COLD INJURIES - Medicine for Mountaineering and Other Wilderness Activities - page 386

Convective heat loss is an almost continuous process, but the amount of heat required
to warm air (the specific heat of air) is so small that little heat is lost by this route in tem-
perate climates. In a cold atmosphere, convective heat loss is greater because more heat is
requiredtowarmthecolderair.However,thegreatestconvectiveheatlossesoccurinwind,
when the air is constantly moving. Even a mild breeze greatly increases heat loss because
the layer of warmed air next to the skin is constantly being replaced with cooler air.
The amount of heat extracted by moving air increases as the square of the velocity, not
in direct proportion to its speed. A wind of eight miles per hour removes four times as
much heat, not twice as much, as a wind of four miles per hour. A strong wind can remove
tremendous amounts of heat. The increased heat loss that occurs with moving air is called
windchill. Table26-1 illustratestheadditionalcoolingproducedbywindinacoldenviron-
ment. For instance, a temperature that poses little threat in still air, such as 15°F (−9.5°C,)
can be life threatening in a wind of 20 to 25 miles per hour (mph) or 32 to 40 kilometers
per hour (kph).