SATURDAY morning, 11:15. Traffic is backed up on the Bay Bridge for five miles, says the radio announcer. Beaches are fogged in along the Jersey coast. If you want to brave the trek to Virginia Beach, the water temperatuire is a chilly 50 degrees.

Meanwhile, in downtown Washington, it's 86 degrees and clear with the humidity low enough to keep your hair from frizzing. In other words, perfect sunbathing weather for The Nation's Capital. So roll over and smear on another coat of Bain du Soleil. Here on your towel in a carefully groomed park, transistor playing Top 40 and shoulders starting to fry, you can almost hear the ocean roar. Or is that traffic?

You're at your favorite urban beach.

Now, there are city parks and then there are urban beaches. A real urban beach is distinguished by the number and dedication of its sunbathing occupants. If more than a handfull of loungers stake out their ground dressed as skimpily as is decent and equipped with the essentials -- lotion, radio, sunglasses, towel, cheap novel -- the turf is an urban beach. Note that water is optional and sand is hardly a prerequisite.Attitude -- The Will to Tan -- is the key factor.

Where to find them? On the shores of the Potomac, by the banks of various man-made ponds, alongside national monuments or by the rushing tide of the G. W. Parkway, the tanning runs the gamut from strawberry pink to burnt almond. This weekend, why not get away from it all and stick close to home? Washington can be lovely when it sizzles.

At the Watergate Beach, a waterfront stretch at Rock Creek Parkway and Virginia Avenue, bike riders frequently pull up 10-speeds, strip down to bikinis and trunks, spread their towels and lather up. The grassy area overlooking the river gets crowded by noon. With all the cocoa butter and Sea & Ski, the air smells like Rehoboth. Only there's no boardwalk honkytonk or noisy Dreamsicle vendors. After soaking up a couple of hours' rays, however, regulars head for life-saving soda machines next door at Thompson Boat Center or across the street at Watergate Exxon.

Nearby, at the famed P Street Beach, the hillside's jumping with disco music and a horde of serious sun-worshipers. The downtown resort above the Parkway is known as a gay men's hangout, but anyone who appreciates a bronzed body fits in. And when you tire of lying flat on your back, the beach is a short jog to the park's parcourse.

For a more subdued sunbathing scene, there's always the Mall, where throngs of tourists provide a Bermuda-short-and-Polaroid parade. Here and there, on blankets dotting the greenery from the Lincoln Memorial right up to the Capitol's steps, scattered bodies bake, minds on ultraviolet rays rather than national landmarks. Discarded jogging shoes and socks at their sides, sunbathers work up a sweat while strenuously doing nothing.

But the Riveria of intown beaches is Constitution Gardens, between the Washington Monument and the Reflecting Pool, opposite the Pan American building. A mini paradise built in 1976 to honor the Bicentennial (on land originally slated for a parking lot), this choice spot is planted with maple trees and beautiful azaleas around a giant man-made pond.Here, urban beach-dwellers can ponder the movements of an occasional cloud or squint at the progress of the meandering kite. Hours pass and the flirtation with burns and blisters begins.

But there's relief in sight. The refreshment stand at Constitution Gardens offers pink lemonade, draft beer, peanuts and popcorn, plus more substantial fare (for those undaunted by their binini figures).

Prefer an island feeling? All along East Potomac Park there's plenty of grassy beach open to sea breezes -- that is, gusts from the Potomac, the Anacostia or the Washington Channel. Hains Point is the closest approximation to an oceanic beachfront within cycling distance of the Capitol: aIt has actual seegulls crusing over Ohio Drive. Bathing beauties join the hordes of bikers, fishermen, picnickers and dogs on the expansive point.

As for the view, you have the choice of postcard scenes: the Potomac panorama, National Airport and the Arlington skyline; the Southwest waterfront and boat marina or Fort McNair on the south. Of course, your eyes will be closed to the sights anyway during the crucial tanning hours (experts say 11 to 2), but if the unthinkable happens and the sky turns gray, you can climb aboard the Lightship Chesapeake, a floating lighthouse built in 1930 that's now an environmetnal center moored in the channel and open for free tours.

Miss the waves? A quick hop across the 14th Street Bridge brings you to the beautified beachfront of Lady Bird Johnson Park. Smell the salt sea air -- or is that someone's carp, too long in the sun? Sunbathers camp out beneath the statue of waves and birds, a memorial to the nation's seafarers who, "upon the waters of the world, gave or give their lives in the performance of heroic deeds." The only heroics going on nowadays, however, are the possible cases of prickly heat risked by unflinching volunteers.

The problem here is parking. It's best to arrive at Lady Bird's beach via the bike path from National Airport, where there are fringe lots available. Otherwise, expect to fight for one of the precious dozen spaces provided along the shoreline, or make the treacherous crossing to the Columbia Island Marina parking lot across the parkway.

Moving southward, on the way to Alexandria's beach at Bellhaven, the D&S Food Store is the last stop for life-support systems: wine and cheese, soda, sunglasses, oils and creams. A few miles past Old Town is Bellhaven, a family beach with a Potomac River vista, picnic tables, grills and two big parking lots. Tireless tanners usually arrive early, even before the peak sun hours, and get situated on reflecting mats and towels. Some bring lawn chairs and inflatable platic pillows. All meditate, eyes closed, on the matter at hand: bronzing. A steady stream of bikers wheel past, riding the eight miles to Mount Vernon, but the hardcore beach bums aren't distracted.

Several narrow beach spots at the tip of Old Town also are crowded with swimsuited bathers who ignore the browsers on King Street. (Hot tip for parched throats: Before settling in, check the Deli on the Strand, between Duke and Prince Streets on Union, for take-out goodies including cold Dr. Brown's sodas and Blue Nun miniatures.)

For strict urbanites, Montrose Park, next to Dumbarton Oaks in Georgetown, offers its share of sunny turf. Gourmet picnics from the French Market on Wisconsin Avenue provide relief after hard hours in search of the perfect tan -- except on Sundays, when the market's closed. Or you can sample the exotic lotions and soothing oils at the Body Scenter, farther up the street.

Heading eastward to Dupont Circle, you'll find the chess players aren't fazed by the sprawling bodies surrounding the elaborate marble fountain. When it gets hot enough, you can pretend the splashing water is an extension of the Atlantic. A lot of neighborhood kids do. In fact, a number of characters here might as well be from Steel Pier -- crapshooters, conga musicians, Frisbee champions, jugglers, peddlers and the usual assortment of local freaks.

Of course, you can melt away at any number of smaller, more secluded city beaches like Glover-Archbold Park on Massachusetts Avenue and Macomb Street; Battery-Kimble Park off Foxhall Road, or tiny Muhlenberg Park off Connecticut Avenue at Ellicott Street. All are public patches of grassy land and sunny exposures, and all require only a bit of imagination to fill in the ocean spray, conch shells and saltwater taffy.

So why make a six-hour commute? As a friend used to say about Atlantic City, "take away the ocean and what have you got?" Another scorcher of a weekend that's well spent at an urban beach.