IT TOOK MONTHS of exhaustive, time-consuming and unnecessarily expensive research, but it was worth it. The outdoor dining habits of fair-weather Washingtonians can now be explained scientifically. The new proven theory is based entirely on two fundamental principles:

First: Whenever there's an abundance of good reasons to dine at one of Washington's more than 100 outdoor eateries (reasons such as pleasant weather, a peaceful setting, cold soups, etc.), it has been found that tourists and natives alike will dine outdoors.

Second: Whenever there appear to be no sound reasons whatsoever, (i.e., loud traffic, subway construction at the next table, and Air Quality Index of 150, swarms of Killer Gnats, no cold soups, etc.), it has been found that tourists and natives alike will still dine outdoors.

To put all this technical jargon into laymen's terms: People in Washington generally dine outdoors whenever they feel like it, and they apparently feel like it often.

Michael Shreck, bartender at the Piccadilly (an English pub-type restaurant near the Chevy Chase end of Connecticut Avenue), was asked why some customers choose to eat their beef-and-kidney pie at Piccadilly's outdoor tables rather than in the climate-controlled intimacy within.

"Damned if I know," he said

Then, there's the American Cafe in Georgetown, perhaps the city's smallest - and oddest - outdoor cafe: two small tables, set in narrow well about three feet below the rest of Wisconsin Avenue, and just a few yards from the ever-busy intersection of Wisconsin and M. "On a nice day, people will come in and ask specifically to be seated outside," said manager Pittman Potter. "This surprises us."

Surprising is not exactly how one would've described outdoor dining in Washington 20 years ago. It was more like illegal. The Washington outdoor cafe wasn't born until 1961, when Bassin's won a long legal battle to overturn ordinances banning such frivolous behavior. Nineteen years later, Bassin's is still serving the masses at 14th and Pennsylvania. Nineteen years later, however, so is everybody else.

There are at least as many kinds of places to go as reasons to look for one. There is the classic European-style sidewalk cafe - where one sits at street's side, sipping wine and eating cheese and generally behaving continentally. You can choose the protection of a canopy, or an umbrella, or to sit unabashedly in direct sunlight (or moonlight). Other places offer a degree more solitude, with seating in a courtyard, or a back patio or porch. Most places offer light fare (soups, salads, sandwiches and such), but there are some that gladly will serve you full-course meals out there. Some outdoor eateries are frequented by pigeons, others by sparrows, and others by lonely-looking (and hence well-fed) beagles.

The attractions of taking a meal in the open air are limited only by one's imagination (and some establishments, you will find, inherently demand a more active imagination than others). The attractions are certainly not limited to the following; PEOPLE-WATCHING

Perhaps the best area in the city for the streetside visual feast is the 19th Street vicinity, downtown - variously hailed in recent times as the New Georgetown, the New Capitol Hill, the New 19th Street, and so on. Call it what you want, but the point is there's never a dull sidewalk. And there are more than a dozen lunch, dinner and late-night drinking spots that will serve you right out there in the midst.

The 15-stool outdoor bar at Rumours (formerly Paradise Cafe, at 1900 M St. NW) faces the aforementioned midst. At lunchtime, it is often filled with men of varying ages, dreesed in business suits or shirt sleeves and loosened ties, depending on the temperature. Rarely do these man bring along something to read - like the folks eating at the tables all around them, they keep busy by watching the passersby, who in turn watch them watching them, and so on into the night.

At night, people-watching is still a valid exercise in this neighborhood. And most of the places that will serve you lunch at noon, in fact, are probably hoping you'll return for a daiquiri at midnight.

Bojangles, disguised as the southwest corner of 21st and M Streets NW, is one of those places. On a street-level terrace above Bojangles proper (the rock music and mostly young crowds are found indoors at the bottom of a staircase descending from a square opening in the terrace), Bojangles' non-dancing customers can be found relaxing at the L-shaped bar and tables dotting the patio. "These people come to talk, maybe have a glass of wine, watch the people on the street," says manager Kevin Tracy. "The people downstairs don't come to talk. They're two completely different crowds." ALONE AMIDST

It is late afternoon, and the two of them are lounging under the canopy outside of Armand's Chicago Pizzeria on upper Wisconsin Avenue.

The young man and woman are quietly sharing giggles over a carafe of Inglenook white. A few feet away, several thousand Maryland-bound vehicles are sharing upper Wisconsin Avenue - not so quietly.

They have the look of love and the afternoon sun in their eyes, and both make them glow all over. The sun, however, also makes them squint.

The air is warm. It is also humid and laced with grit, pollen and assorted carbon derivatives. In the distance, soft rock music can be heard. In the immediate area, however, it's drowned out semi-regularly by passing Metrobuses and cement trucks.

Though they sit amid this sensory chaos, however, no one is suggesting that these two aren't happy. Nor that they aren't ever-so-patiently waiting the full 20 minutes for Arlington's specialty, the Chicago-type deep dish pizza. Nor that they probably would prefer, in fact, to wait 30 minutes or more.

They are obviously tuned to each other's wavelength, so to speak, and the interference be damned.

This peculiar human ability to manufacture one's own ambiance is displayed not only at Armand's, but at dozens of places similarly situated along such formidable four-laners as Connecticut Avenue (particularly above Calvert Street) and Pennsylvania Avenue (from Washington Circle to well past the White House). ALONE ALONE

About a block from the Eastern Market Metro stop, on Eighth Street SE, there's a little place called Patio Cafe. Customers are required to check their urban hustle at the door. Past the tables inside, past the upright piano, the bar and the kitchen, there's a back door that leads to a small, sunny courtyard dotted with Mexican-tiled tables and a few people, all of whom seem to be talking so quietly. You are surrounded on two sides by the walls of a furniture store, on another by the restaurant itself, and on the fourth side by a brick wall high enough to hide the yard of the woman who lives next door but not the wash she's hung to dry from the porch.

It's extremely easy to pause at Patio Cafe for a glass of wine and some light lunch and find yourself calling in sick at work for the rest of the day. GARDENS OF EATING

In Georgetown, where the sidewalks are as precious as the proverbial moderately priced house, Mr. Smith's on M Street has solved the problem - as have others - by putting its outdoor seating in a garden out back. Half the garden is enclosed, half is not. When it isn't raining, you can have sauteed mushrooms and quiche in either half. Feeding cottage fries to pigeons, however, is possible only in the open half.

The Iron Gate Inn, someone claimed, does not exist; it's just a cruel joke played on hungry travelers along N Street, merely a small signpost someone put out next to the headquarters of the General Federation of Women's Clubs. Wrong. The Iron Gate Inn is alive and well and serving Middle Eastern foods at the end of the wide-arched alley. Part of the restaurant is indoors, in what used to be the carriage house of General Nelson A. Miles (a figure, if you didn't know, of the Spanish and Indian Wars), but part of it is in a pleasant garden shaded by trees and the dome of St. Matthew's Cathedral. Before you complain that the salad is kind of rustic, make sure it isn't just a fallen leaf in the bread dish. AND SO FORTH

Suburbs and sidewalk cafes do not seem to mix. Outdoor eateries rely heavily on high-density environments and large numbers of pedestrians. Suburbs rely heavily on sprawl, and on the automobile.

"Gee, outdoor dining, eh?I don't know . . . I really can't think of anyplace," they said at the Montgomery County Chamber of Commerce. It was mentioned that the Olney Inn used to serve food and drinks out on the patio. It was also mentioned that the Olney Inn, unfortunately, burned down earlier this year.

There are several places to dine outside in Virginia - and more are expected to appear (particularly in higher-density jurisdictions such as Arlington and Old Town Alexandria), in the wake of the state Alcoholic Beverage Control Board's recent relaxation of its "drinking in public" laws to allow sidewalk cafes. One of the newest is Alexandria's Old Town Holiday Inn, where the colonial-style restaurant three weeks ago put a 10-seat bar and 52 tables out in the courtyard, in the center of the hotel complex on King Street. The fare is mostly salads, sandwiches and strolling musicians (at night), and a hotel spokeswoman says the business has so far "gone beyond our wildest dreams."

Gadsby's Tavern, over on Old Town's North Royal Street, has a garden out back for magnolia-side dining.On a nice day, the Alpine Restaurant (at Lee Highway and Glebe Road in Arlington) serves about half of its Italian fare to customers who sit in a side courtyard. And the Fish Market, down near the foot of King Street in Old Town, has a second-floor balcony that seats six - three tables of two each - and provides an unbeatable view of the Potomac, the streets below and the classical banjo performance inside.

It isn't possible to list the entire collection of D.C.'s more than 100 outdoor eating spots, but it is possible to list, in no particular order, the following non-scientific cross section.