The Moose Fire started near Salmon, Idaho on July 18, spreading to over 3,000 acres (1,200 ha) by the following morning. During the day on the 18th, smoke from this fire spread to the east and northeast into Montana, driven by aggressive fire activity and upper level winds.
Fire activity decreased overnight, with the smoke plume not reaching nearly as high into the atmosphere. As the sun set, the mechanism for warming the ground ended, cutting off vertical mixing and causing cooler air to sink to the surface.
This phenomenon is referred to as a nocturnal temperature inversion. Generally speaking, it is expected that the temperature will cool with higher altitude. A pattern of cooling as altitude decreases is a reversal, or inversion, or that pattern.
Returning to the Moose Fire, rather than smoke being carried east by upper level winds decreased fire activity and the presence of the nocturnal inversion instead forced the smoke downward into the Lemhi Valley.
Over time, the smoke continued to seep downward until it reached the Snake River Plain causing smoggy conditions in cities like Idaho Falls, Pocatello, and Rexburg. Smoke from a fire near Twin Falls also added to the smog in that area.
Nocturnal temperature inversions are common. They can be most obvious in areas with a lot of terrain such as Central Idaho where valley floors can be noticeably cooler than mid-elevation locations during the early morning hours.
Other temperature inversion patterns also exist. Most notably for the Columbia Basin is winter inversions, which develop under calm conditions from approximately mid November to mid February because the sun angle is so low in the sky that it doesn’t heat the ground enough for vertical mixing. This is the cause of the long lasting fog that the Tri-Cities and surrounding areas get during those months.
This afternoon (July 19), daytime heating is likely to mix at least some of the smoke up off the ground with upper level winds pushing it east toward Montana and Wyoming. Still, everyone within about 150 miles (240 km) of Yellowstone may experience worsened air quality due to fires burning in the region.
This article was written on my phone. I apologize for any formatting issues that may come from this.
