A wonderfully phony ad came in the mail the other day -- a flyer for some gadgety new stuff selling itself as superwarm long underwear. The cover showed a bunch of men and women gathered at a busy, half-frozen duck pond, surrounded by snow and decked out in their new multicolored longjohns. It was great, except that the ducks were fakes -- those decorative decoys so favored on rich folks' mantelpieces. They had preposterously orange feet and were scattered around in wooden poses. There were scores of them. To make the scene more realistic, the ad agency evidently thought something ought to be flying around; so it got a bunch of pigeons and turned them loose. The result is a photo that anyone who has spent any time outdoors could greet with nothing short of a guffaw. It doesn't give you a lot of confidence, either, in the expensive, miracle-fabric longjohns. As someone who spends a good deal of time outdoors in the winter, I have developed some firm notions about appropriate cold-weather garb; the cornerstones of my philosophy on warmth are three primitive, hard-to-find fabrics -- wool, rubber and leather. The wool keeps you warm. The rubber keeps you dry; you don't wear it unless it's rainy or snowing. The leather, sometimes in combination with rubber and always in combination with wool, is for your hands and feet. Adequately supplied with wool, rubber and leather, you can spend a day outdoors -- even in such sedentary pursuits as bird- watching or goose-hunting -- and withstand just about any cold the mid-Atlantic weatherman throws at you. Goose down, duck down, 60-40, Polypropylethylstremenol and the other plastics are fine in certain circumstances, but for sustained exposure to wind and cold, give me the old standbys, in layers. Layering is the key. Say it's a day that starts off at 10 degrees with 10 knots of north wind, then warms to 48 during the day and cools back down to 20 that evening: I'll wear duofold turtleneck- style long underwear (layer of cotton next to the skin and wool outside). On the torso I'll top that with a thin wool Army shirt left me by my father after World War II, then a heavier wool Woolrich shirt-jacket, then an Icelandic (the best) wool sweater I bought on sale at Woodies for $30. On top of it all I'd wear a wool-lined 60-40 shell jacket, which will shed rain but is not waterproof. With any threat of precipitation, I'd carry along my rubber camouflage rain jacket from L.L. Bean, which came with a guarantee that it was "100 per cent waterproof." Over the duofold bottoms I'd wear either two pairs of medium-weight wool trousers or, if it was scary cold, one such pair covered by a thick pair of woolly pants. On the feet, three pairs of woolen socks. If I planned to walk I'd wear insulated leather construction-type boots; if I was mostly going to be sitting I'd wear my mighty Sorel boots, which have leather uppers, rubber bottoms and a thick felt liner. The hands require wool inserts with leather gloves over them. For the head, two woolen watch caps and, if it's going to be really cold, a woolen face mask. The beauty of this system is that as it warms up you can shed the appropriate layers so that you're always comfortable. I carry a daypack and stuff discarded layers in it, out of the way. When walking you can shed layers, and when you stop you put them back on. If it rains you have the rain gear, but even if wool gets a little wet it retains warmth, unlike any plastic or down fabric. Goose-down coats are terrific when conditions are right; a single wool shirt with a goose-down jacket over it will keep you w It's rare that urban and suburban types step out for a long stay in the cold without access to any warmth. So they don't need to think too carefully before dressing. But some of the sweetest days I've spent have been in a marsh or on a mountain or along a river in the dead of frigid January, when no one else was around. It would be a pity to cut days like that short because some wacky advertisement oversold cold-weather gear. Being dressed properly for the cold is a satisfaction in and of itself. Outdoors types are forever sizing up others they meet and checking out their cold-weather gear. The people who really know their stuff, I think, are construction workers. They don't care how they look, just how they feel, and they have a particular need for clothes that hold up under a strain. Look around a construction site this winter. You won't see too many goose-down jackets.