A frosty reception

Wednesday 21 April 2010, 10:39

Derek Brockway Derek Brockway

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You will often hear me and other weather forecasters referring to 'ground frost' and 'air frost' during our weather bulletins but probably wondering what the difference is?

At night when the skies are clear, the ground surface cools down rapidly and heat from the sun, escapes quickly into the atmosphere.

The air in contact with the ground cools down a lot but because air is a good insulator i.e. the higher up you go, the lesser the cooling effect.

So on a calm, clear night, a thermometer on the ground will normally read several degrees colder than one positioned a few feet higher up.

Meteorologists measure standard temperatures (the ones you see on forecasts) at a height of around 1.5 metres or 4 feet above ground in a Stevenson screen.

So, if the forecaster says "a low of 3 Celsius 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 Celsius, but the forecaster still warns of a 'ground frost' and the need to scrape the frost off your car windscreen in the morning.

This is because the ground can reach freezing while the air temperature remains above.

The next few days are looking fine and settled with plenty more sunshine.

However, the nights will turn cold with more ground frost and in some rural areas a slight air frost is likely too, with temperatures falling just below freezing.

Gardeners should continue to cover up any delicate plants or bring them indoors just in case.

Derek


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