Frost is a deposit of ice crystals that forms on surfaces when the surface temperature falls to 0 °C or below and the dew point (or frost point) is also below freezing. The delicate white crystals — called hoar frost — grow by deposition (water vapour transitioning directly to ice without passing through the liquid phase). Frost should not be confused with frozen dew: dew forms first as liquid and then freezes, producing a smooth, clear ice coating.
Frost forms preferentially on surfaces that cool fastest through radiative cooling: car roofs and windscreens, grass, and exposed metal objects. Clear skies, light winds, and high humidity favour frost formation. In valleys and hollows, cold air drains downhill (katabatic flow) and pools, creating frost hollows where temperatures can be 10–15 °C lower than on surrounding hillsides. Forecasters distinguish between an air frost (temperature measured at 1.25 m in a Stevenson screen falls below 0 °C) and a ground frost (temperature at grass level falls below 0 °C, while air temperature may remain above zero).
Frost is a critical hazard for agriculture: late spring frosts can destroy blossoms and young shoots of fruit trees, grapevines, and vegetable crops. In Spain, frost damage is a significant concern in the Ebro valley, Castilla y León, and highland areas of Andalusia and Extremadura where olive, almond, and vine crops are exposed. Farmers use various protection methods: wind machines (mixing warmer air from above the inversion), sprinkler irrigation (the latent heat released by freezing water keeps surfaces near 0 °C), smudge pots (less common now due to pollution), and row covers. Accurate frost forecasting — incorporating surface type, terrain, wind, cloud cover, and humidity — is a specialized and commercially valuable branch of applied meteorology.