hoar

Hoar is a term with two related senses in English. As a noun, it appears in hoarfrost (also written hoar frost), the form of frost that develops when water vapor deposits as ice crystals on surfaces in freezing conditions. As an adjective, hoary describes something grayish-white with age or something venerable or ancient.

Hoarfrost forms when moist air is cooled enough that water vapor sublimates directly into ice, depositing on surfaces that are at or below freezing. It often yields feathery, white crystals on vegetation, fences, windowpanes, and other objects, especially after clear nights with little wind. The crystalline patterns depend on humidity, temperature, and the roughness of the surface, and hoarfrost is typically more conspicuous on fine, plant-like structures than on smooth surfaces.

The adjective hoary derives from Old English and is related to the modern word hoary, meaning aged or whitened by age. In contemporary usage, hoarfrost is the preferred term for the frost phenomenon, whereas hoary is used to describe something old, gray, or venerable. The form hoar alone as a noun is rarer and mostly encountered in historical or literary contexts.