Windchill and the risk of tissue freezing

Low air temperatures and high wind speeds are associated with an increased risk of freezing of the exposed skin. P.A. Siple and C.F. Passel (Proc. Am. Phil. Sec. 89:177-199, 1945) derived their windchill index from cooling experiments on a water-filled cylinder to quantify the risk of frostbite. Their results are reexamined here. It is found that their windchill index does not correctly describe the convective heat transfer coefficient (h(c)) for such a cylinder; the effect of the airspeed (v) is underestimated. New risk curves have been developed, based on the convection equations valid for cylinders in a cross flow, h(c) proportional to v(0.62), and tissue freezing data from the literature. An analysis of the data reveals a linear relationship between the frequency of finger frostbite and the surface temperature. This relation closely follows a normal distribution of finger-freezing temperatures, with an SD of 1 degrees C. As the skin surface temperature falls from -4.8 to -7.8 degrees C, the risk of frostbite increases from 5 to 95%. These data indicate that the risk of finger frostbite is minor above an air temperature of -10 degrees C, irrespective of c, but below -25 degrees C there is a pronounced risk, even at low v.