The Museum of Natural History loomed in the background, its pterodactyls a magnet for the gawking throng, while on the sidewalk outside Danny Avrutick played Bach on his fife for a knock-kneed youth in an "I'm a Washington Play Boy" T-shirt.

The conditions were perfect yesterday for that phenomenon of nature known as tourism in Washington.

Between the big spike in honor of George Washington and the big thimble where laws are made, a disparate body of visitors and regulars frolicked on the Mall. Shirtless men lay supine, snoring their satisfaction under a sun that warmed to a thermostatically perfect 72 degrees. Teen-aged girls in jumpsuits tittered and bopped while the boys accompanying them swaggered, yapping.

And they're doing it in big numbers. There are already indications that a lot more tourists are going to be here this year than last year.

More is not always more, however. Cabbie Ronald R. Jones observed, dismally, that part of the tourist season is given over to cheapskates. "The DAR ladies and the older ladies who come for the cherry blossoms, I call them the golden grandmothers . . . . Then you get the kids. They spend their money on junk. That's what they spend their money on. They ride buses. They don't get in my cab."

From golden grandmothers to tacky teens, the Washington tourist crowds number more than 12 million a year, according to Marie Tibor, a spokeswoman for the Washington Convention and Visitors Association. And that's not counting all the visitors who come here for business or to glom onto government. That's 12 million people in pure Kodak country. These tourists spend a billion dollars in the Washington economy, D.C.'s Chamber of Commerce says, making tourism the second largest industry here after the federal government.

No estimates are available for the number of tourists in the city this weekend but Tibor said April is normally the third best month for tourism, after July and August. The weekend weather forecast calls for increasing cloudiness today, with a 50 percent chance of rain by nightfall; Sunday is expected to be cloudy with scattered showers.

"We're having a very, very good tourist season," said Tibor. "Smithsonian visitor inquiries are up 31 percent over last year. The Tourmobile bus was doing so well that they opened their Mount Vernon tour a week or two ahead of the normal . . . . The industry executives out there are very, very positive about this tourist season."

Not everybody is gleeful, though. For some Washington residents, the funny-looking folks from out of town are a big pain in the Mall. They crowd the restaurants, hog the parking spaces and are liable even to trample a complacently snoring resident who thinks he's found peace between the big spike and the big thimble.

Dave Uhe, 16, a Sioux Falls, S.D., high school student, noted that his visit to Washington so far has been pretty swell -- "It's big. It's pretty impressive" -- but he has noticed a certain shortness of temper among the locals.

"Most of the people are pretty nice," he said, "with the exception of a few people who are working in the stores. They get pretty snippy at this time of year, I guess. It's high tourist season and they're under a lot of pressure."

Uhe said he and his friends went to see "42nd Street" at the National Theatre and were instructed to proceed to the third level. They mistakenly went to the second level and got chewed out by a tourist-weary native.

"She said, 'Go to the third level. Whatsamatta, can't you count?' " Uhe recalled.

Ah, well, with all those paddle boats plying the pretty Tidal Basin, the flowers in bloom around the Capitol and the sun shining brilliantly on a post card-perfect day, why dwell on anomalous rudeness?

One large tourist in cut-off sweatpants, splayed out on the steps of the National Gallery, had no beef with the world on Friday. He donned his dark shades, leaned back and fell asleep.

No beef, either, from Sue Walrath, one of 150 people touring here with the Mohawk, N.Y., Cub Pack 44. "Breathtaking," she gasped, standing in front of a packed Tourmobile bus. "You can feel your heart speed up so you're actually breathless."

Next to Walrath was 13-year-old Billy Thomes with his Starship Enterprise model, recently purchased at the Air and Space Museum, and his dad, Bill Thomes. Thomes Sr., the designated "cub master," told an inspiring story of how the cub pack raised $7,500 selling stocking stuffers and M&M candies so they could come to Washington and buy ro- cketships, T-shirts and hats that say "Space Shuttle."

Judging by the pageant on the Mall, it is a given among the young that touring means buying. What do they buy? According to George Lucas, veteran vendor along Constitution Avenue, they buy "T-shirts. Same thing, always is. Yeah, glasses sell pretty good, too."

Rich Hahn, 14, of Reading, Pa., bought himself a super-appropriate souvenir of Washington, a Poo Poo Cushion that he was operating at high volume on the sidewalk Friday. Rich's friend, Steward Willman, 15, was sporting handcuffs on his belt and glacier glasses over his eyes.

Glacier glasses feature a distinctive flap at the sides, designed to bar the glare of the sun. For use in Washington, though, they offer the complete elimination of peripheral vision -- always a plus in a crowd when your friend is operating his Poo Poo Cushion.

Tourism in the nation's capital: a phenomenon of nature.