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Fog: Why it's been a soupy December

Philly region getting a heavy dose of a Carl Sandburg favorite -- fog.

If you have the sensation that you've been driving through the clouds on several recent mornings, don't worry, no one is spiking your coffee.

For at least the eighth day this month, fog settled upon parts of the region again Thursday morning, with visibilities dropping to a quarter-mile in Blue Bell.

Officially, "dense fog" – defined as visibility under a quarter mile --has been reported twice so far at Philadelphia International Airport. That's already above the monthly average.

And, yes, it is related to the very strange December weather that is about to resemble April's.

One factor has been the dominance of Pacific air across the country, says Paul Pastelok, the long-range forecaster at AccuWeather, in State College, Pa., where the fog Thursday morning resembled gray soup.

"I've been here 20-some years now," he said. "This is the worst fog I've seen in December. You literally couldn't see to make a right turn.

Missing so far have been those drying, fog-chasing winds from the northwest.

Instead, the mild air from the Pacific, light winds, low sun angles, longer nights, and unusually high dewpoints – the temperatures at which water vapor magically comes out of hiding – have conspired to generate a harvest of ground clouds.

Complicating matters has been the warm air aloft. Warm air rises over cooler air, but when it encounters a warmer layer it gets trapped; that's an inversion.

Essentially, the clouds of fog get trapped near the surface for several hours before the weakened pre-solstice sun can burn them away.

And that increases the likelihood that you'll get stuck in traffic.