Dense fog may creep back Wednesday. It’s that time of year, and at least it isn’t wildfire smoke

No, that wasn’t more Canadian wildfire smoke you saw Tuesday morning, but plain, old fog, an evocative sign that it is indeed October and that Halloween isn’t far away, even if it will continue to feel like June during the afternoons this week.

“It kind of adds to the spooky feeling,” said Alex Dodd, a lead meteorologist at the National Weather Service Office in Mount Holly. Unfortunately, he added, it also can be quite dangerous, particularly this variant, and it’s likely to return Wednesday.

“I think it will be very similar to what we had,” said his colleague, Patrick O’Hara.

» READ MORE: This time of year fog can be a frequent visitor

In some parts of the region, visibilities dropped to about one-sixth of a mile around daybreak Tuesday, according to the weather service.

Although it felt summery Tuesday with highs racing past 80 and an encore expected Wednesday, conditions are expected to be ripe for fog in the morning, with temperatures falling into the 50s, the winds calm, and the air full of moisture.

Fog basically is a cloud that forms on the ground.

On windless nights, conditions are ripe for “radiational cooling,” in which daytime heating escapes into space.

» READ MORE: It will look like October in the morning, and feel like June in the afternoon

As the temperature drops, it approaches the “dew point” — the temperature at which water vapor is coaxed out of hiding and condenses into droplets.

In the case of fog, the droplets are tiny — about a thousandth of an inch (about as much snow as Philly had last winter) and all but weightless. A cubic inch of air can hold 10,000 or more of those droplets.

Once fog forms, it typically persists until the sun burns it away, unless the wind somehow beats the sun to it.

In October, dense fog is about twice as more likely to form than in September or in the summer months, based on weather service data.

Conditions in the air and on land can become ideal in the fall for those ground clouds as the nights become longer, said Dodd.

The cooler air interacts with water and land surfaces that still are harboring some of summer’s residual warmth, Dodd said, creating those vision-obscuring droplets.

Fog provides atmosphere for the season that we associate with gothic tales, cauldrons, and goblins, and is arguably preferable to the acrid wildfire smoke that was so prevalent over the region during the summer.

But fog can be downright dangerous.

A study by the AAA Foundation for Traffic Safety cited fog as a factor in 10,000 fatal crashes nationwide for the 20-year period ending in 2009.

This variant of fog can be especially hazardous because it can vary so much from place to place and is difficult to forecast. The splendid afternoons — the readings were due to reach the mid-80s Wednesday, and near 80 Thursday — belie what conditions might be like in the morning.

“It’s so localized,” Dodd said. “In Center City, it would be like what fog?”

Driving into a fog bank can reduce visibility to near zero.

“That variability almost makes it worse,” he said. “When it’s widespread fog, the threat is very visible.”