The scorching heat and unbearable humidity of summer in Washington are here, but something is missing: tourists. While the city stews, unprecedented numbers of its air-conditioned hotel rooms are empty.

A spokesman for the Hotel Association of Washington said yesterday that some hoteliers may be approaching the breaking point. Occupancy for the first five months of the year, said Leonard Hickman, is off more than 11 percent.

Fewer than 63 percent of the city's more than 16,000 hotel rooms are filled, the lowest level since the association started keeping records in 1946.

"I'm sure nobody is going to announce they're going to go into bankruptcy in December until they do it," Hickman said. But, he added, "How long can this go on? Some of those hotels are spending a fortune in advertising."

Mayor Marion Barry noted the downward trend in city tourism at a press conference yesterday, saying that potential visitors have less money to spend on vacations to the capital city and blaming that on the Reagan administration's economic policies.

Hickman and others in the city's tourist industry agree, saying that harsher economic conditions and erratic weather this year have conspired to dampen the tourist season.

"I imagine everybody has the same problems, with money and gas and all that," said Rita Bobowski, press spokesman for the Smithsonian Institution's Air and Space Museum, one of the world's most popular museums, where attendance this year is down more than 15 percent. "I don't think we have a handle as to whether it's the local people or the out-of-towners" who are visiting the museum less, she said.

Attendance at all Smithsonian museums is off more than 10 percent, from 9.1 million visitors during the first five months last year to 8.2 million in the same period this year. Smithsonian spokesman Lawrence Taylor said the attendance drop has not adversely affected museum programs or caused layoffs among staff.

According to Hickman, however, hoteliers have "laid off more people than they ever have. They've cut every cost they can cut and still stay open."

Many hotels are advertising special rates and package deals in an attempt to stave off financial disaster, Hickman said. Others in the industry similarly are making unusual promotion efforts.

Tourmobile, the company that transports visitors around monuments in its fleet of 22 blue-and-white articulated buses, for the first time has begun running television advertisements. Business there is off 11 percent as of the first of the month, down nearly 600,000 passengers from last year.

Company spokesman Phyllis Tate said an unusually cold winter and springtime snowstorms made a tenuous economic climate even worse. Tate and others are hoping the business decline will at least level off during the rest of July and August.

"We're not panicking here. I don't see the end of tourism on the horizon," Tate said.