Key Biscayne, Fla. ? The late afternoon sea was flecked with whitecaps, and the purple flags snapping in the brisk wind meant there was a danger of Portuguese man-of-war in the water. But for the moment, Stephen Leatherman only had eyes for the sand between his toes.

“It’s a 3.5 in terms of color. Not pure white, but an off-white, light in color,” declared the barefoot professor from Florida International University as he ambled along the shoreline at Crandon Park, a public beach near Miami. “In terms of softness, it’s pretty soft, but there are all these cobbles and pebbles. That takes away from the score.”

Leatherman, a coastal scientist who has taken the tongue-in-cheek alias “Dr. Beach,” has gained national notoriety through his proposition that America’s beaches — like wines, restaurants, golf holes, cars or places to live — can be rated with scientific objectivity and mathematical precision, then ranked to sift out the best.

Every Memorial Day weekend, Leatherman issues his list of 10 top family beaches in the United States, a group that he maintains guarantee “a memorable summer vacation.”

Some in the tourism industry say that making this increasingly cited list is akin to having a stock touted in the columns of The Wall Street Journal or a vintage lauded by Wine Spectator.

In 2002, Dr. Beach proclaimed Port St. Joseph Memorial State Park in Florida’s Panhandle — with its fine, sugar-white sand and high dunes — to be America’s finest.

“As the publicity went from Buffalo to Baton Rouge, many people made side trips to visit, mentioning the ranking we’d received,” said Anne Harvey, park manager. “There are now 185,000 visitors a year, which is an increase of 100 percent over the past eight years. (Leatherman) certainly had an impact on us.”

“Absolutely it helps,” said Carolyn McCormick of North Carolina’s Outer Banks Visitors Bureau. “People rely on books and magazine articles and the recommendations of other people when they travel, and many people rely on those lists.”

Leatherman’s methodology employs a five-point grading scale and 50 distinct criteria, from width at low tide and sand quality to smell, lifeguards and crowds. His subjects are the 650 major public recreational beaches that he has identified from the Eastern Seaboard to the Hawaiian islands.

“I’ve been to all the beaches in the U.S. once, and some many times,” said Leatherman, 56, who received a doctorate in environmental sciences from the University of Virginia and is director of Florida International University’s laboratory for coastal research. On average, he visits 100 beaches each year and tries to see all likely candidates for the top 10.

His perennial high-scorers tend to be in Hawaii, Florida, California, North Carolina, the eastern tip of Long Island and Cape Cod, Mass. Once a beach is proclaimed No. 1, it is retired from the competition.

All the No. 1 picks since Leatherman began his annual rankings in 1991 have been in Hawaii or Florida, where the professor finds the warm, clean and clear waters especially welcoming for ocean bathing. Last year’s national winner was Kaanapali Beach on Maui. Although they have never been his top selections, Leatherman does like Santa Rosa, Calif., for its seaside amusement park and Venice Beach for its unrivaled “funky” diversity of human fauna.

“There is no better beach in the U.S. to see and be seen,” he said.