Frost

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

Let’s start with what might appear to be an even more basic question - why does it warm up during the day and cool down at night? Because of the sun, you cry! But the air is too tenuous to be warmed directly by the sun to any significant extent. In fact it’s the ground surface that heats up, warming the air from below - just like heating a pan of water on your electric hotplate.

So on a calm, clear night a thermometer on the ground surface will normally read several degrees colder than one just a couple of metres above.

Meteorologists measure standard temperatures (the ones you see on forecasts) at a height of around 1.5 metres above ground. So if the forecaster says "a low of 3 degrees tonight" they mean the temperature at about shoulder height.

Air 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 sometimes stay above freezing. This often happens in early autumn, when the soil still retains some of its Summer heat. Normally though the temperature at ground level will be significantly colder!

Ground frost
Sometimes the air temperature at night dips to 3 or 4 degrees, but the forecaster still warns of a 'ground frost' and the need to de-ice your car in the morning. This is because the ground can reach freezing while the air temperature remains above.

Hoar frost
The white crystals seen on the grass on a cold morning are neither a ground nor an air frost. This is a 'hoar frost', which forms when the air cools and water condenses onto the grass.

Hoar frost is very different from frozen dew, which 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 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!