hoarfrost

Hoarfrost is a form of frost that occurs when water vapor in the air deposits directly as ice crystals onto surfaces, rather than first condensing into liquid water. This deposition happens when the air is saturated and the surface temperature is at or below freezing, typically during cold, clear nights.

Formation occurs under calm, clear conditions when heat radiates away from objects faster than the surrounding air can supply it. This leads to very cold surface temperatures and high humidity near or at 100 percent. The resulting crystals are often feathery, needle-like, or dendritic, radiating from the contacted surface and giving vegetation, fences, windows, and other exposed objects a white, delicate coating.

Hoarfrost is most common in temperate and polar regions and at high elevations where winter nights are long and still. It is distinct from rime frost, which forms when supercooled liquid droplets in fog freeze on contact with surfaces; hoarfrost forms directly from water vapor deposition, not from liquid water.

Impacts of hoarfrost are usually weather-related. Heavy hoarfrost can weigh down delicate plants or thin branches and create slippery surfaces for pedestrians and vehicles. The frost typically disappears with sunlight as surfaces warm.

Etymology-wise, the term hoarfrost derives from old usage describing a feathery, white frost. In everyday usage, it is sometimes referred to simply as white frost or frost flowers, though frost flowers more commonly describes other ice-crystal formations.