If you sense that a cloud has been hanging over our lives recently, that is no illusion.

For the fourth consecutive morning on Friday, dense fog — that which reduces visibility to a quarter-mile or less — was detected officially at Philadelphia International Airport, the National Weather Service reported.

And we have a shot at making it five in a row Saturday morning, said Mike Gorse, meteorologist with the weather service office in Mount Holly, with more fog creeping in “on little cat feet,” to invoke Carl Sandburg’s memorable metaphor.

Fog banks basically are very low-level clouds composed of countless, suspended, all but weightless water droplets.

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But along with repelling sunshine and darkening moods, fog also is a notorious safety hazard for all forms of transportation.

Government data for the 20-year period ending in 2009 cited it as a factor in about 10,000 fatal crashes.

'Tis the season

October, a bridge period between the warm and cold seasons, historically has been the most fog-prone month in Philadelphia.

A big reason for that is that the ocean water temperatures are still chilling from the heat of summer, said Paul Walker, senior meteorologist at AccuWeather Inc.

For example, the surf temperature off Atlantic City on Friday morning was 66, and 67 on the bay.

It so happens that mornings this week have featured weak winds with an easterly, onshore component, luring some of the moist maritime air landward to interact with the cooler surfaces on the mainland.

Conditions have been ideal for low-level cloud building.

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What’s more, inversions — that’s when temperatures above are warmer than temperatures below — have formed overnight, trapping the clouds at the surface. To rise, warm air needs cold air above it.

The puny winds have been almost no help in dispelling the fog. The sun hasn’t been much help, either.

The sun’s ray are becoming ever more oblique and weaker and they aren’t doing a very good job of burning it off, Gorse said.

The end is near

A cold front is due to plow through the region Saturday, which should be the last of the recent run of 70-plus afternoons. Sunday will feel November-ish, with highs only in the 50s.

The front also should extinguish the fog. So on Saturday, Walker said, we’ll have one more opportunity “to walk into the clouds.”

It is unlikely that they’ll be missed.