Warm upper air traps smog over valley

An inversion has settled over Grand Junction, as shown in this view looking east from Colorado National Monument. The Bookcliffs and Grand Mesa are barely visible in the distance. At the monument’s Visitor Center at 10 a.m. Friday, the temperature was 27 degrees, and the temperature at the west entrance was 12 degrees.



You may have been less than excited to see an old acquaintance sneak back into town this past week.

A weather-related inversion much like its notorious cousin, a similarly unwanted guest that stayed too long last year, has crept back onto its perch, hovering just above the Grand Valley.

Inversions occur when warm upper air traps cold air and contaminants beneath it, casting the valley in a smoggy state for those on the valley floor.

Friday’s inversion was substantial, topping out at 8,500 feet, said Mike Chamberlain, a meteorologist for the National Weather Service in Grand Junction.

Twice a day, at 4 a.m. and 4 p.m., workers at the National Weather Service run up a balloon outfitted with a device to measure the depth of the inversion, Chamberlain said.

The Grand Valley’s latest inversion may clear out with dropping temperatures slated for Monday.

However, the inversion may reappear thicker than before by midweek as the weather stabilizes.

Colder weather or changes in weather such as wind or a storm can help move out an inversion.

Inversions are common in valleys in Western states, but intensities of inversions can vary depending on an area’s geography, Chamberlain said.

That may explain why the high temperature in Montrose on Friday was 40 degrees. Grand Junction’s high temperature on Friday was 26 degrees, and Thursday’s high was 30 degrees.

Similarly, Moab, Utah, which is lined with mountains and ridges, has not enjoyed the warmer temperatures often associated with the high-desert area.

High temperatures for the next couple of days there are expected to be in the 30s, with a high of 23 degrees Monday.

An accumulation of snow on the ground last year didn’t help alleviate the inversion that settled in for the bulk of December and well into January.

The current inversion also may linger as long as the white stuff remains on the ground.

“You can’t get any warming up if the snow doesn’t melt,” Chamberlain said.


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