The temperature fell to a pleasant - and record - low of 55 degrees early yesterday as the Washington area continued to enjoy an unusually cool month of June.

The official 55-degree reading at 5:40 a.m. at National Airport was one degree below the previous record for the date set more than a century ago in 1873. The normal low for June 26 is 67 degrees.

The cool temperatures and low humidity of the last several days are part of a massive highpressure system pouring fresh Canadian air into the upper Atlantic coastal area - an area frequently gripped by 90-degree heat and muggy humidity by this time of summer.

National Weather Service forecasters said temperatures should gradually rise during the rest of this week, reaching normal levels in the mid-to-upper 80s by tomorrow and Friday.

In the meantime, enjoy the cool. The month of June so far has maintained a daily temperature average of about 72 degrees, almost three degrees below the normal average for the month as a whole.

Fifteen of the first 25 days of June were below average, with early morning low temperatures falling into the 50-degree range a half-dozen times.

On the high end of the thermometer, the temperature has reached 80 degrees or more on only 15 of the last 25 days.Normally, the temperature reaches the 80s every day in June.

Another phenomenon: the temperature has not yet hit 90 degrees this year - even though normally it should reach 90 at least once in April, twice in May and eight times in June.

The highest mark so far this year has been 89 degrees, once on May 10, and once again on June 10. High daytime temperatures generally, however, have been far below that level, averaging in the upper 70s and low 80s. CAPTION: Picture, With a cool breeze blowing, John Rees of Great Falls sits wrapped in his serape and listens to the Army band yesterday at the Jefferson Memorial. The Washington Post