Anyone who has experienced a cold winter has most likely experienced the phenomena of wind chill. Wind chill is the sensation that is caused by the combined effect of temperature and wind, and in turn makes cool temperatures feel much colder. This happens because the wind rids of the layer of heat that the body creates over the skin, called the boundary layer, and by doing so makes the body lose heat faster due to it trying to replenish this boundary layer. Wind also makes people feel colder by evaporating any moisture on their skin - a process that draws more heat away from their body.
The system designed to calculate wind chill is based off of a mathematical model created by Environment Canada, and tested in Toronto, Ontario. It uses data based off of 12 volunteers who went through clinical trials in a refrigerated wind tunnel to see how the model held up in a real-world situation. The results of the testing helped create the official index for wind chill, displayed in Figure 2.
10 km/h - vanes begin to spin, wind is felt on face.
20 km/h - leaves begin to blow.
30 km/h - tree branches begin to sway.
40 km/h - large flags flap strongly.
50 km/h - umbrellas become hard to use.
Although it may seem like this index may just be for convenience, it is really much more than that. This information was developed for peoples' safety. It is used to be able to tell when to wear more protective layers, and how to dress in cold, windy situations. Most importantly, it tells people when they are in danger of frostbite: at wind chill temperatures below
Windchill also amplifies the conditions for potential hypothermia (which occurs when a person's body temperature is under 35°C) which can be fatal. It is always important to dress warmly with many layers in these situations to maximize safety, and try to never be in such conditions for an extended period of time.