Wednesday, December 17, 2014

When No Snow Is a Good Thing

Smog over the Salt Lake Valley this morning as viewed from the Avenues
So far this year, the air quality in the Salt Lake Valley has been far better than the previous two years.  Although it is on the upper east bench and thus is a cleaner observing site compared to those on the valley floor, air quality sensors at our mountain meteorology lab near Red Butte Canyon show that PM2.5 concentrations over the past month have only briefly exceeded 30 ug/m3.

Source: MesoWest
Differences in the severity, frequency, and persistence of poor cool-season air quality episodes over the Salt Lake Valley aren't the result of changes in emissions, but instead meteorology.  Although we've had another dry start to the inversion season this year, we haven't had a big cold intrusion since mid November to establish a cold pool over the valley.  In addition, we've had enough weak systems come through to stir things up from time to time.  But, perhaps most importantly, we haven't had any significant snow cover on the valley floor.  Yes, I admit it.  This is one time when no snow is a good thing.

Compared to a snow-free land surface, snow has two major influences on the exchange of energy between the Earth's surface and the atmosphere.  First, it reflects more sunlight back to space.  Second, it has a very low thermal conductivity, so it effectively insulates the atmosphere from the ground (or alternatively from houses or buildings if you are in a residential or commercial area).  Collectively, these two effects result in stronger and more persistent inversions than would occur during non snow-covered periods.  

When the right conditions exist, we can still get inversions without valley snow cover, even some strong ones.  However, the lack of snow cover has probably helped some with the air quality so far this winter.  

Events and Announcements:

I'll be talking about Secrets of the Greatest Snow on Earth, snow, weather, and who knows what else tomorrow (Thursday Dec 18) on KUER's RadioWest show at 11 am.  Tune in to 90.1 FM or catch the live stream here.  

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