The Washington Post - 12/10/12, Jason Samenow

D.C.’s historic monuments are barely visible from less than a mile away at 11 a.m. Monday (National Park Service)

Since Saturday night, widespread fog has covered large parts of the Washington, D.C. metro region. The explanation is pretty simple: the ground has been relatively cold compared to the air trying to sneak in from the south.

We’ve had a prolonged episode of what’s known as “advection fog”. It’s caused by “the horizontal movement of warm moist air over a cold surface” says the National Weather Service [2]. Note: advection sounds like a fancy word, but it really just means moving air.

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Fog blankets D.C., cuts visibility [4]