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Wind Chill

Watch and listen to the latest World and UK weather broadcasts
UK weather chart showing the direction of wind chill currents.
Winter cold snaps make us all realise the effect the wind can have on temperatures.

Key Points
  • Wind-chill is a measure of the amount of heat lost from the skin as the wind blows across it.
  • When the temperature is 0°C, a ten knot wind will make it feel like -5°C does in calm air.
Also in this Series

Wind Chill - Effects
Frostbite
Wind

Also in BBC Weather

Winter High Pressure
Frost

bbc.co.uk Links

BBC Health - Frostbite
BBC Health - Hypothermia


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The BBC is not responsible for the content of external websites.

Even when temperatures are about average, strong northeasterly winds can make it feel bitterly cold in the UK.

Such a wind is often described as a 'lazy' wind, as it does not appear to go around you, but straight through you instead! In such conditions weather forecasters often mention "the wind-chill". But what is it?

Wind-chill is a measure of the amount of heat lost from the skin as the wind blows across it. Strictly speaking, the comparison is between an unclothed individual moving through calm air at a brisk walking pace and that same individual moving through wind.

Early wind-chill development was based on conditions likely to be encountered in Antarctica.
Whilst it is not recommended that you try this for yourself, the comparison does have a serious basis. Early wind-chill development was based on conditions likely to be encountered in Antarctica. Frostbite is a major hazard to anyone working in such areas. It was found that wind speed was critical to formation of frostbite; whilst it was possible to work in temperatures below minus 40 Celsius, winds of as little as three or four knots can make a dramatic difference.

So in calm air a temperature of 0 Celsius feels like 0 Celsius. But if there is a wind of ten knots blowing it will feel the same as minus five Celsius feels in calm air. As the wind increases to 25 knots, the wind-chill falls to minus 12 Celsius. A wind of 40 knots gives a wind-chill of minus 16 Celsius.

But remember, the wind-chill will not make the air temperature alter. The wind-chill factor also takes no account of humidity. When the air temperature is well below freezing the amount of moisture that can be held in the air is very low. This is one reason why skiers often find that they are quite comfortable skiing in a tee-shirt and jacket despite temperatures of minus ten Celsius or even lower, as long as there is no wind.

There is more than one way of calculating a wind-chill index. Met Office forecasters used to use the index devised by R.G. Steadman. While forecasters found it accurate for UK conditions, it measured 'full-body wind chill' and was thought to exaggerate wind chill as it excludes heat generated by metabolism, heat retention by clothing and heat loss due to respiration and conduction.

The JAG/TI algorithm is now used, which measures 'face only wind chill' and is a Canadian method. The Met Office use this method as it has been clinically tested, it is simple to use and based on current research.

The typical weather scenario for referring to wind-chill temperatures is a wintertime east to northeasterly wind...
The typical weather scenario for referring to wind-chill temperatures is a wintertime east to northeasterly wind, driven across Scandinavia or Russia, on the southern side of a high pressure system. Such winds are cold because of their track across the mountains and tundra. They can often be strong too, because low pressure over Southern Europe can result in a very tight 'squeezing' of the isobars.

So next time you hear a weather forecaster start talking about "severe wind-chill" you will know this is not best time to go streaking through the streets!





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