"The fog comes on little cat feet. It sits looking over harbor and city on silent haunches and then moves on."

Poet Carl Sandburg knew what he was talking about. This fog seemed to appear out of nowhere last night. It socked us in overnight, and is taking its sweet time burning off this morning. It has affected much of the Southeast, forcing ground stops at airports across the region, including BWI, while air traffic controllers wait for the visibility to improve. School systems on the Eastern Shore delayed their openings this morning. Dense fog advisories remain in effect west of the Chesapeake Bay.

National Weather Service meteorologist Luis Rosa, out at the Sterling forecast office, explained that yesterday's rain contributed a lot of moisture to the atmosphere. More humidity moved into the region on southeast breezes from the Chesapeake Bay and the Atlantic Ocean.

After the cold front passed by, skies cleared and, as night fell, radiational cooling brought air temperatures down to the dew point - saturation. That's when the water vapor in the air began to condense, forming droplets and - fog. As I look at our weather instruments here at The Sun in downtown Baltimore, the temperature (at 10:25 a.m.) is 43, and the dew point is 43. So the humidity is 100 percent. And it's still quite foggy around the Maryland Penitentiary.

What we need now is solar heating. As temperatures warm, the fog droplets will begin to vaporize, and the fog banks will "burn off." That should leave us with a sunny, mild day, and a fine weekend.