If it's too cold, Larry Bercowitz, 34, a street vendor, bundles up. If it's too windy, he huddles against a building. He wears dark glasses and a straw hat to combat the sun.But in the rain there is no defense for a man who sells flowers made of chicken feathers for a living.

So far, May 1978 has been a particularly depressing month for Bercowitz and the thousands of other "fair weather people" who prefer clear sunny skies to the dark, rainy days that have dominated this month.

According to the U.S. Weather Station at National Airport, eight of the first 15 days of May have been marked by measurable rainfall. As of 9:30 yesterday morning, about 3.11 inches of rain had fallen this month. On the average 3.68 inches of rain falls on Washington the entire month of May.

"This time of year people are looking for a sense of renewal, hope and brightness," said Dr. Peter G. Angeles, a local psychiatrist who moted that rainy springs produce large numbers of depressed people. "What they are getting instead is rain."

Chet Hendricks, lead forcaster for the U.S. Weather Service, predicted that thundershowers and intermittent rain will continue in the Washington area until probably tomorrow night.

Hendricks said the daily rains in this region since Saturday have been due to a low pressure system which stalled over Virginia and has been staving off clearer weather.

By Thursday, Hendricks said the slow-moving low pressure system will move off the Delaware-Maryland cost into the Atlantic.

From Saturday until yesterday morning, 1.86 inches fell in the Washington area, compared to between two and three inches in the nortern Shenandoah Valley and two to 2 1/2 inches of rain along the Chesapeake Bay.

Gody Rivera, a weather service specialist at National Airport, said most people think of April as the month with the heaviest rainfall ("April showers bring May flowers"). But in the Washington area, May is the third wettest month, next to August which is number one and July, number two.

Although area police said no emergencies have been created by recent downpours, there have been numerous instances of flash flooding of streets and streams and thousands of potholes have been washed out by the rain. Some of those potholes were reportedly responsible for morning traffic tieups.

Charlie Williams, D.C. Department of Transportation highway construction engineer, said crews were working yesterday - sometimes in the rain - to repair potholes. But he said not much can be done until the weather clears.

Mayor Washington's Office of Emergency Preparedness received 12 reports of flooded street intersections. But Richard Bottorff, assistant director of plans, said that in all cases the flooding quickly receded when the shower ended.

As for Larry Bercowitz, he was last seen in an afternoon shower, dismantling his display table and covering his chicken feather and plastic flowers with a green canvas.