A Harbinger of Things to Come

Looking south from the Avenues Foothills, 1250 MDT October 19, 2013

Although we are not yet in the inversion season, we are now nearly a full month past the fall equinox, days are shorter than nights, and the sun is getting low on the horizon.  As a result, we're simply not able to mix out the air over the Salt Lake Valley very vigorously, and the net result is a touch of smog.  The sounding taken from the Salt Lake City airport yesterday afternoon shows that the surface-based mixed layer extended to about 750 mb (~8500 ft), where a stable layer basically put a "cap" on the valley atmosphere.  The base of that cap was very near the visual top of the densest smog in the photo above.

Source: SPC

The difference between yesterday and a full blown wintertime smog event is the height and strength of the inversion or stable layer.  Yesterday's stable layer was fairly high, so pollutants were dispersed through a fairly deep layer and there was some transport of air through mountain passes and other gaps.  The stable layer was also weak, so there probably was some exchange of air between the valley and the cleaner airmass further aloft.

With a weak upper-level trough approaching last night and passing later today and tonight, I suspect we  see enough flow and cooling aloft to bring an end to this temporary smog event.  Nevertheless, it is a harbinger of things to come.