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Mass: Blame the Boa Constrictor-Like High Pressure for Fogtober

Tim Durkan

Why did we have such a long stretch of fog? Blame the inversion, says KPLU weather expert Cliff Mass.

Foggy days in the fall aren’t uncommon in the Northwest, but the recent long stretch—the so-called “Fogtober” and “Fogmageddon” that Mass said will finally leave us Sunday—is quite rare.

And Mass says we can thank a phenomenon called an inversion, which brought about an unusual range of temperature—what Mass calls “summer and winter at the same time.”

Inversion = Warmer Air Higher Up

Mass says the recent inversion was one of the strongest he’s ever seen. Temperatures at the surface near sea level were in the 40s, while a few thousand feet up, temperatures hit 60s, 70s, even lower 80s at times.

“It was an amazing contrast,” said Mass, professor of atmospheric sciences at the University of Washington. 

Why Do Inversions Occur?

“We had two things happening at once,” said Mass.

First, there were no clouds to keep the heat sealed on the surface, he said.

High pressure overhead caused air to sink and prevented clouds from forming. And without a cloud cover, the surface radiated heat into space through infrared radiation.

“So the surface can radiate heat to space and cool down, [like] if you think of refrigeration coil on the surface,” Mass said.

But, he added, the same principle doesn’t hold true higher in the atmosphere: “It can’t radiate the heat as well as the surface can. So that’s one reason.”

The other reason has to do with sinking air. It turns out air sinks stronger higher up than near the surface.

“If you have stronger sinking aloft, that causes warming,” he sad.

So you have more warming as you have stronger sinking aloft than at the surface. That also strengthens the inversion, Mass said.

Why Did it Last So Long?

The high-pressure air that fuels inversions are slow, stable, and strong not unlike a boa constrictor, says Mass.

"Just as a boa constrictor envelops and tightens on its prey, so is the high pressure over us tightening and strengthening the inversion over our heads!" he said. “The thing is, with the high-pressure state in place, the air was pushing down, compressing the temperature difference between the surface and right above. Day after day, it got stronger."

Inversions are very stable features of the atmosphere, said Mass, and “that means it’s hard for the atmosphere to mix through an inversion; it keeps the atmosphere very layered. So when you’ve had strong inversion, it prevents any kind of mixing that would destroy the fog layer."

As the inversion sank and got lower and lower from 1,500 feet to 1,000 feet, to even around 900 feet, it pushed down, compressing the foggy cold layer below.

Fogtober did have one silver lining, says Mass: the beauty of the fog itself.

“The beautiful thing about fog is that it flows like water, and it’s following the cold air below. So as it moves over buildings and over hills, you can see the undulations, the almost liquid nature of the fog as it moves around objects and hills,” said Mass. ”So it’s actually very, very photogenic.”


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.  

Bellamy Pailthorp covers the environment for KNKX with an emphasis on climate justice, human health and food sovereignty. She enjoys reporting about how we will power our future while maintaining healthy cultures and livable cities. Story tips can be sent to