Fog covered the Potomac River Friday (Jenn Vogel via Flickr)

At one spot in the Washington area, it got so cold this week that it set a record. At the same spot in the same week, it got warm enough Friday to match a record. The difference was as small as five days but as large as 71 degrees.

Startling as that swing was, the story of Washington's weather on Friday may have been something more difficult to describe with numbers. In fact, it was a symbol of indistinctness. It was the fog.

Whether white or gray, a blanket of fog covered waterways in the area. It concealed the surface of the Potomac River in the morning, and it persisted into the evening to bedeck the banks of the Anacostia River.

It "was kinda spooky," tweeted Taylor Jackson, who watched the fog roll in from the Anacostia near Nationals Park.

Science has its explanations: warm air laden with moisture, streaming over icy rivers, is a recipe for condensation and fog.

Less susceptible to the quantitative precision of thermodynamics was the intangible sense of beauty, romance and mystery created over the area by the cloak of fog.

Morning fog surged under the 14th Street Bridge and ran through the arches of the Arlington Memorial Bridge.

Meanwhile in the realm of the measurable, the top temperature of 70 degrees at Dulles International Airport tied a record for warmth there for Jan. 12. Only five days earlier, the low of minus-1 degree set a record there for the date for cold.