Tourism in Washington is up sharply from last year, according to industry officials who credit the increase to improved economic conditions that make a trip to the nation's capital once again possible for many who had to forgo vacations during the recession.

Linda St. Thomas, a press spokeswoman for the Smithsonian Institution museums, said yesterday that attendance this year may surpass the all-time record of 24 million visitors set in 1976, the bicentennial year. Attendance for the first seven months of 1983 was up by more than 14 percent over the same period in 1982, which was a poor year for the Smithsonian, she said.

Occupancy at Washington hotels and motels is up 4 percent this year over last year, said Leonard Hickman, executive vice president of the Hotel Association of Washington. And spokesmen for both Tourmobile Sightseeing and Gray Line Sightseeing Tours said the number of passengers this year has increased by between 3 percent and 4 percent over last year.

"We're sold out constantly. Pretty soon we won't have enough buses," said Louise Snyder, a press spokeswoman for Gray Line.

Many tourism officials credit the economic recovery with the upturn in the city's vital $1 billion tourist trade, which fell off sharply last year when the nation was mired in a deep recession.

"The economy is loosening up, and more people feel they can take that vacation that they have had to put off. It's as simple as that," said Phillip Wagner, manager of the Holiday Inn downtown on Rhode Island Avenue NW.

Tom Mack, president of Tourmobile Sightseeing, said that "the desire of people to travel is up. The economy is giving them room to breathe and take those trips, whether big or small."

The improved economy was a major factor for the Stell family from Redline, Pa. They licked red, white and blue popsicles in front of the Lincoln Memorial yesterday during a pause in the midst of a one-day sightseeing tour of Washington.

Brad Stell said that for years he had wanted his three sons to see the changing of the guard at the Tomb of the Unknown Soldier at Arlington Cemetery, a ceremony that Stell saw as a boy and never forgot. But only this year, said Stell, has the family had the money to make the trip to Washington.

"We came here because moneywise we can do a lot and there are so many places to see," said Connie Stell, who added that the family still cannot afford to take long trips but could manage the drive here.

Tourism industry officials say that the number of families touring is on the increase. To meet their needs, Wagner said, hotels are promoting more family packages and discount rates to boost occupancy in the city.

According to a Smithsonian study, most tourists visiting Washington come from the northeastern part of the United States. They usually drive here, coming from within a 500-mile radius of the city.

"There are many sites right around you that can be enjoyed without the expense," said Marie Simon, who drove from her home in Breezewood, Pa. "Otherwise it becomes too expensive to travel."

Simon and her sister, Genene, were standing in line yesterday to tour the Washington Monument. "When I drive, I can save a little to splurge on souvenirs. It's the only way I can seem to make my money stretch," she said.

Simon also credited the economic recovery with her opportunity to travel. "At about this time last year, I couldn't afford to go anywhere. This year, I had a little bit more money. Someone up there must have been watching out for me," she said.

At the the Smithsonian's Air and Space Musuem, one of the world's most popular museums, there were 1.4 million visitors this July, up from 1.3 million in July of last year, St. Thomas said.

At the Hirshhorn Museum of Sculpture and Art, she said, attendance was up by 20,000 visitors for July over the same month last year. At the Museum of Natural History, July attendance was 695,000, up from 668,000 for July of last year; for the Museum of American History, July attendance was 662,000, up from 567,000 last year, she said.