Winter’s Temperature Inversions Often Cause Stagnant Air, Fog And Black Ice

Dec 29, 2017

If you’ve ever hiked to the top of a mountain and found yourself shedding layers of clothing because of increasing warmth, you’ve experienced a temperature inversion. They’re strongest in winter and can come with some unpleasant side effects, such as the freezing fog and declining air quality the greater Puget Sound region recently experienced in mid-December. Temperatures in the lowlands were near freezing at the time, but hit 55 degrees at Hurricane Ridge in the Olympics.

“Temperature inversions are situations in which (air) temperature increases with height,” said KNKX weather expert Cliff Mass, a professor of atmospheric sciences at the University of Washington.

Their name comes from the reversal in normal conditions that they represent.

“Normally, temperature decreases with height,” Mass said. “When it increases with height, it’s the opposite of what we normally see. That’s why we call them ‘inversions.’”

And he says the ones that happen close to the surface of the earth almost always suppress circulation of the atmosphere, because they’re extremely stable configurations.

“When you have colder, denser air near the surface and warmer, lighter air aloft, that suppresses any tendency the atmosphere has to mix.  So that has big implications,” Mass said.

Stagnant Air

First of all, inversions tend to cause poor air quality and steadily worsen it, because they lock in pollutants.

“Inversions tend to keep pollutants near the surface, so if people are burning wood or even running their heating, all those pollutants are going into the lower atmosphere and they’re not mixing out,” Mass said. “So air quality tends to get worse and worse as these inversion situations continue.”

Fog And Black Ice

Mass says another common side effect of inversions is fog, because of the predominance of cold, dense air near the slightly warmer earth.

“And it’s often kind of moist air, trapped near the surface. And so, as the inversions stick around, the situation gets foggier and foggier and foggier,” he said.

That can be bad for aviation and cause challenging driving conditions, especially since the fog can freeze onto roadways or sidewalks and become black ice.

“As we get freezing fog that can be very dangerous,” Mass said.   

 If You Need Fresh Air, Go Up

Mass says in the wintertime, inversions tend to form when we have high pressure overhead and relatively weak winds. So a trip up above the inversion could be a pleasant excursion.

“The inversions tend to be only hundreds of feet to maybe a few thousand feet deep,” he said.  “So you can go up either into the passes, or you might want to go up one of the local mountains, like Cougar Mountain or one of the high hills around here, you can often get above the inversion.”

Change in Temps Can Range from 10 to 40 Degrees

The Seattle Times recently reported about an inversion in which there was a difference in temperature of more than 20 degrees between the lowlands and a hiking destination on Mount Rainier.

“It’s not unusual to have situations in which the temperature is ten, fifteen, even twenty degrees warmer at say 3,000 feet than at the surface,” Mass said.  

In fact, he had a student who went through all the records at the weather service radiosonde site at Quillayute, a balloon launch site on the Olympic coast.

“He found the largest temperature difference that he could find for the last 50 years was on the order of 40 degrees Fahrenheit, between the surface and 3,000 feet,” Mass said. “Can you imagine that? 40 degrees warmer!”

Why Are Inversions Strongest In Winter?

Inversions can happen year-round, but they’re strongest in winter because that’s when there’s the most potential for cooling of the surface more than the atmosphere above. When the skies clear out, which is often associated with high pressure, Mass says the long nights can allow lots of heat to radiate into space.

“And so the earth cools down and cools the atmosphere next to it,” he said. “Winter’s the great time, you have a lot of night and if you happen to get high pressure that gets rid of the clouds aloft, you can lose a lot of the energy at the surface. And that helps produce the inversion.”

Inversions can also happen in summer, but they tend to be weaker.

“And those occur when we have cool marine air off the ocean, which moves inland while there’s still warmer air aloft,” Mass said.

Summer inversions tend to be most prevalent in August and September, he says. 

Weather with Cliff Mass airs at 9:02 a.m. Friday, right after BirdNote, and twice on Friday afternoons during All Things Considered. The feature is hosted by KNKX 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 podcasts of Weather with Cliff Mass shows, via iTunes or Google Play.