Weather wise

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IF THE forecasters are right - which they are more than 75 per cent of the time - the spring in our weather is about to lose all its bounce. An Arctic blast from Greenland, they say, is going to bring about a drop in temperature of as much as 10 degrees over the next couple of days, bringing snow and sleet to much of the country.

Under such conditions, anyone with a scientific approach to life will head immediately to the formulae for calculating wind-chill factor, for wind-chill is what makes you feel colder than it really is.

In still, cold air, the human body will heat the air around it creating a sort of duvet of warm air that insulates us. If we don't move, or if we wear heavy clothing to trap it, this duvet will greatly slow down the chilling effect of the cold air on our bodies. In windy conditions, however, our duvet is constantly being blown away and we lose energy (so get cold) through the constant need to create new duvets by warming the air with which we come into contact.

So the first thing to do when you go out in the cold is to decide which of the various formulae to use for calculating the wind-chill. There are two principal ones on offer. The simpler is: H=(0.14+0.47pounds v)(36.5-T) where H is the calories lost per second by each square centimetre of exposed skin, v is the wind speed in metres per second and T is the temperature in degrees Celsius. The final term, 36.5-T represents the difference in temperature between the air (T) and dry skin (36.5).

If you are fussier, or have clammy hands, you may prefer the alternative formula:

H=(10.45+10pounds v-v)(33-T)

Both formulae appear to have been the result of experiments in the Antarctic with thermometers, rather than with human beings in windswept Wimbledon. Since neither takes into account the relative humidity of the air, or the sweatiness of the human, the three-significant-figures accuracy of them is largely spurious.

For all practical purposes, a good enough guide to wind chill is obtained by subtracting one from the temperature in degrees Celsius, then taking away the wind speed in metres per second.

t Christian Aid yesterday launched a special El Nino relief appeal to raise pounds 500,000 to enable workers already engaged in relief operations particularly in Peru, Papua New Guinea and Somalia, to expand their efforts to allow emergency relief from the effects of drought and flooding.

For details, phone 0345-000333.