SO FAR THIS YEAR, summer has been on its best behavior. Perhaps it will strike you as unwise to call attention to the city's good fortune this early in the season, with so many opportunities still ahead for heat waves, inversions and air-pollution alerts. But we believe in giving credit where credit is due. Frequently, in the past, when we have complained about Washington's weather, nothing whatever has been done about it. But this year has been markedly less oppressive and smoggy than usual. The system has responded to our grievances, and we want to take note of it.

Speaking of air pollution, you may have noticed that the air in and around the city has been relatively breathable in recent weeks. Compared with the usual atmosphere in June and July, it has seemed pristine. We asked some of the experts whether the efforts to cut air pollution here and throughout the country aren't beginning to pay off. The experts reply that you can't prove it yet. It's true that the amount of exhaust pollution per car is falling rapidly, but the numbers of cars, and the distances that they are driven, are rising. The opening of Metro's Blue Line has taken a lot of buses out of central Washington, but the rush-hour traffic isn't any less dense.The experts say that the relatively low pollution readings over the past month have been mainly meteorological good luck - winds that blew the right way, for once, and rain that came at the right moment.

But perhaps it does not strain optimism too far to suggest that all the expensive and elaborate new controls are at least keeping things from getting worse. There are the exhaust devices on the cars; there are the smaller cars with lighter engines; there are the devices to clean up the smoke from the utilities' generating plants; there is Metro. For a decade, summer air quality here was getting steadily and dramatically worse. It is now possible to hope that, at the least, the city may be approaching the turnaround.

A couple of years ago Congress passed the Weather Modification Policy Act, which established - did you doubt it? - the Weather Policy Modification Board. Last week the board published its report. By the 1990s - contingent, of course, on somewhat more generous congressional appropriations - it may be possible for the weather modifiers to diminsh the force of hurricane winds, and to increase snowfall in certain areas. But increasing the snowfall in Washington is not likely to be high on your list of priorities, unless you are 10 years old and have a sled.

We support the proposed Prevailing Winds Act, under which it would be declared national policy for the wind to blow on Washington mainly from the east from June 15 to Sept. 15. That would constitute another important element in President Carter's energy program, since an east wind would reduce the need for artificial air conditioning, in turn alleviating the country's dependence on foreign fuel oil. It would bring the city a succession of cool, gray days like those that we've had in the past few weeks. The land would grow as green as Ireland. It would be good for roses and, we venture to say, for dispositions.