And after this long stop, the skiing itself has become more difficult. By the time you push off downhill, your muscles have cooled and tightened so dramatically that they no longer contract easily, and once contracted, they won't relax. You're locked into an ungainly, spread-armed, weak-kneed snowplow.

Still, you manage to maneuver between stands of fir, swishing down through silvery light and pools of shadow. You're too cold to think of the beautiful night or of the friends you had meant to see. You think only of the warm Jeep that waits for you somewhere at the bottom of the hill. Its gleaming shell is centered in your mind's eye as you come over the crest of a small knoll. You hear the sudden whistle of wind in your ears as you gain speed. Then, before your mind can quite process what the sight means, you notice a lump in the snow ahead.

Recognizing, slowly, the danger that you are in, you try to jam your skis to a stop. But in your panic, your balance and judgment are poor. Moments later, your ski tips plow into the buried log and you sail headfirst through the air and bellyflop into the snow.

You lie still. There's a dead silence in the forest, broken by the pumping of blood in your ears. Your ankle is throbbing with pain and you've hit your head. You've also lost your hat and a glove. Scratchy snow is packed down your shirt. Meltwater trickles down your neck and spine, joined soon by a thin line of blood from a small cut on your head.

This situation, you realize with an immediate sense of panic, is serious. Scrambling to rise, you collapse in pain, your ankle crumpling beneath you.

As you sink back into the snow, shaken, your heat begins to drain away at an alarming rate, your head alone accounting for 50 percent of the loss. The pain of the cold soon pierces your ears so sharply that you root about in the snow until you find your hat and mash it back onto your head.

But even that little activity has been exhausting. You know you should find your glove as well, and yet you're becoming too weary to feel any urgency. You decide to have a short rest before going on.

An hour passes. at one point, a stray thought says you should start being scared, but fear is a concept that floats somewhere beyond your immediate reach, like that numb hand lying naked in the snow. You've slid into the temperature range at which cold renders the enzymes in your brain less efficient. With every one-degree drop in body temperature below 95, your cerebral metabolic rate falls off by 3 to 5 percent. When your core temperature reaches 93, amnesia nibbles at your consciousness. You check your watch: 12:58. Maybe someone will come looking for you soon. Moments later, you check again. You can't keep the numbers in your head. You'll remember little of what happens next.