Many landmarks and monuments, hotels and houses, bridges and boulevards in the Washington metropolitan area appeared to vanish for much of the day Sunday, as a blanket of impenetrable fog settled over the region.

At 7 p.m. Sunday, the National Weather Service issued a dense fog advisory that remains in effect through 9 a.m. Monday.

In the advisory, the weather service warned that “poor visibilities will make driving hazardous through early Monday morning.”

According to the weather service, the advisory meant that visibilities would frequently be reduced to less than one quarter mile.

It advised motorists to slow down, use low-beam headlights and “leave plenty of distance ahead of you.”

However, the weather service said matters would likely improve by mid-morning Monday. It said it expected that a cold front moving through the area would banish the fog.

Of course, the fog that hung over the area on Saturday night was expected to dissipate at some point Sunday. But it did not seem to work out that way.

Throughout the day on Sunday, visibilities often fell to a quarter mile or less, as reported by weather service observation sites at airports around the Washington region.

At times, visibility was restricted to about 200 yards.

Those on the Washington side of the Potomac River near the Kennedy Center could barely see the trees on the opposite bank. The buildings of Rosslyn were invisible in the mist.

At Reagan National Airport, visibility dropped from a quarter mile at 10 p.m. to an eighth of a mile at 11 p.m. However, the fog lifted a bit by 1 a.m. to give a visibility of a mile and a quarter.

Visibilities in the same range were reported for much of the day at Dulles International Airport.

The fog was not confined to the Washington region. It was widespread along the East Coast, affecting airports in Philadelphia and New York.

At 1 a.m. Monday, visibility at Philadelphia International Airport was given as a quarter mile. The same visibility was reported at that hour at New York’s Kennedy International Airport.