White frost is a type of solid ice that forms directly from water vapor found in the air. White frost forms when the humidity is above 90% and the temperature is below 18 degrees Fahrenheit. White frost grows against the wind, since the facing winds have higher humidity than leeward air. The wind must be light or the delicate icy structures become broken. The structures have big, interlocking, needle-like crystals. This phenomenon occurs very rarely. White frost is considered an intermediate stage between actual hoar frost and rime, and is sometimes confused for either or both of these elements.