A lot of people can't believe in winter until the first snowstorm. The melting flake on the tip of the tongue, the flawless frosting on the roof of the house, the white mound where the evergreen shrub used to be - this makes them believers at last.
Some of us can't believe in winter until the first solid ice has formed on the nearest pond - ''hard, dark, and transparent,'' as Thoreau described the ice on Walden.
The first skim ice is a stage rehearsal - a tentative run-through for the real thing. Under a barrage of stones cast by small boys from the shore, this brittle experiment disintegrates into splinters and glittering shards and slowly dissolves back into its original element.
Skim ice is to real ice what a papier-mache model is to the completed building.
The moment of verifiable winter is near for the ice-watcher when air bubbles begin to appear in the thickening ice - those pure white holes as beautiful, varied, and mysterious as the markings in a child's marble.
If the ice-watcher lives near enough to the pond, he or she may hear the evidence of solid ice before seeing it. For, as Thoreau observed, those bubbles, trapped like tiny stars beneath the frozen surface, explode like ''little air-guns'' and provide the ''whooping of ice'' in a pond at night, ''as if it were restless in its bed and would fain turn over.''
But just as summer doesn't really begin for the sometime ice-watcher until he or she immerses as a swimmer, winter doesn't really arrive until he or she skates on the ice.
No matter how many times it happens over the course of how many winters, the small wonder does not cease to amaze. One actually stands on this lovely dark improbability that, until three or four frosty nights ago, was water.
This is winter.
The first dive of the first summer, the first wobbly glide from shore of the first winter - these are frontier adventures that stay in one's mind forever. And every annual reentry into winter on skates becomes a reunion with that younger self.
The repetition never seems to stale the pleasure of making the first tracks on fresh ice, as if the world had just been invented. Every winter the very mature adult feels sheer infant's delight in spraying up snow shavings with the first sharp stop.
There is a sort of tribal confirmation of winter on the first big skating day of the season when enough people gather on the pond so that the sound of a hundred skates speaks like the eerie ''om'' of a continuous bass beneath the treble of human voices.
This is the stuff Currier and Ives prints are made of.
But the final and complete ratification of winter must occur during the first skate at night. The pond, surrounded by ordinary suburban domesticity in daylight, becomes at night a floating universe on its own. Nothing seems to exist but stars overhead and ice beneath, with those other stars frozen in it. All is windless stillness as the skater pushes away from the shore, from the lights and the people.
Sun? Earth? Water? These elements seem like fantasies from a dream. The world has become pure air-and-ice.
Following a cloud of condensed breath, the skater skates alone into the center of this universe.
At that moment, the skater believes in winter, and almost nothing else.