A deposit of interlocking crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plants, wires, poles, etc. The deposition of hoar frost is similar to the process by which dew is formed, except that the temperature of the frosted object must be below freezing. It forms when air with a dew point below freezing is brought to saturation by cooling.
Frost that has developed over a long time from water vapor, usually forming large crystals. When this happens in snow, it forms depth hoar or pukak.
Deposits of ice having a crystalline appearance, generally assuming the forms of scales, needles, feathers or fans. Formed by sublimation of water vapor from surrounding clear air.
A deposit of ice crystals in the form of scales, needles, feathers or fans.
A particular kind of frost. Hoar frost forms when water vapor hits a freezing surface. It freezes immediately, leaving spikes called hoar frost. The temperature of the air is usually around freezing when there is hoar frost, but the ground must be a lot colder than freezing. The reason the air is warmer than the ground is because the air must be warm enough to carry water vapor.