|The air is still polluted, but these clouds are actually a help rather than a hindrance|
Below is a loop of upper-air soundings collected each morning at the Salt Lake City airport over the past week. These soundings are plotted as a "skew-t" so a line of constant temperature slopes upward to the right. Note the two-stage evolution of the inversion event. First, the upper levels warm dramatically while the morning surface temperatures remain relatively steady. This occurs in the first 5 frames of the loop (note how the red line shifts rightward from about 800 to 600 mb). Second, a mixed layer forms very near the surface in the last 2 frames, as indicated by temperatures cooling with height to an level just above 850 mb.
|Morning upper-air soundings from the Salt Lake City airport from 5–12 Feb 2016|
Here are a couple of still images to further highlight this point. On the morning of 8 February, the inversion was based right at the surface. In such a situation, there's very little vertical mixing of the airmass. Pollution is trapped right at or very near the valley floor.
|Morning upper-air sounding from the Salt Lake City airport on 8 Feb 2016|
This morning (11 Feb), however, the inversion base is elevated and sits at 836 mb (5680 ft), almost 1500 feet above the valley floor. Below the inversion, there is a shallow mixed layer that is allowing for some vertical mixing of pollutants.
|Morning upper-air sounding from the Salt Lake City airport on 11 Feb 2016|
The growth of such a mixed layer is very common following the formation of low clouds during inversions in the Salt Lake Valley. Radiative cooling at cloud top drives turbulence that mixes the layer in and below the clouds. The end result is what meteorologists call a cloud-topped mixed layer. If you are in the Salt Lake Valley this morning, you are not in the inversion, you are beneath it, in the cloud topped mixed layer.
What does this mean for pollution? Well, it means that at night there's more mixing through a deeper layer, which reduces PM2.5 concentrations. The presence of the mixed layer also slows the long-term rate of rise (NOTE: Concentrations are still high, so this is no reason to roast marshmallows over a fire tonight). For example, PM2.5 levels at Hawthorne last night were not as high as they were the previous night. In addition, there is a flattening of the long term trend.
As discussed in the previous post, PM2.5 concentrations at University of Utah sensors are running higher than those aw Hawrhorne, but they also suggest a flattening out over the past 24 hours or so.
None of this means the air quality is good. We are still in unhealthy territory. What it does mean is that the rate of increase along the valley floor will probably slow. Those of you on the upper benches may see more sustained high PM2.5 levels in this scenario as the base of the inversion lifts and you are more continuously enveloped in the gunk. Perhaps this isn't a silver lining for you.
Science wonks can see Pataki et al. (2005) for more info on these cloud-topped mixed layers and their influence on mixing and transport in the Salt Lake Valley.