MANY WASHINGTONIANS are keeping their cool under straw hats this summer.

And they are not alone. Men and women all over the country are snapping up cowboy styles, boaters and bowlers, panamas and padres, big brims and small, in straw and other media. It's good news to hatmakers and retailers who find their only headache in the fact that straw and other materials in short supply have made hats more expensive.

Burt Champion, of the Millinery Institute of America, pegs the big boost in hat wearing to the end of fancy women's hairdos and the availability of Chinese straws for the American market. The hats got simpler and more wearable at the same time, too, he says.

"Hats are selling well not only because they look good," says Marta (Chi Chi) Labarraque of Woodward & Lothrop, "but because people are far more aware of staying out of the sun as a good health measure. Now hats are utilitarian as well as fashionable."

Helga Orfila, wife of the secretary general of the Organization of American States, likes the practical aspect of keeping the sun off her face. "But it also lets me wear a more simple hairstyle under the hat," she said.

Unlike the last big hat wearing period of the 1940s, hats for all seasons are no longer so rigid. Many of the big successes, even in straw, are flexible enough to be worn brim up and down, decorated or not, and sometimes even rolled up and stuffed in a bag. According to Joe Grassi, mens hat buyer for Raleighs, women are buying a number of hats the stores have for men. "We sell many of our hats in small sizes, obviously to women," says Grassi, who believes that women tend to individualize their styles by adding different color ribbons.

"If you want to make a fortune, make a cowboy hat," insists the millinery trade spokesman Champion. "There is no limit to the price range. The world has gone crazy for cowboy hats."

John Eason, retired from the United States public health service currently owns about ten hats, usually buying a new one each season. "I've always worn them because I'm bald," he said, revealing his shiny head as he tipped his hat and walked on.