hoar frost

noun2

1. •
   Originally, any frozen dew forming a white deposit on exposed surfaces.

   • He [the Lord] geueth snowe like woll, & scatereth yͤ horefrost like ashes. — 1535
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.