Wind Chill… What’s in a number? In this case what is behind the number and how is it best utilized?
Wind chill is the cooling effect on the skin caused by a combination of temperature and wind. The number stands for the amount of cooling that the skin on the face experiences and we feel. The number that we are given is a temperature equivalent to the cooling sensation that your skin would feel in a calm wind. As an example, if the air temperature is -10 degrees and the wind chill value is -20, your face will feel as cold as if the wind is calm and the air temperature was -20 degrees. The wind chill number is not an actual temperature but a sensation of coldness. As a result it is not stated with a degree symbol. In the given example the weather conditions would be given as the temperature is -10 degrees with a wind chill of -20.
How Does Wind Chill Affect You?
To understand the effect you need to be aware that our skin has a thin layer of warmed air next to it. The air is warmed by our body heat. This thin layer acts as an insulating layer between the cold air and our skin. Wind tries to remove this heated layer and the body tries to replace it. It is a kind of tug – a war. We make it and the wind pulls it away. As long as the body is winning the contest our skin remains comfortable. However, when the wind has the upper hand the body cannot keep up the skin starts to cool. This process gradually removes energy from the body which over a time period can lead to other cold related problems such as frostnip, frostbite and hypothermia.
Another part of the cooling process is caused by evaporation of moisture from the skin. Wind increase the rate of evaporation and as the evaporation process causes cooling; the skin experiences another heat loss factor.
The concept of wind chill is not a new idea. The first experiments with heat loss from the skin started back in the late 1940’s. The initial methods were, by today’s standards, rather simplistic but they did serve as a sound starting point. Over the years many other methods and experiments, including tests on human volunteers, have been explored and the results are the numbers that we now have. As noted, today’s number are given in a temperature equivalent but you may remember when the values where in watts per square meter. An example number from earlier times might have been 1500 watts per square metre. It is much easier for us to relate to the present values. Wind chill does not have any effect on objects. Therefore you car in the driveway does not suffer from wind chill it just loses heat faster when there is a wind. Wind chill does not affect the actual temperature of the air.
The numbers that we see are the result of some rather complex mathematics. Values are calculated for temperatures from 10 degrees and below with a wind velocity of 4.8 kph or greater.
Because wind chill values are based on calculations they are best used as guide lines for outdoor activity. For instance the wind chill value does not take into account the effect of direct sunshine, which can have a warming factor even on a cold day. The current calculations do not consider the effect of humidity. Besides being used as guides to personal behavior the numbers are used by organizations to decide if outdoor activities should be cancelled. Examples might be whether school buses are going to be running and if recess at school will have to be an indoor activity on that day or the scheduling of warm up breaks for outdoor workers.
Be aware, prepare and enjoy being outdoors.
