White frost is a solid deposition of ice which forms directly from water vapour contained in air. It forms when there is a relative humidity above 90% and a temperature below -8 °C (18 °F). The developing heat is then transferred to air by convection. White frost grows against the wind direction, since arriving windward air has a higher humidity than leeward air, but the wind must not be very strong in order not to damage the delicately built icy structures. These structures resemble a heavy coating of hoar frost with big and interlocking crystals, usually needle-shaped. This phenomenon occurs quite rarely: it is considered a kind of intermediate stage between hoar frost and rime and it is often confounded with these.
