Thursday, December 6, 2012

Haze, Smaze, and Fog

A foggy morning in the Avenues
Photo taken 7:20 AM MST 6 Dec 2012
The weather has now sunk temporarily into deep depression as fog formed over the Salt Lake Valley overnight.  The seeds for the fog were sown Tuesday night and Wednesday when rain fell from mid-level clouds into a shallow cold pool (a.k.a., inversion) that was present over the Salt Lake Valley.  This jacked up the relative humidity near the valley floor and led to wide spread haze within the cold pool yesterday.  The haze was very apparent late yesterday afternoon and was topped in places by shallow stratocumulus or "scud" clouds.

Looking southeast toward the Wasatch Mountains from the Avenues
at 5:15 PM MST 5 Dec 2012 (click to enlarge)
Usually I bristle at the use of the word haze during cold-pool events.  Usually, the poor low level visibility is produced by pollution, which should be called smog.  Haze forms only at higher relative humidities when condensation can occur on microscopic, water-soluable particles that are floating around in the atmosphere.  These particles can be natural, but they can also be produced by human activity.  One could use the term "smaze" to describe haze that forms in these polluted environments, as was the case yesterday.  

The sounding from yesterday afternoon tells the story pretty well.

The surface temperature and dewpoint were 47ºF and 45ºF, respectively, which equates to a relative humidity of 93%.  This is high enough for haze particles to form, but not high enough for those particles to grow into larger cloud droplets.  Thus, clouds formed only in a few places at the top of the cold pool where there was just enough turbulence, lift, or moisture for the atmosphere to reach 100% relative humidity.  

Above the cold pool, the relative humidity was lower and the air less polluted.  The visibility in this layer, which extended to cloud base, was outstanding.  

Overnight, fog and low clouds formed over the valley.  These can be seen from the photo from the top of Snowbird, which is framed well by ice on the lens.  

Source: Snowbird
Don't worry, the inversion is weak.  The fog will probably lift and dissipate today, and we should be further stirring things up the next couple of days as cooler air moves in.  

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