This marks the third day this month, and the second consecutive day, that dense fog was observed officially at Philadelphia International Airport.
Fog is a frequent visitor around here, but dense fog, defined as the stuff that reduces visibility to a quarter-mile or less, is less common than one might think.
On average, Philadelphia has experienced about 14 such days annually, 1.5 of those in November. Thus, Philly already has observed twice the monthly average.
While fog can be a travel hazard, it is quite a fascinating phenomenon. Essentially, it is a ground cloud formed by condensation droplets.
The droplets consist of gravity-defying lightness. When you drive through fog, you experience something similar to that of a pilot taking off or landing into an overcast.
What was different about today's variety was its tenacity, Carl Sandburg notwithstanding.
In Carl Sandburg's famous "Fog," the poet noted that after approaching on "little cat feet," the fog "sits looking over harbor and city, and then moves on."
In "The Love Song of J. Alfred Prufrock," T.S. Eliot's fog arrived in the evening, "curled once about the house and fell asleep." Presumably, however, it woke up in the morning.
That was not the case today.