Increased wind speed results in increased heat loss. This is called the wind chill factor. You can walk outside in shirt sleeves in minus 40°C if it is absolutely calm but you may require the thickest of layers at minus 5°C if the wind is above gale force. Even the wind generated by walking can cause frostbite; a gale at minus 40°C is dangerous.
|Freezing cold||below −25°C|
|Bitterly cold||−25 to −15°C|
|Very cold||−15 to −10°C|
|Cold||−10 to 0°C|
|Very cool||0 to 10°C|
|Cool||10 to 15°C|
To calculate the wind chill factor use the following equation which is derived from the Nomogram of Rees, W.G. Polar Record 29 (170):229-234 (1993). The formula is:
[ -32 Ws /25 - Ts ] Twc = 2 [ ----------------- + Ts ] [ 2 + 1.5 Ws / 25 ]
where Twc = wind chill temperature, Ts is still air temperature and Ws is wind speed in metres/sec.
To make this easier, you can use the wind chill calculator.
The National Weather Service in the United States has developed a different formula for measuring wind chill. The NWS Wind Chill page has further information.