When the mercury reached its high of 86 degress here yesterday and began sliding slowly downward, Washington weather history was made.

This year, for the first time in the 108 years that records have been kept, the city passed July 9 without th thermometer ever reaching 90 degrees, the popular symbol of summer and sweltering.

Civil War hero Ulysses S. Grant was in the White House in 1871, the year record-keeping began, when the city experienced 189 days of below 90 temperatures before summer asserted itself on July 9 and the mercury broke through to 91.

The traditional sentence of heat and humidity usually imposed by summer on the city has been stayed this year by what weather forecasters describe as an unusual movement here of cold air from Canada.

Winds that sweep across middle and upper Canada at altitutdes above 18,000 feet have this year taken a more southerly course, favoring Washington With the refreshingly cool air whose blessings are normally restricted to those in more northerly latitudes.

Of course, Washington cannot yet be described as the icebox of North America. Although the temperature has yet to reach the magic 90 mark, it has reached 89 - on two days, June 10 and May 10.

In June, the average temperature was only about two degrees below normal. Yesterday's high temperature of 86 was only two degrees below the normal high of 88. What has distinguished this summer, however, according to forecasters, is the lack of the usual hot spells and heat waves.

That lack appears to be reflected, utility spokesmen say, in the decline this year in the use of air conditioning.

Potomac Electric Power Co. bills for July "are going to be lower." said company spokesman Dawn Fleming.

At the pool at the District's Francis Recreation Center, assistant manager Alvin Wilson doesn't need a thermometer to know the temperature is lower.

On many recent eveings, he said, only about 25 persons have been using the 50-by-25-foot pool, which in the past "was filled."

No 90-degree-or-more day has been recorded here since last Sept. 19. On the day the termperature reached 94. The next day, the high reading was 79 and it hasn't reached 90 since.

Moreover, forecasters say they don't know when it will. The outlook for today and the next four days envisions no 90-degree readings.

But forecasters have no doubt that we will see 90 degress again."We always do," said forecaster Jeffrey Bowman. CAPTION: Picture, Joggers, like these resting and running on the Monument grounds, have found this year's weather ideal. By Frank Johnston - The Washington Post