Behind 30 hours of fog: warm air moving over cold ground, "advection fog" Posted at 12:04 PM ET, 12/10/2012 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. Note: advection sounds like a fancy word, but it really just means moving air. Left: Temperatures at 11 a.m. Notice the cold air to the north (mid-40s in south central Pa.) and warm air to the southeast (near 70 in Salisbury, Md.). Right: Weather map at 9 a.m. shows warm front draped over central Va. - wedge of cool air is north of front. Fog forms on the north side of the wedge/front - where cool air and warm air intersect near the ground. (National Weather Service) The website Haby's Hints provides a somewhat deeper explanation: The setup for advection fog will often include an advection pattern bringing in warmer and more moist air from the south. The set-up for the ground surface will be a snow covered ground or a saturated ground that has been chilled by cold temperatures before the winds shift back from a southerly type direction. The circumstances described are exactly those we've had in the D.C. area. The cold air near the ground has come courtesy of cold air damming, where high pressure to our north and its clockwise circulation has driven a wedge of cool air southward. At the same time, warm air has been trying to make a push northward. The intersection of warm air with cold air has resulted in condensation, producing 33 hours of fog in the last 38 hours at D.C."s Reagan National Airport. Related: Fog blankets D.C., cuts visibility This afternoon, as the infux of warm air strengthens, the lingering chilly air near the surface will weaken (or get mixed out) and the fog will burn off. That's already happening, as visibilities have improved from just 0.13 mile (220 yards) at 1 a.m. to about a half mile at noon. Check out the scene to our west along the Skyline Drive and the adjacent Shenandoah Valley where fog has lifted... Web cam image from Big Meadows along the Skyline Drive looking into the Shenandoah Valley around noon. National Park Service