When your Jeep spins lazily off the mountain road and slams backward into a snowbank, you don't worry immediately about the cold. Your first thought is that you've just dented your bumper. Your second is that you've failed to bring a shovel. Your third is that you'll be late for dinner. Friends are expecting you at their cabin around eight for a moonlight ski, a late dinner, a sauna. Nothing can keep you from that.

Driving out of town, defroster roaring, you barely noted the bank thermometer on the town square: minus 27 degrees at 6:36. The radio weather report warned of a deep mass of arctic air settling over the region. The man who took your money at the Conoco station shook his head at the register and said he wouldn't be going anywhere tonight if he were you. You smiled. A little chill never hurt anybody with enough fleece and a good four-wheel-drive.

But now you're stuck. Jamming the gearshift into low, you try to muscle out of the drift. The tires whine on ice-slicked snow as headlights dance on the curtain of frosted firs across the road. Shoving the lever back into park, you shoulder open the door and step from your heated capsule. Cold slaps your naked face, squeezes tears from your eyes.

You check your watch: 7:18. You consult your map: A thin, switchbacking line snakes up the mountain to the penciled square that marks the cabin.

Breath rolls from you in short frosted puffs. The Jeep lies cocked sideways in the snowbank like an empty turtle shell. You think of firelight and saunas and warm food and wine. You look again at the map. It's maybe five or six miles more to that penciled square. You run that far every day before breakfast. You'll just put on your skis. No problem.

There is no precise core temperature at which the human body perishes from cold. At Dachau's cold-water immersion baths, Nazi doctors calculated death to arrive at around 77 degrees Fahrenheit. The lowest recorded core temperature in a surviving adult is 60.8 degrees. For a child it's lower: In 1994, a two-year-old girl in Saskatchewan wandered out of her house into a minus-40 night. She was found near her doorstep the next morning, limbs frozen solid, her core temperature 57 degrees. She lived.

Others are less fortunate, even in much milder conditions. One of Europe's worst weather disasters occurred during a 1964 competitive walk on a windy, rainy English moor; three of the racers died from hypothermia, though temperatures never fell below freezing and ranged as high as 45.

But for all scientists and statisticians now know of freezing and its physiology, no one can yet predict exactly how quickly and in whom hypothermia will strike--and whether it will kill when it does. The cold remains a mystery, more prone to fell men than women, more lethal to the thin and well muscled than to those with avoirdupois, and least forgiving to the arrogant and the unaware.