When most people think of a frost, they think of Jack Frost, a white coating on the ground after a cold night, but this is only one type of frost. There is also 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 all temperatures are measured similarly. 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.

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.

Neither a ground nor an air frost are 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 zero degrees 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 hoar frost is a delicate icy structure. Clear skies help all kinds of frost to form. When the night is cloud free, heat can escape from the earth quicker, without being 'blanketed' by the clouds, allowing the temperatures drop enough to form a frost. It is also the origin of the proverb 'clear moon, frost soon'.

The difference between the different types of frosts are 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!

Other features in the Weather Basics series:

- Clouds
- Day and Night
- Fog and Mist
- Fronts
- Frost
- High Pressure
- Humidity
- Jet Streams
- Lightning
- Low Pressure
- Mirages
- Northern Lights
- Rain or Showers?
- Thunder
- Watercycle