Winter weather woe: Inversion traps air and triggers air-quality advisory in Boise area
Thanksgiving is a time for gratitude. But in the Treasure Valley, it’s also a time for annoyance: It marks, unofficially and approximately, the start of inversion season.
And indeed, inversion season has begun, right on time. The Valley is particularly susceptible to inversions during the final month of fall and for most of the winter because of its geological status as a valley between the Boise and Owyhee mountains.
This inversion has pushed the National Weather Service in Boise to issue what Treasure Valley residents have come to know but not to love: an air stagnation advisory. It took effect Friday and is expected to last at least through next Thursday, Dec. 5.
While the weather phenomenon has created issues for the Valley, high temperatures are not expected to exceed 45 degrees.
What is an Inversion?
Jackson Macfarlane, a meteorologist with the National Weather Service in Boise, explained that an inversion is caused by the high pressure moving in and making the mid levels of the atmosphere — between 5,000 and 15,000 feet above ground level — warmer.
“It’s actually a pretty simple process assuming that your surface temperature stays the same, in this case 40 degrees,” Macfarlane said. “If you’ve got warming in the mid levels, as that temperature warms, eventually it’s going to cause an inversion.”
“We tend to get our inversions under high pressure in the winter, usually characterized by steadily, slowly sinking air, which inhibits any condensation,” Macfarlane said. “We’ve got very faint clouds going on right now, because in most levels of the atmosphere, it’s far too stable for condensation to occur.”
This means the inversion traps stagnant air and low-level fog near the surface, causing breathing problems for those particularly sensitive to the pollutants. The air stagnation advisory issued Friday morning covers an area much larger than the Treasure Valley. It includes portions of South Central, Southwest and West Central Idaho and Northeast and Southeast Oregon.
Macfarlane said the inversion does not always bring low cloud coverage and fog, but the air at the surface is going to remain stagnant for a while. The advisory is in effect until 9 p.m. Thursday, Dec. 5, though it may be extended as the Weather Services receives new information.
“We put out those air-stagnation advisories in collaboration with the Idaho Department of Environmental Quality, with the message that there’s an enhanced risk for pollutants to stay trapped near the surface,” Macfarlane said.
How will the weather be this weekend?
While the inversion is expected to linger for a full week, the forecast this weekend is expected to be calm, with sunny and cold temperatures into Monday.
Friday evening, the low temperature was expected to reach 25 degrees. The winds were forecast to be calm with clear skies.
The forecast for Saturday called for a high of 39 and a low of 24, with clear skies with light wind all day. Sunday should be sunny with a high of 40 degrees and a low of 23.
The forecast for Monday calls for a high of 41 and a low of 24.
This story was originally published November 29, 2024 at 4:33 PM.
