The windchill temperature index provides a measure of how wind can speed up the rate of heat loss from exposed skin. Wind speed that is equal to or less than three miles per hour is considered calm. Only exposed skin is affected by windchill, and it is of greater significance when it is cooler. Windchill effect can be negated if the climber is properly dressed and prepared for the weather conditions.
Adapted from Wilderness Medicine by Paul Auerbach
Light Gray = Frostbite within 30 minutes at this temperature and wind
Dark Gray = Frostbite within 10 minutes at this temperature and wind
Black = Frostbite within 5 minutes at this temperature and wind