Washington — At Dulles International Airport, long ranks of cars sporting ''Member of Congress'' license plates sit patiently in the VIP parking lot, waiting their owners' return from home districts.
Watergate's Jean Louis restaurant, where dinner for five is the cost of a small mortgage payment, is closed until after Labor Day. Too many expense accounts are out of town.
Motorcade sightings are down substantially, according to unofficial observers. Rush-hour traffic is light enough to be only marginally annoying.
Washington, unlike Paris in so many other ways, shares with the French capital a quasi-official August torpor. By common consent, it seems, all important politicians, bureaucrats, and lobbyists leave town at the same time. Presumably, they flee secure in the knowledge that nothing important will happen while they are gone, since everyone who is anyone is also on vacation.
President Reagan is on his way to Santa Barbara, where he will ride horseback and perhaps clear a little brush. Members of Congress, who spend most of their free time running for reelection, are back home chucking their districts under the chin. Many lobbyists and news media chiefs are on Nantucket polishing their tans.
But it is a little-remarked fact that Washington not only survives, but thrives in August, like a matron who kicks off her sensible shoes and runs barefoot in the grass.
This August has been particularly pleasant, as a ''cold'' snap has sent temperatures plunging into the mid-80s. There are free concerts everywhere: Fridays, military bands play at the Jefferson Memorial; Sundays, there is jazz by the canal in Georgetown.
Beds of marigolds edge city parks with orange. At dusk in the Washington Cathedral garden, fireflies, like tiny Christmas lights, flicker against dark green boxwood.
And the city is far from empty. While lawyers pour out of town, tourists are pouring in. The visitors, for the most part, do not care that the government is shut down. There are more important things to see: the set of ''M*A*S*H,'' for instance.
Two weeks ago, the Smithsonian's Museum of American History opened a small exhibit of relics from the television show ''M*A*S*H,'' an action the curators probably now regret. As of 3 p.m. on Monday, 30,391 people had lined up to see the dressing gown of Nurse Margaret (Hot Lips) Houlihan, Corporal Klinger's Toledo Mud Hens cap, and ''The Swamp,'' residence of Capt. Benjamin Frankin (Hawkeye) Pierce. The pace of attendance sets a record for any Smithsonian special show.
The line to get in the exhibit is often hours long. One evening earlier this week it stretched down the hall, wagged a few times, and ended in front of the giant flag that once flew over Fort McHenry. A tourist couple, pulled along by two young daughters, warily approached the end of the waiting crowd. They were from Michigan, and weary. ''All we've heard from the girls all day is, 'Is this the building with ''M*A*S*H'' in it?' '' sighed the mother.
Inside the exhibit, the prime topic of conversation proved to be which cot in The Swamp belonged to Hawkeye, and which to his stuffy roommate, Maj. Charles Emerson Winchester III.
A woman from West Virginia, apparently deep in thought, paused before a placard discussing the show's antiwar themes. Turning to her daughter, she wondered aloud, ''Which ran longer, 'M*A*S*H,' or 'Bonanza'?'' (For trivia buffs , it's the latter.)
Of course, a true upper-crust Washingtonian wouldn't dream of visiting a place so tourist-swamped as the Museum of American History. Residents tend to frequent the quieter, non-Smithsonian art museums, such as the Phillips (now closed for renovation) and the Corcoran.
If you think this reflects sophisticated taste, consider that the Corcoran's big draw this summer has been an exhibit featuring funny photos of a dog. The show, of works by New York video/photo artist William Wegman, ''is swamped all the time,'' says Robin Asleson, a Corcoran education department employee.
Man-Ray, Wegman's now-departed Weimaraner, modeled for many of the exhibit's photos and film clips. In the name of art, Man-Ray is shown dressed as an elephant, wearing high heels, wrapped in gold tinsel, and covered with flour, among other things. All the while, the noble dog stares soulfully at the camera, as if to say, Why me? If Rembrandt had painted a dog's face, it would have looked like Man-Ray's.
The Wegman exhibit closes August 28. Those who fled Washington will never know what they missed.