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 removes a layer of heat that the body creates over the skin, called the boundary layer. This causes the body to lose heat faster because it's attempting 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 the table below.
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.
The index below was developed by Environment Canada to determine different temperatures at certain wind speeds. Refer to the wind speed reference images above to visualize what the speeds might look like outdoors.
Although this index may seem like it's just for convenience, it is really much more than that. This information is crucial for peoples' safety. It can inform people 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.