The second day of 1982 was so mild paddlers Pat, Steve and Janet couldn't stay home. Blue skies, gentle breeze and temperatures in the 40s lured them to the Potomac near Carderock in canoes and a kayak.

"Beautiful," they said from their lunch stop, a rocky island in midstream. But Dick Bridge, former safety chairman of the Canoe Cruisers Association, has a different descriptive term. He calls such warm winter days "killers."

Years ago Bridge and his son Charles were paddling the same stretch of river on a warm Jan. 11. Bridge is a top-notch boatman, but when the canoe came through Yellow Falls it lurched into an eddie and capsized.

The Bridges discovered a new dimension in cold, experiencing firsthand the debilitating, life-sapping effects of hypothermia.

"We pushed the canoe to shore and were out of the water in 15 minutes," said Bridge. "I felt okay, but when I went to undo the dry pack to get fresh clothes, I could barely work my fingers."

He was in the second stage of hypothermia, when the heat engine that is the human body starts shutting down blood supply to the extremities in order to husband warmth for the vital organs.

Had he and Charles been alone, with no spare dry clothes or fire-starting gear, they would have had one recourse to avoid freezing--run for civilization. But even as they depended for survival on their muscles, those muscles would be shutting down. They would be weak and stumbling; the cooling-down process, abetted by wind and wet clothes, would have been irreversible as long as they stayed outside.

Fortunately they had a partner in another boat, Dan Sullivan, and dry clothes and matches. With Sullivan's help they opened the pack, changed, started a fire, warmed up and were able to paddle out.

Eight marines and a sailor who tried to cross the Potomac near Quantico on a springlike day in March 1968, were not so well prepared. Water temperature was around 40 when they spilled their war canoe; it took days to find the bodies. These strong men, perhaps in the best physical shape of their lives, were sudden victims of hypothermia.

My wife and I joined the little winter run on the Potomac Saturday. As we soaked up sun on our lunch rock I leafed through a booklet, "Hypothermia, Killer of the Unprepared," by Theodore G. Lathrop, M.D. It brought home vividly the dangers of cold, particularly when I looked up how long we could expect to survive should we fall in the 35-degree water swirling around our island. The book said we'd last 15 to 30 minutes.

There are no national tallies on annual loss of life by hypothermia, but veteran mountain climbers, hikers, paddlers, hunters and other outdoors people recognize it as a serious winter peril in any temperate climate.

Almost every description of hypothermia includes the word "insidious." It is insidious because victims don't expect it, don't recognize it as it develops, and by the time they are aware of their circumstance it's often too late to do anything about it.

Hikers who start shivering imagine they can "tough it out" to their destination; paddlers are embarrassed to change clothes among strangers if they get wet; mountaineers hate to halt the party and make camp early simply because they wore the wrong clothes. But in almost every case, the best reaction to developing hypothermia is to stop, get shelter and generate warmth however possible.

Those who ignore early warning symptoms can quickly exhaust their bodies' heat-producing capacity, and soon begin a downward temperature slide that can end in tragedy.

The symptoms of a declining core (rectal) temperature, as described by Lathrop, are:

* 99-96 degrees--Intense shivering; impaired ability to perform tasks.

* 95-91 degrees--Violent shivering; difficulty in speaking, sluggish thinking and amnesia.

* 90-86 degrees--Strong muscular rigidity replaces shivering; exposed skin may become blue and puffy; muscle coordination affected, producing erratic, jerky movements; general comprehension of the situation dulled, but victim generally still able to maintain posture and the appearance of psychological contact with his surroundings.

* 85-81 degrees--Victim becomes irrational, drifts into a stupor; muscle rigidity continues, pulse and respiration are slowed.

* 80-78 degrees--Unconsciousness, erratic heartbreat.

* Below 78--Failure of brain's cardiac and respiratory control centers; cardiac fibrillation; probable edema and hemmorrhage in lungs; death.

The problem is that the slide from shivering to muscle-stiffening to unconsciousness can be sudden--often, as in the case of the marines in the canoe, a matter of minutes.

The goal of winter outdoorsmen is to keep their bodies producing heat and to slow the escape of that heat through their clothes. To produce heat the cold body needs extra high-energy foods (sweets, proteins, carbohydrates); to keep the heat from escaping requires insulation in the form of wool, down and waterproof/windproof outer shells.

Cotton clothes have a tendency to act as a "wick" if they get wet, drawing heat away from the body. Wool, by contrast, retains about half its heat-retaining qualities, even when wet.

Wind and wet are the catalysts for hypothermia. Wind acts to strip away the warm layer of air that, if undisturbed, would cling to the body. Even a 2 mph breeze begins stripping away the insulated air layer from an unprotected body, and the outdoorsman is in a "very cold" range, according to Lathrop, at 40 degrees if the wind is blowing 25 mph.

The thermal conductivity of water, according to Lathrop, is 32 times as great as still air, meaning wet clothing extracts heat 32 times as fast as dry clothing.

An unprotected head is an outdoors disaster in winter, wasting half the body's heat production at 40 degrees and three-fourths of heat production at 5 degrees.

The last half of Lathrop's booklet describes tragic deaths from hypothermia. Usually the victims set off for what they expected to be routine little jaunts, and something happened to make the voyages far more demanding than expected.

The message of these incidents is that no winter venture is without risk. The time for experimenting outdoors--for exploring new trails, paddling new streams, testing equipment--is the summer, when nature smiles.

In winter, walk the proven trails, carry rain gear, shelter and fire-building equipment, bring spare clothes and strong friends. This isn't Alaska but it doesn't have to be. It can turn 40 degrees, windy and wet anywhere.