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.
A frost day is a period of 24 hours in which the minimum temperature in a thermometer screen is equal to or below 0°C (32°F). Occasionally the temperature can remain below freezing all day, known as an ice day.
Ice day is a period of 24 hours, beginning from 9:00 am, throughout which the maximum air temperature remains below 0°C. The average number of ice days per winter varies between 1 to 5 for much of England and Wales and 2 to 6 for Scotland. However, there are winters without a single ice day and winters, such as 1962-63, when Birmingham recorded 17 of them, for example.
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.