THERE'S A CERTAIN northerly winter wind that makes Washington's ice-white monuments seems screeching cold; when the great expanse of the Potomac carries quick white horses on each tan wave, and the flags strain frantically. On such a day, I find myself thinking automatically, "what a hell of a day at sea."

But the fact is, on many such days between October and March, people are gathering at sailing clubs and marinas, rigging their little boats, pulling on rubberized clothes or scuba wet suits and preparing to do the thing called frostbiting.

Frostbiting has never really been defined, and probably never will be. It is an excess of enthusiasm for sail-racing carried into an absurd season. It is an act of aggression against logic. It is a group revolt against comfort. It is an affront to the human circulatory system.

Yet for all this, frostbite sailing grows in popularity, as does all sailing, and its adherents walk with a certain strut and talk with just a hint of condescension - they may often be heard describing nonfrostbiters as "summer sailors" or even using the ultimate insult in racing circles: "turkeys."

The centers of frostbiting are Annapolis and Washington, West River and Oxford, Md., and a score of other harbors and boat clubs that provide all-winter weekend racing for members ang guests. But frostbiting is a fraternity of its own beyond the boundaries of club membership. No one is turned away who has a boat in a recognized frostbiting class and whose fingers can stop shaking long enough to sign a waiver exonerating the host organization from blame if he or she should drown nor die of exposure.

Each weekend small hosts gather to race, sometimes literally breaking ice to launch their boats.

I had heard much more than I had ever seen of frostbiting before I tried it myself. Frostbiters tend to be self-announcing, to put mildly, but after some discussion about types of boats, I purchased a singlehanded dinghy of the Laser class, one of the most popular, for $850. I became absolutely devoted to this little vessel after a season of winter racing. She was fast enough to make one whoop with joy before the wind, complicated enough to promise new hopes of victory and light enough to put on a car-top rack. And then my education began.

It is a lucky thing that my boat is designed to be righted after a capsize by merely stepping on the centerboard. It soon became clear that expertise in recovering from disasters was one of the prime elements in winter Laser sailing; some of the better sailors could scramble around the topsides of their capsizing craft, jump on the center-board and jump back in the cockpit without even getting wet.

The next investment was a full wetsuit, which cost $120. This frogman suit covered me from neck to toe and made me forget the dire statistics I had read about human life expectancy when immensed in water colder than 40 degrees (it is about 25 minutes).

Before I got caught up in the competitive aspects of racing in a 20-boat fleet, each outing was an adventure apart. Some of the winter days were rare things, sun and shadow over the white sails of the fleet (usually far ahead) the bright and empty bay and river, and the cold heavy air of winter pushing across the water, unused and unseen by all but this very few.

There were also absolutely rotten days when the wind would not blow and the cold penetrated right to the bones. For, without movement and action, frostbite sailing is agony with no escape. There were absolutely rotten days when gear would break and capsizes would come one after the other until beet-red hands refused to obey desperate commands and exhaustion made the simplest maneuver too hard, too long.

And there was sailing on the Potomac against the worthies of the Potomac River Sailing Association at the Washington Sailing Marina.

Frostbiting on the national river is a thing apart. Aside from the possibility of running into a submerged icebox, a waterlogged oak tree or even a corpse, there is National Airport, producing wind conditions that a fleet of computers could not unravel or record.

The racing course in the winter lies directly across the landing approach for the largest passenger jets. Sailing there for the first time, one is first deafened and stunned by an artillery barrage of shrieking and bellowing jetblast, followed by a downdraft stinking of incompletely burned kerosene that capsizes some craft and sends others spending ahead. The local worthies talk darkly of the "airport lift" and ascribe winning races to a thorough knowledge of the airline schedules on Saturday and Sunday.

Certainly Potomac River frostbiters suffer patiently some of the worst sailing conditions in the world just to continue their favorite sport, and any publicity they receive lays heavy stress on their apparent "insanity, the truth is simply that they are sailing fanatics, like most winter sailors.

They are also doubtless better sailors than their summer-only colleagues, because sailing, like golf, requires constant practice to maintain top racing form. I never reached this top form, despite many a bruise and icicle, but there are those who have.

Most of my racing was done at the Severn Sailing Association, whose prime mover in winter racing is the internationally known sailor Dr. Stewart Walker. Walker is a stubby, fiery little man with grand shoulders and an abundance of energy, who, with a few other men and women now in their 50s and 60s, established frostbite racing at Annapolis' SSA when sailing was a little known sport for the rich and social.

Walker continues to race all winter and is renowned for demanding that race committee proceed to the course in any weather whatsoever. He has been known to drive his boats beneath the surface of the water before easing sail.

Walker had already risen to the top when my frostbite career began, but if there was one member of the fleet I could depend on beating, it was an amiable and loose-limbed scientist named Michael Guhin. In his alte 30s, Guhin had become well known for his dogged determination to finish each race, no matter how poorly he placed. This is a polite way of saying he was a prime, full-feathered turkey.

Through every winter outing, Guhin fought on, grinning and cursing, while more facile sailors skimmed to the lead. His second frostbite season he was less in evidence and it seemed he had drifted away from the ice-edge waters.

But if any case can convince me that frostbiting's struggles are not in vain, it is Guhin's, for after all, serious efforts in sport are toward one end - increasing skill and performance.

It was after a year's absence from winter sailing that I next encountered Guhin, and it was in a piece in a yachting magazine telling of the world's championship in the Star class. Now the Star class is one of the most complicated, fast and high-strung boats sailed in competition, and the leaders of the class are considered among the world's top sailors for that accomplishment alone; it is a viola to the Laser class kazoo. The winters of the great prize were Guhin and another Annapolis sailor, skipper Jim Allsop. I could have fallen right out of the cockpit of my Laser with surprise.

Last year's turkey had become a swan!