In the early hours of Monday, in a city already facing uncertainty as it approached the looming fiscal cliff, things became decidedly more obscure and less clear cut.

It was the fog. Fog was settling over much of the Washington region. Fog was making things difficult to see very far or very high. It became a challenge to navigate, to be certain of one’s bearings, to spot landmarks and obstacles.

Just before 2 a.m., Monday visibility at Reagan National Airport had been cut to about 220 yards. That was even lower than the mile-and-a-half that was reported about 11 p.m. Sunday.

At Dulles International Airport, visibility was about a half mile. In Baltimore it was about a quarter mile.

In traffic camera video, automobile headlights appeared to be indistinct patches of light as they loomed out of the mist on the Capital Beltway’s inner loop near the George Washington parkway.

Fog was reported over a large part of Virginia, from Richmond to Roanoke to Leesburg.

The National Weather Service issued a dense fog advisory shortly before 11 p.m. Sunday. The District of Columbia and much of the surrounding area came under the advisory.

The weather service said the advisory would continue until about 9 a.m. on Monday.

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

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

In an explanation of what was happening, the weather service said that cool air was trapped near the ground. This is an inversion from what’s normal, in which air becomes cooler as elevations increase.

However, the weather service said some atmospheric developments could prevent the inversion from persisting long into Monday.