By definition, hoarfrost, or white frost, is 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.

The deposition of hoar frost is similar to the process in which dew is formed, except that the temperature of the befrosted object must be below freezing. It forms when air with a dew point below freezing is brought to saturation by cooling.