Many Washingtonians who took their cue from the thermometer rather than the calendar shed their coats and enjoyed themselves in brilliant sunshine yesterday as the mercury reached 60, probably for the last time in the 1970s.

Temperatures are expected to slide down toward the 40 degree range -- normal for this time of year -- as 1979 ends and the New Year begins, according to the forecasters of the National Weather Service.

As they saw it yesterday, the 1980s will be inaugurated on a day of rather routine weather conditions.

The high temperature, according to forecaster John Forsing, will be in the low 40s, right around normal, and well below the record of 68 degrees recorded on New Year's Day in 1966.

Nor is there any immediate indication that the low will plunge anywhere near the 14 degrees below zero recorded in 1881. That was only one degree above the city's all-time low of 15 degrees below, recorded Feb. 11, 1899.

As currently envisioned, Tuesday will be neither dark and gray nor bright and sunny. Ambiguous and equivocal might be apt descriptions if they were used to characterize the weather.

Clouds will arrive in the area late tonight, and are expected to linger through Monday, but they "should be breaking somewhat" on New Year's Day, said forecaster Forsing.

No rain or snow is expected here at least through New Year's Day. December 1979 has been a dry month, and is expected to end that way.

So far, only .85 inch of precipitation has been recorded at the official measuring station at National Airport, well below Washington's December average of 3.04 inches. There have been only three drier Decembers in the last 41 years -- 1965 with .47 inches, 1955 with .22 and 1938 with .71.

By contrast, four inches was recorded last year, 6.03 inches in 1973 and 6.54 in 1969.

Splendid as yesterday was, with bright skies, brilliant sun and little of the wind that had lashed the area earlier this week, the day's high temperature of 60 degrees was four degrees below the record for the date.

That record was set 86 years ago, in 1893, on a day when President Grover Cleveland, according to contemporary news accounts, returned to the White House from a leisurely three-day cruise on the lower Potomac River and the Chesapeake Bay.

"The weather," said The Washington Post for Dec. 30, 1893, "was beautiful."

Guns and ammunition were taken aboard the Navy lighthouse tender used by the president "in order that the party could defend itself if attacked by ducks," the Post's reporter wrote, "but the warm weather kept the bay and river clear of the webfeet," and not a shot was fired.

On his return to the White House, it was reported that cleveland "looked remarkably well and his complexion was considerably tanned."