A frost occurs when the temperature falls below 0 degrees Celsius or 32 degrees Fahrenheit.
On clear still nights the Earth loses a lot of heat out into space, and the ground cools down very quickly. The air in contact with the ground cools down as well and if the temperature drops below freezing then you have a ground frost.
An air frost occurs when the temperature at about 2 metres above the surface falls below freezing, but since the air is an insulator of heat it doesn't fall as quickly as air in contact with the ground. So it frequently happens that you get a ground frost but not an air frost.
On television the temperatures that are shown are always the air temperature, so this is why you will sometimes hear the presenter talk about a frost with all the temperatures on their map above freezing.
Hoar frost is a term used when, as the air cools down below freezing, it condenses out some of the excess moisture in the form of ice crystals. So the whiteness of a frost doesn't show its severity, but only how moist the air was. Incidentally, if the air condenses out the moisture at temperatures above freezing we see it as dew.
There is no such thing as black ice although we hear a lot of reports of it on the roads .It is simply a frost producing a thin transparent layer of ice on the roads, which are normally a dark colour, and hence you can see the colour of the road surface through the ice. If the road surface was orange then it would be an orange frost and it is particularly dangerous because by seeing the colour of the road you think there is no layer of ice on it.
There are several degrees of severity for frost with a slight frost 0C to minus 3.5C, a moderate frost minus 3.5C to minus 6.6C, severe minus 6.5C to minus 11.5C and a very severe frost below minus 11.5C.
Wind speed does affect the severity of a frost, and although a strong wind can prevent a frost by slowing down the night time cooling, once the temperature has fallen below 0C then a strong wind can make it very penetrating and damaging.
Related Links:
- Weatherwise - The Basics - Dew and Frost
- Weather Basics - Frost
