Low air temperatures and high winds are associated with an increased risk of freezing exposed skin. Such injuries may result in extensive loss of duty-time and can also require long medical treatment. Cold weather can be expected in many regions where conflicts demand military presence. Hence, tools for predicting the risk of skin frostbite is helpful in order to reduce cold weather casualties. Siple and Passel (1945) exposed bare skin to different climates and observed at what combinations of air speed and temperature skin freezing occurred. In addition, they performed cooling experiments on a water-filled cylinder from which they derived the wind chill index (WCI). They reported that an increased risk of frostbite was prevalent at a WCI above 1400 [kcal/m2oh)]. Finger frostbite at considerably lower WCI values than 1400 has also been reported. These exposures were, however associated with snow in the air or with the skin wetted. Another factor of importance is presence of solar radiation. It has been found that frostbite rarely occurs in intense sunshine in spite of high WCI-values. Though the use of the wind chill index has been widely spread, its foundations have been questioned from time to time. WCI has been re-examined and was found not to describe the convective heat transfer correctly. Based on a corrected heat transfer model an analysis of finger frostbite data, presented in the literature, revealed that the frequency of tissue freezing was related to the skin surface temperature. Risk curves have been developed that predicts reasonably well both the frequency of experimentally developed finger frostbite and casualty rate among soldiers during field exercise. These risk curves have been extended, now also taking into consideration solar radiation and skin wetness. The effect of acclimation, expressed as time stayed in cold regions, can also be included in the prediction of the risk of tissue freezing.