Frost is a weather condition that occurs when the prevailing temperature falls below 0°C (32°F). It is recognized by the icy deposit that forms but if the air is very dry this will not occur.
In weather forecasting grades of severity of frost are distinguished as slight (-0.1 to -3.5°C), moderate (-3.6 to -6.4°C), severe (-6.5 to -11.5°C), and very severe or hard (below -11.5°C).
Air below 0°C is defined as air frost, measured at a height of 2m (6ft). Ground frost occurs when the air at ground level is chilled below freezing point. Ground frost is measured at 5cm (2in) above the ground. Hoar frost the fluffy deposit of minute ice crystals on grass and brickwork, occurs on calm, clear nights when condensation takes place after freezing. Black frost, as the name suggests, is a thin sheet of frost without the white colour usually associated with frost.
Frost hazard on roads is most common in maritime climates, such as in Scandinavia and the British Isles, where rainfall is regular and winter temperatures oscillate unpredictably around 0°C, and while salt is an effective de-icing agent it damages both road surfaces and vehicle bodies.
Frost is also an agricultural hazard; in 1971 the annual frost loss in the USA was estimated at USD 1.1 billion.