Severity of frost
Frost occurs when the temperature of the air in contact with the ground, or at thermometer-screen level, is below the freezing-point of water (‘ground frost’ or ‘air frost’, respectively). The term is also used of the icy deposits which may form on the ground and on objects in such temperature conditions (GLAZE, HOAR-FROST). Since the sensation of cold depends not only on air temperature but also on the accompanying wind speed, the fourfold classification of frosts used in forecasts of this condition in the United Kingdom is varied with wind speed. Thus, frost is classified as ‘slight‘, ‘moderate‘, ‘severe‘, or ‘very severe‘ for screen temperature ranges of 0.1 to -3.5°C, -3.6 to -6.4°C, -6.5 to -11.5°C, or below -11.5°C, respectively, if the accompanying wind speed is less than 10kt; and for screen temperature ranges of -0.1 to -0.4°C, -0.5 to -2.4°C, -2.5 to -5.5°C, or below -5.5°C, respectively, if the wind speed is 10kt or more.Met Office Glossary (1991).