Because of the vagaries of Washington's weather, this summer has been among the least polluted here in recent years.

The Metropolitan Washington Council of Governments has not announced any air pollution alerts thus far in 1978, though it has reported 11 days on which its air quality index has climbed to 100 or higher, a level officials termed "very unhealthy."

Not since 1974 have there been so few days when COG's index reached 100, and not since 1971 have so many days gone by without an air pollution alert. (In 1974, only 11 days reached or surpassed 100 on COG's index, but there was one four-day pollution alert that year from July 8-11. In 1971, the first pollution alert was announced on July 22).

National Weather Service forecasters attribute this year's comparatively pollution-free early summer here to an unusual weather pattern. In broad terms, they say, cold air from Canada continually has blown across the Washington region, preventing air stagnation here and causing rainfall that helps cleanse the sky of pollutants.

Nevertheless, environmental researchers and planners warn that Washington's more customary pollution may yet blanket the area before the summer has ended, and they express certainty that severe pollution will engulf the region, at least intermittently, for years to come.

The Chesapeake & Potomac Telephone Co. announced yesterday that it would start including COG's daily air quality index reports as part of its recorded weather information, obtained by dialing WE 6-1212. Since 1975, C&P has included air quality reports in its weather message only when COG's index reached 100 or pollution alerts were announced. Now, index readings will be included in the weather message starting at 10 a.m. on weekdays and 1 p.m. on weekends, a C&P spokesman said.