$14\mathrm{.}19$ In Chapter $11,$ we explained the concept of windchill factors. We said that the heat transfer rates from your body to the surroundings increase on a cold, windy day. Simply stated, you lose more body heat on the cold, windy day than you do on a calm day. The windchill index accounts for the combined effect of wind speed and air temperature. It accounts for the additional body heat loss that occurs on a cold, windy day. The old windchill values were determined empirically, and a common correlation used to determine the windchill index was

WCI$=(10\mathrm{.}45-V+10\sqrt[\phantom{2}]{V})(33-T_a)$

where

WCI$=$ windchill index $(kca\frac{l}{m^2}*hr)$

$V=$ wind speed $(\frac{m}{s})$

$T_a=$ ambient air temperature $(C)$