Hoar Frost Proper
(Section 3.2.1.4.4)Definition: Hoar frost proper: A deposit of ice that generally assumes the form of scales, needles, features or fans and which forms on objects the surface of which is sufficiently cooled, generally by nocturnal radiation, to bring about the deposition of the water vapour contained in the ambient air.
Hoar frost proper is ordinarily deposited on objects at or near the ground, mainly on their horizontal surfaces. Hoar frost is observed especially during the cold part of the year when the air is calm and the sky is clear.