Wasatch Weather Weenies

During winter, the Salt Lake Valley often sees pollution episodes with elevated particulate matter due to the presence of low-elevation cold pools that persist due to the limited solar heating.  This enables particulate matter that forms from the emissions from automobiles, industry, home heating, and other sources build up within the cold pools, resulting in poor air quality.  This poor air quality is reflected in elevated levels of particulate matter with a diameter of 2.5 micrometers or less, or PM2.5.  Such events are known colloquially as inversions. Because the PM2.5 is typically trapped in the cold pool, one can usually find clean air by moving to higher elevations.  The depth of the pollution varies from event to event and sometimes from hour to hour as the cold pool sloshes around in the valley.  At times, the benches may be in the clear air.  At other times, elevated PM2.5 can extend higher.   An example, from December 15, 2025, is shown...

The pollution layer over the Salt Lake Valley remains today (Saturday) but clearly shallowed over the past few days. The photo below was taken at 0743 MST Tuesday.  You can compare that with the one below taken at 0745 this morning. So, some temporary relief in some upper-bench areas.   On the valley floor, there was also some improvement, with PM2.5 this morning at Rose Park being lower than it has been in a few days. All of this is evidence of top-down mixing with the pollution being mixed out from the top down.  Sadly, it has not all been removed on the valley floor.  What remains after today we will probably be stuck with again for a while.

As discussed in the previous post , a weak upper-level trough was expected to move over northern Utah yesterday, but it was unclear if it would crack the inversion (I was skeptical).  Indeed, looking at things this morning, I'll call it "stirred not shaken." Impacts on the pollution are varied.  In the Avenues foothills, there was clear improvement and evidence that the pollution is shallower, as sometimes happens in a weak partial mixout event in which things are eroded from the top down.   On the other hand, a look at satellite imagery shows evidence of fog or low clouds in portions of the Salt Lake Valley, Tooele Valley, and the area from Bountiful to Layton.  Instead of being bright like the mountain snow, the fog and low clouds look grey, which may reflect that it is thin, patchy, or simply mixed with all of the smog that is still stuck on the valley floor.  Source: College of DuPage.  Image from 1611 UTC/0911 MST 16 Jan 2026. As I worked on this ...

Just another day in the paradise that is the Wasatch Front.  The latest west-facing image from the University of Utah shows a blanket of smog over downtown Salt Lake City.   Source: https://horel.chpc.utah.edu/camera_pages/wbbw.html It's tough to get above it without going to the mid elevations.  I hiked this morning to the top of the Avenues foothills and it was unclear if I was completely out of it even at 5700 feet.   Recent station (filled squares) and mobile (filled circles) PM2.5 observations show values in the valley generally between about 28 and 45 ug/m3.   Source: https://utahaq.chpc.utah.edu/ The threshold for unhealthy for sensitive groups is 35.5 ug/m3, so we are seeing values that are near or above that.  The DAQ sensor at Hawthorne Elementary is at 41.3 ug/m3 as of 2 PM.   Is there hope for a mix out?  The best chance is probably on Thursday and Friday.  On Thursday there is a weak upper-level trou...

If you want to build a persistent valley cold pool in northern Utah, often referred to colloquially as "an inversion," the best way to do it is to have a strong cold surge followed by the development of an upper-level ridge. The cold surge provides the cold air at low levels while the ridge then increases the temperatures aloft.  During the winter, there's insufficient heating from the sun to warm the cold air at low levels, so one ends up with a persistent valley cold pool that is topped by a stable layer or inversion that prevents the dispersion of air pollution. This process is even more effective if there is snow on the ground.   The past two weeks, however, provided something a little different.  On December 6 we had an atmospheric river move through the area (indicated by green below...note that the meteograms for temperature and wind do not quite line up vertically).  This was followed by a period with occasional mid and high clouds, somewhat enhanced sou...

The sun had not yet crested the Wasatch this morning when I awoke and when I looked out the window I saw this odd band of white extending at low levels through the western Salt Lake Valley.   My first thought was what the hell is that.  It's too shallow to be a lake band and it doesn't look like fog.  I took a look at the weather observations and the relative humidity in that area was less than 60%.  What could it be.  I then looked at PurpleAir thinking it might be dust, but even then the PM2.5 numbers didn't seem very impressive.   I kept digging, soliciting the input of some meteorological friends.  Eventually, with some help of specialized observing sites, satellite imagery, and weather cams, it became clear that it was blowing dust originating from the exposed Great Salt Lake bed east of Antelope Island.   A quick look at the evidence.  The most damning was a video of web cam images from the MesoWest web cam at Syracuse ne...

During the most recent inversion event, fog developed over the Great Salt Lake, eventually expanding to cover most of the lake and environs.  Below is a series of images from the NASA Terra satellite, which flys over roughly mid morning on December 4th, 6th, and 8th, the latter being yesterday (Sunday).   It was on the 4th that fog first became evident, with a couple of regions of fog extending westward from near Antelope Island over the southern bay of the lake. By the 6th, fog covered most of the lake.   And finally, by yesterday (Sunday, 8 December), the lake was nearly entirely covered and spread into populated areas that included the northern Salt Lake Valley and portions of Davis County.   Given the limited lowland snow cover in this instance, it is likely that the evaporation of water from the lake played an essential role in fog formation in this event, leading to fog development and persistence mainly over and near the lake. I often tell stude...

We are currently mired in the first major inversion and air quality episode of the season, although I don't think it's been as bad as it could have.   First, the valley is mostly snow free.  This likely leads to a bit more energy input into the atmosphere (due to less sunlight being reflected back to space and increased ground heat flux) and perhaps a bit of a decrease in particulate matter generated by photochemical processes (although this is not my area so I'm speculating).   In the case of the former, the soundings released at the Salt Lake International Airport at 0000 UTC each afternoon are too late to capture the structure of the convective boundary layer that develops each afternoon.  The convective boundary layer is the part of the atmosphere in which surface heating by the sun leads to vigorous vertical motions (the updrafts are sometimes called thermals) and mixing.  The depth of the afternoon convective boundary layer varies from meters to...

Last night, the cold pool beneath the inversion got shallower and the upper Avenues emerged from the smog.  I took the photo below from near 13th Avenue on my way into work.  It shows the clear skies at upper bench level, but also the thin lens of smog enveloping downtown and the lower valley.   Upper air soundings collected at the airport yesterday morning and this morning show very subtle but important changes.  Yesterday morning, the inversion base was elevated and near 860 mb (about 400 to 500 feet above the valley floor.  Beneath the inversion was the valley cold pool that contained the fog and low-level stratus clouds.  The flow was weak in and above this layer and didn't reach 20 knots until about 775 mb (about 3300 feet above the valley floor).   Source: SPC This morning, however, the inversion is based right on the valley floor and temperatures below about 800 mb (about 2500 feet above the valley floor) have warmed, whereas they have...

Salt Lake City descended into Inversion Hell with the development of low clouds and increasing PM2.5 levels. Today's MODIS imagery, taken around 1:30 PM local time, showed the coverage quite well with widespread low clouds (in some inastances fog) over much of the central Great Salt Lake, Skull Valley, Tooele Valley, northern Salt Lake Valley, and Davis County.   During the satellite overpass above, the avenues foothills were cloud free, but that changed at around 4:30 when the clouds pushed into the upper Avenues. The fog and low clouds would only be depressing if they weren't also accompanied by poor air quality.  Observations from Hawthorne Elementary along 700 East in Salt Lake City show a major increase in PM2.5 concentrations today to 45-49 ug/m3, solidly at Unhealthy for Sensitive Groups levels.   The high temperature today at the Salt Lake City International Airport was only 32˚F.  For comparison, the maximum temperature on Mt. Baldy (11,000 ft) abo...

As wonderful as the snow we have had since yesterday is (14" at Alta Collins as of 8 AM), what was really fascinating about this storm is how it affected the valley cold pool and associated pollution that has been resident over the Salt Lake Valley the past few days.   The valley cold pool is the layer of cold air that resides in the Salt Lake Valley during what are known colloquially as inversions .  I prefer the term cold pool because not all inversion events feature a true inversion in which temperature increases with height (the atmosphere can still be quite stable when the temperature decreases slowly with height but is not inverted).  In addition, in some instances the inversion is elevated, so it is not really correct to call the cold air beneath it the inversion.   OK, enough semantics.  Valley cold pools can be deep or shallow.  They can mix out quickly or slowly.  Then can be eroded by surface heating (which warms the cold pool) or...