First some definitions. An inversion is a layer in the atmosphere in which temperature increases with height. Within a valley or basin, a cold pool is the layer of cold air beneath that inversion. Inversions vary in strength, depth, and altitude. The current inversion has been characterized by an elevated inversion rather than one that is very near the valley floor. In addition, the cold pool beneath the inversion has featured high relative humidity. This has led to a cold pool that has not only filled with pollution, but has featured haze (small water droplets that form in a high humidity environment), and clouds.
This morning those clouds largely fill the Salt Lake Valley, and the contrast between valley and mountains couldn't be larger. I awoke this morning to a depressing scene, as bleak and grey as you will see in Salt Lake City.
Meanwhile, the weather at Snowbird was stunning, with the view from the top of Hidden Peak showing a sea of stratocumulus over the Salt Lake Valley.
This morning's sounding shows a multi layered structure from the valley floor to crest level. From the valley floor to about 814 mb (about 6500 feet) the atmosphere was well mixed. That means pollution is not confined to a very shallow layer near the valley floor, but is actually mixing through a depth of about 2000 feet. At 814 mb is the base of a weak inversion. This inversion sits at the top of the stratocumulus layer. Then there is a second, much stronger inversion, based just below 700 mb (10,000 feet), which is being produced by the upper-level ridge that is building in.
There are some interesting things happening, however, in the Salt Lake Valley on smaller scales. When looked east this morning, I saw a bright spot along the east bench where the clouds and haze were thinner.
A look at the surface observations showed light easterly flow at several east bench locations.
That easterly flow appeared to be disrupting the clouds and possibly contributing to the entrainment of drier and cleaner air into the valley cold pool. Although the PurpleAir PM2.5 values are sometimes too high when the relative humidity is high, they are very useful for identifying pollution patterns over the valley and show lower PM2.5 indicies along the east bench of the northern Salt Lake Valley, especially north of Big Cottonwood Canyon. These lower values extend eastward across downtown Salt Lake City. This is consistent with the extension of easterly and southeasterly flow to the Salt Lake City airport in the station map above.
Consistent with this easterly flow and associated entrainment of cleaner air, the air quality at Hawthorne Elementary has dropped significantly, all the way down to about 12 ug/m3.
|Source: Division of Air Quality|
|Source: Utah Division of Air Quality|