in still air. In most temperate countries such temperatures usually are the result of heat loss by radiation at night, so those types of frost sometimes are called radiation frost.

Types of frost include crystalline hoar frost from deposition of water vapor from air of low humidity, white frost in humid conditions, window frost on glass surfaces, advection frost from cold wind over cold surfaces, black frost without visible ice at low temperatures and very low humidity, and rime under supercooled wet conditions.

The size of frost crystals varies depending on the time they have been building up and the amount of water vapor available. Frost crystals may be clear or translucent, but, like snow, a mass of frost crystals will scatter light in all directions, so that a coating of frost appears white.

If a solid surface is chilled below the dew point of the surrounding humid air and the surface itself is colder than freezing, ice will form on it. If the water deposits as a liquid that then freezes, it forms a coating that may look glassy, opaque, or crystalline, depending on its type. Depending on context, that process also may be called atmospheric icing. The ice it produces differs in some ways from crystalline frost, which consists of spicules of ice that typically project from the solid surface on which they grow.

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Wikipedia