... You twist on your flashlight, and its cold-weakened batteries throw a yellowish circle in the snow. It's right around here somewhere, you think, as you sift the snow through gloved fingers. Focused so intently on finding the bail, you hardly notice the frigid air pressing against your tired body and sweat-soaked clothes.

The exertion that warmed you on the way uphill now works against you: Your exercise-dilated capillaries carry the excess heat of your core to your skin, and your wet clothing dispels it rapidly into the night. The lack of insulating fat over your muscles allows the cold to creep that much closer to your warm blood ...