hoar frost

hoar frostnoun/ˈhɔː fɹɒst/

• 1

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

  • He [the Lord] geueth ſnowe like woll, & ſcatereth yͤ horefroſt like aſhes.
  • [W]hen vve have, as vve very frequently have, put mixtures of ſnovv and ſalt into vials, and left them in the open Air, vve generally obſerv'd, that the outſide of the Glaſs vvas cas'd vvith ice, or covered vvith hoar froſt, directly over againſt that part of the inſide of the Glaſs, vvherein the frigorifick mixture vvas.
• 2

  climatologymeteorologynatural-sciencesWater 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.

  • On Sunday night, January 23, ſeveral things vvere laid out at the Obſervatory, ſuch as ſheets of brovvn paper, pieces of boards, plates of metal, glaſſes of ſeveral kinds, &c. vvhich all began to contract hoar-froſt ſeemingly as ſoon as each body had time to cool dovvn to the temperature of the air. The ſheets of brovvn paper being ſo thin acquired it ſooneſt, and vvhen beheld in candle-light they became beautifully ſpangled over by innumerable reflections from the ſmall cryſtals of hoar-froſt vvhich had parted from the air.
  • This striking fact, of the lower temperature of the surface on which dew and hoarfrost are forming, it was always found difficult to explain; for the transition of a body from the state of vapour to the fluid or solid form is always accompanied with an evolution of heat, […] The surface is not warmer in consequence of the formation of the dew or hoarfrost, but the dew or hoarfrost is deposited because the surface is previously cold; and its low temperature, compared with that of the air above, is owing to the discharge of heat by radiation.