A deposit of interlocking ice crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plant stems and leaf edges, wires, and poles. Also, frost may form on the skin of an aircraft when a cold aircraft flies into warm and moist air or when it passes through air that is supersaturated with water vapor. Hoarfrost is formed similarly to the way dew is formed except that the temperature of the frosted object must be below freezing. Frost forms when air with a dew point below freezing is brought to saturation by cooling. In addition to its formation on freely exposed objects (air hoar), hoarfrost also forms inside unheated buildings and vehicles, in caves, in crevasses (crevasse hoar), on snow surfaces (surface hoar), and in air spaces within snow, especially below a snow crust (depth hoar). Hoarfrost is more fluffy and feathery than rime, which in turn is lighter than glaze. Hoarfrost is designated light or heavy (frost) depending upon the amount and uniformity of deposition