What is an inversion? What lies behind the fog in the Treasure Valley

BOISE, Idaho – Dense fog has settled in the Treasure Valley, causing limited visibility, icy roads and colder than usual temperatures – all thanks to a weather phenomenon called an inversion.

• An inversion traps cold air in the valley while warmer air remains up in the mountains. This dynamic creates favorable conditions for the formation of dense fog on the valley floor.
• However, those who venture into the mountains can escape the darkness and enjoy the sun and clearer skies above the layer of fog in the valley.
• Submit your inversion photos here!

(The following is a transcription of the full broadcast story.)

A dense fog has settled in the Treasure Valley, and although the fog is thicker west of Meridian, visibility is limited and temperatures are colder than usual in most areas.

This is all thanks to a weather phenomenon called inversion, where cold air settles at lower elevations and actually gets warmer as altitude increases.

“When you think about the mountains, you usually think it will be colder up there than in the valley. But when an inversion occurs, it actually gets warmer in the mountains and colder down in the valley,” said National Weather Service meteorologist Spencer Tangen.

The effects of an inversion go beyond just cool mornings. Tangen said dense fog forms when temperatures and dew points clash, leading to slick roads and even the risk of black ice.

Courtesy of NOAA

“When we are in a reversal, you can see thick fog, which affects visibility when driving. It can also trap pollutants near the ground, so air quality often deteriorates because the pollutants have nowhere to go,” Tangen explained.

But there is a silver lining – if you’re willing to climb for it.

I drove to Bogus Basin, where the ceiling of the inversion ended. The sun shone brightly above the fog, providing an impressive view of the clouds and the valley below.

“You know, it’s actually pretty good when you’re in the mountains. If you go skiing and want to get away from the clouds, it’s usually very sunny during an inversion,” Tangen added.