Hoarfrost forms through deposition: water vapour transitions directly to ice crystals on surfaces whose temperature is below 0 °C. It differs from frost caused by freezing of already-deposited liquid water (frozen dew).
Formation conditions
Clear nights (maximum radiative loss), light winds (allowing cold air to settle near the ground), and sufficient moisture are required. Hoarfrost forms delicate white needle, feather, or fan-shaped crystals. In agriculture, hoarfrost on crops can cause damage by freezing intracellular water, especially dangerous during flowering.