Weather: H = (10.45 + 10pounds v - v)(33-T) so start shivering

As we desperately struggle with the instructions on our central heating boilers, it is little consolation to know that it is not as cold as it feels. The wind chill factor makes low temperatures feel even colder.

In the late 19th century, they used wet-bulb thermometers to measure how cold it felt. While a thermometer with a dry bulb gives an accurate measurement of air temperature, it does not take account of the perceived cooling effect of the wind. We feel cold when it's windy because of the heat lost through evaporation of moisture from the surface of our skins. In the summer, that is how sweating helps regulate our body temperature. Body moisture uses some of the heat-energy of the body as it turns into water vapour. On a muggy, breezeless day, the air becomes saturated and can hold no more moisture; so we feel hot and sticky. Even a slight breeze, however, can keep the air moving enough to provide a constant stream of unsaturated air that can let us evaporate and cool.

In the same way, a thermometer whose bulb is wrapped in a damp cloth will register a lower temperature that a dry-bulb thermometer. The constant evaporation of water removes energy from the surface of the thermometer and cools it down.

In the 1920s, formulae were calculated that combined the readings of both wet-bulb and dry-bulb thermometers to arrive at temperatures that accurately reflected the subjective assessments of trained subjects as they walked between rooms kept under different conditions of temperature and air movement. In the 1950s someone came up with the "discomfort index", which was soon renamed (presumably to sound more scientific) as the "temperature humidity index".

These, and later formulations, took into account air temperature, wind speed and humidity to arrive at a measure of how cold or hot it really felt. When the air is cold, however, it can hold less water vapour, and atmospheric moisture becomes a negligible consideration. That's where formulae for calculating the wind-chill factor come into their own. We try to keep warm by cloaking ourselves in a layer of warm air, but the wind keeps blowing it away. Just how cold it makes us feel depends, of course, on the amount of bare flesh exposed and the thickness of clothing worn as well as atmospheric conditions, but despite that, highly accurate formulae have been calculated to measure the effect of wind on the heat lost by the human body.

If you just want a rough guide, you subtract one from the actual air temperature (in degrees Celsius), then take away the wind speed in metres per second. If you want a more accurate measurement, then you can try the formula given in the headline.

This measures the heat K, measured in kilogram calories, lost each hour by one square metre of exposed skin, where T is the air temperature and v is the wind speed in metres per second. The (33-T) term represents the difference between the air temperature and that of the dry skin itself, which is normally around 33C. The wind-chill temperature is calculated as the temperature, under precisely specified conditions of light or zero wind, at which the body's heat loss would be identical to that in the conditions that actually exist.

At a temperature of 10C, a wind of 7 metres per sec (just over 15 mph) brings the wind-chill equivalent temperature down to about 2C, while if the true air temperature drops to 5C, the same wind speed makes it feel close to minus five.

In cold and windy conditions, the main function of clothes is to provide a windless environment for the body. So in the current conditions of cold, north-westerlies, the best advice, as you already knew, is to wrap up warm and keep the pounds v away from your H.