Some blame the weather for it; some say its just a fad that reflects a longing for the romanticism of old Europe and of leisure living represented by brightly colored umbrellas and potted plants.

The rising popularity of sidewalk cafes in the District of Columbia,usually considered a European invention, has baffled restaurant owners, who are unable to reason why eating amidst blowing dirt and dust seemingly unbearable heat and traffic noise can be so attractive.

In a city where the restaurant business is so competitive that only half the number of restaurants that open each year survive,restaurateurs are learning side-walk cafes can be the marketing edge between success or failure.

"Personally, I don't like to sit outside in the heat and smell the exhaust fans while I eat." said Fran O'Brien,owner of Fran O'Brien's at 1823 L Street NW.

"But outdoor cares help the restaurant business because they are big marketing factor in letting people know that this is a restaurant when they see others eating outside, and by attracting people who would come in on impulse,"he said."They (sidewalk cafes) can make the difference for some restaurants between making it or not."

The phenomenon of the sidewalk cafe in D.C. is a recent one.

In the late 1905s when Harry P. Zitelman presented his idea of opening the first sidewalk cafe here, his idea was initially opposed by city officials who said outdoor cafes would cause more problems than they were worth.

At a public hearing in 1961,the city's deputy highway director said the spread of sidewalk cafes would stop pedestrian traffic and would make people take to the streets where they probably would get run over.

, The fire chief said that,tables and chairs would get in the way of fire hoses and city sanitary engineers said street cleaning trucks spraying water might accidentally wet down customers.

, The public health director cited other hazards-blowing dust and debris in food, insect invasions and unwanted rodents pestering diners.

But it was the police chief who characterized the sidewalk cafe, now a seemingly innocent place for food among the flowers, as a "potential source of disorder."

He said pedestrians might brush up against patrons or vice versa, resulting in open-air fights and that side-walk cafes would provide a favorable setting for "ladies of easy virtue."

D.C. Commissioner Walter N. Tobriner however, was instrumental in pushing of commission approval of Zitelman's idea.

Tobriner said he couldn't understand how, with all the calamitous events foreseen by city department heads, Europeans have been able to operate sidewalk cafes for generations.

Today there are 85 sidewalk cafes. And these outside establishments, once considered the bane of D.C., have become a delight to many diners, a boon to businessmen and a growing source of revenue for the city.

Zitelman's 2 1/2-year battle with city government to allow him to open the first sidewalk cafe at Bassin's Restaurant, 14th Street and Pennsylvania Avenue NW, opened the door in the early 1960s to a European concept of open-air eating. In the past several years it has become almost as American as the apple pie some of the cafes serve.

At Fran O'Brien's sidewalk cafe recently, Kathy Gee sat sipping a drink while watching the late afternoon traffic and passersby and speculated tha the popularity of sidewalk cafe was due to a remembrance of the past.

"The remanticism of sidewalk cafes appeals to me," Gee said. "My boyfriend is in Europe now, and I can imagine him sitting in a sidewalk cafe in France or Germany. I think its sad that for so long, we've tended to disregard and forget the European culture."

, Joe McLaughlin, a waiter, said he has seen relationships patched up and couples get back together at the sidewalk cafe of P.W.'s Saloon.

At the Paradise Cafe one evening manager Tom Wilson said a man ordered three bottles of champagne when ayoung woman agreed to marriage.

"Sidewalk cafes have become places to go where there is a casual atmosphere where people can relax. By day, its a place for leisurely lunches and by night, a place to go to without the hassle of Georgetown dress codes, crowds or traffic problems," Wilson said.

L.W. McKibbin of the city's Public Space Permits and Records branch, the office that grants permits for sidewalk cafes, said while he considers the popularity of sidewalk cafes a fad, he doesn't think there is much demand for them.

"It's a spur-of-the-moment thing, "McKibbin said. "Competition has gotten tough in the restaurant business, and I have a lunch that sidewalk cafes are an attempt to expand and to relate to the crowd."

The sidewalk cafe, set up on property owned by the city, is a way of expanding the size of a restaurant without having to pay a substantial increase in rent, restaurant owners said.

The rent that the city charges is based on a complex formula of 4 percent of the assessed value of the property abutting the sidewalk times the square footage of the sidewalk space rented.

Last year, the city received about $450,000 in rent payments for private use of sidewalk space. Much of that income came from sidewalk cafes.

The sidewalk cafe at Kramer's bookstore at 1517 Connecticut Ave. NW, for example, occupies about 570 square feet of public space and was rented from the city for $1,225,5 0 this year. The rate was considerably cheaper than the charge for the combined bookstore,bar and restaurant inside.

John Piles, a spokesman for Restaurant Management Inc, whose partners own the Paradise Cafe, P.W.'s Saloon, The Greenery and Rocky Raccoon's, said the rental of outside public space for his firm's sidewalk cafes represents about 10 per cent of the restaurants' total rent during the year.

At the Paradise Cafe, the extra space outside nearly doubles the restaurant's seating capacity.

Restaurant owners said they would like to build temporary fixtures that would enclose sidewalk cafes for year-round use. Currently only a few sidewalk cafes are open beyond the usual late April-to-October season.

The Paradise Cafe recently installed heaters and plastic windows to provide protection and shelter during colder temperatures.

But the D.C. Public Space Committee in June ordered the cafe to remove the large windows in 30 days because they were not a part of the cafe's original design and because the committee considered the windows permanent fixtures, prohibited in the leasing agreement for public space.

Owners of the cafe have challenged the committee's order and the window remain, waiting a decision by the city's Department of Economic Development.

Other cafe owners said they also would like to modify their cafes for year-round business.

"I was thinking of enclosing my sidewalk cafe, too," said O'Brien. "But with this new ruling, that idea was stopped."

Piles said a study by his firm indicates the city could earn $5 million to $10 million each year in rentals and residual benefits, including increased taxes, if sidewalk cafes were allowed to have some type of enclosure.

Regardless of the Department of Economic Development's ruling on enclosures for sidewalk cafes, owners expect the rising trend to continue.

"I fought for sidewalk cafes for nearly three years, and now everybody is coming in on the gravy," said Zitelman, who retired and sold Bassin's in 1976.

"When restaurant owners asked me how my cafe was doing in the 60s when there were few of them, I told them then that it was no big deal and that the cafe wasn't doing well.

"I wasn't being honest, I admit, but I didn't want any competition," Zitelman said. "There is only one thing now that I resent," he said. "When I go to a sidewalk cafe and order a beer, they make me pay for it."