In late march a humid wind blowing from the Northeast at 15 knots through lightweight cotton clothing has a chill factor of 55 degrees below zero. Prolonged exposure can result in the permanent loss of some illusions, among them the one that says spring occurs somewhere in the vicinity of the month of March.

"Beware the Ides . . . ," Julius Caesar was told, but it looked like a nice spring day, we suppose, and he gave the bodyguards the morning off. People have since learned to be more wary about accepting the spirit of spring in their hearts.

This doesn't mean you can't enjoy those beautiful days that present themselves here and there, but you don't make a fool of yourself about them. When someone starts rhapsodizing about the weather, the birds, the flowers, you think of daffodils bending before a gray, nasty wind and say, "Yes, but they're talking about sleet tomorrow." And you don't pack the wool clothes away.

Nobody, of course, falls for the vernal equinox anymore. The equinox occurred this week and was reported witb the cynical comments that have become commonplace. An equinox doesn't know warm from cold or a crocus from a dead car battery. It is taken no more seriously than the appearance of tourists, blue-lipped, beside the Tidal Basin.

Some few people still believe that baseball means spring. But baseball has developed a wanderlust in the direction of winter, poking farther down into April and farther up into October. It's generally accepted now that when you see the commissioner in his box seat at such times he's wearing longjohns under his light cotton clothing.

Some see the return of the swallows later in April as evidence that it's spring. These birds are thought to have scouted out the situation for any possible spells of cold weathr before committing themselves for the season. Don't count on them. If swallows were that smart, they wouldn't have to eat bugs.

May Day? We recall some in recent years that have been colder than Stalin's heart. Daylight Saving Time? Nothing is more chilling than a hard, brightly lit evening when the temperature is 44 degrees. How about May 15, which garden books suggest as the safe date to free your house plants from indoor confinement? Well, maybe you can start to loosen up a little by then, remarking to close friends who won't betray a confidence, "Looks like it might finally be here."

But the extremely prudent (if there can be such a thing) will wait for Memorial Day. Even if that gives them only a few weeks of spring before summer comes, at least they won't have had their hearts broken.