Why the Fall Brings Dense Fog, and Why That’s a Good Thing

Oct 15, 2013

Late September and early October are the foggiest time of the year in the Northwest, according to Cliff Mass, KPLU weather expert and professor of atmospheric sciences at the University of Washington.

Why is that? Mass says fall in the Northwest brings all the right ingredients.

“A few things are happening. Most of our fog here is radiation fog, and that’s associated with relatively clear skies that allows the Earth to radiate heat to space, and the Earth cools the atmosphere near it to saturation, and that produces fog,” said Mass. So here are the ingredients that trigger those dense-fog mornings. 

1. Longer Nights

“We have much, much longer nights now, so there’s a long period for the temperatures to cool down and the fog to form,” said Mass.

2. Rain

“Another thing is that we’re starting to get rain again, and that’s moistening up the surface,” said Mass.

3. Stable Air

There’s one other thing, says Mass: the atmosphere is more stable during the fall.

“And by that I mean the atmosphere doesn’t get mixed up as much, because we tend to have warm air above and cooler air near the surface,” he said.

1 + 2 + 3 = Fog Bragging Rights

It turns out our conditions are so ripe for dense fog that the foggiest place in the lower 48 is right here in western Washington.

“And what I mean by fog, I mean dense fog. Cape Disappointment is well ahead of any other place,” said Mass.

Why Super-Dense Fog Is a Good Thing

Mass says what he calls “super-dense fog—when you can barely see across the street or the house down the way” is often a good sign.

“To get really, really dense fog, we can’t have a lot of dense clouds overhead; the fog actually has to be relatively thin,” he said. “So what happens is if it’s relatively clear above, you’re able to get intense cooling near the surface. That produces the densest fog.”

So what super-dense fog usually means is that a sunny afternoon is likely ahead, says Mass.

But there is one thing to keep in mind: predicting the time of the burn-off is a very difficult thing.

“We’re getting better at it, but that is a very difficult problem: when, exactly, will the fog burn off? And that is a million-dollar question for the airlines,” said Mass.

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The weekly KPLU feature "Weather with Cliff Mass" airs every Friday at 9 a.m. immediately following BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KPLU Environment Reporter Bellamy Pailthorp. Cliff Mass is a University of Washington Professor of Atmospheric Sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to a podcast of “Weather with Cliff Mass” shows.