hoar frost

Meanings

1. Originally, any frozen dew forming a white deposit on exposed surfaces.
2. Water vapour which has undergone deposition or desublimation (“transformation directly into ice crystals without first turning into liquid water”) when the air is cold and moist to form a white deposit on exposed surfaces.

Etymology

PIE word *prustós From Middle English hore frost, horfrost [and other forms], from hor (“grey; greyish-white”, adjective) (from Old English hār (“grey”), ultimately from Proto-Indo-European *(s)ḱeh₃- (“darkness; shadow”)) + frost (“cold spell, freezing weather, frost; hoar-frost; rime”) (from Old English frost, forst, ultimately from Proto-Indo-European *prustós (“frost”)). The English word is analysable as hoar (“greyish-white; white”, adjective) + frost.