You will often hear weather forecasters referring to ground frost and air frost, but how do they differ?

The temperatures recorded by the Met Office are those that are measured in Stephenson screens. This is a standard screen that ensures temperatures are measured in the same way all over the world. The screen is just over a metre above the ground and air at this height can have a different temperature from that of the ground.

Air frost and ground frost
When the temperature in the Stephenson screen reaches zero, there is said to be an 'air frost'. Even with an air frost, the ground can still stay above freezing. This often happens in early autumn, when the soil still retains some of its Summer heat. Equally the ground can fall below freezing when the air does not.

Sometimes the air temperature at night dips to 3 or 4 degrees, but the forecaster still warns of a 'ground frost' overnight and the need to de-ice your car in the morning. This is because the ground can reach freezing without the air.

Hoar frost
Neither a ground nor an air frost is described as the white crystals seen on the grass on a cold morning. This is 'hoar frost', which forms when the air cools and water condenses onto the grass. It is actually dew if it forms at temperatures above freezing, but if it forms below freezing it is hoar frost.

Hoar frost is very different from frozen dew. Frozen dew is dew that has frozen after it has formed. They are very different to look at, frozen dew looks like frozen water droplets, and a hoar frost is a delicate icy structure.

Frost severity
There are several degrees of severity for frost with a slight frost 0°C to minus 3.5°C, a moderate frost from minus 3.5°C to minus 6.6°C, severe frost from minus 6.5°C to minus 11.5°C and a very severe frost below minus 11.5°C.

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 0°C then a strong wind can make it very penetrating and damaging.

The difference between the different types of frosts is very important to some people including farmers, who need to know when their plants are at risk, and motorists, who need to know when there are icy patches on the roads. To others though, all types of frost mean that it will be very cold!