Valley Fog, Hoar Frost, and Inversion

Fog has settled thick in the Salt Lake Valley. Cache valley has had days of fog.

December 7th, 2017

NASA MODIS

December 12th, 2017

NASA MODIS

Freezing fog often causes hoar frost which is the thick white frost that deposits on trees. If the fog droplets are supercooled, meaning the liquid water is below freezing, it will freeze on contact to surfaces. My brother took this picture from Logan:

Photo Credit: Nathan Blaylock

Oh, and the air is getting pretty bad out there. Check out the PM 2.5 trends across northern Utah. Yuck.

airquality.utah.gov

Strong inversion and persistent cold air pool: January 2017

I'm looking at the inversion strength of the cold air pool in northern Utah between January 25 and February 5, 2017. This occurred during the Utah Winter Fine Particle Study (more about that study here: https://www.esrl.noaa.gov/csd/groups/csd7/measurements/2017uwfps/).

Inversions occur when the upper level air is warmer than the air below it. This makes it difficult for air to mix vertically and dilute pollutants from the surface. One way to determine the strength of an inversion is to compare the potential temperature at 700 mb (approximately the height of the mountain peaks) and the potential temperature at the surface. In the top figure in the graph below, I show potential temperature from the HRRR analyses at 700 mb (red) and the surface (blue). The 12-hr HRRR forecast is also shown in yellow. The observed potential temperature from the Salt Lake City radiosonde are plotted as black dots. The bottom figure is simply the difference between the upper level and surface level potential temperature, labeled "Surface Temperature Deficit."

For the most part, the HRRR analyses correspond well with the balloon observations, except the HRRR doesn't mix out as early as was observed on February 4th.

Below shows the potential temperature at the surface and 700 mb, as well as the differences from the HRRR model analyses for the course of the event. The right panel shows the difference between surface and 700 hPa potential temperature. Areas that turn dark red is where the inversion is strongest.

This is bad...

Have you taken a look outside? You can't see much. This morning we were socked in with dense fog, plus the pollution levels are in the unhealthy range. PM 2.5 concentrations in downtown Salt Lake are over 100 micrograms/m3. That's bad.

Utah county has been quite a bit cleaner than Salt lake County, but I looks like the pollution from Salt Lake is being transported south. Winds are light from the north. This may explain the sudden spike in PM 2.5 concentrations at the Lindon air quality site.

This mornings sounding shows how strong this inversion really is. The ground is cold, below freezing, while the air temperatures in the mountains are above 10 C. Since cold air sinks, all that pollution we emit at the surface stays at the surface. 

WBB tug-o-war

Interesting case during this inversion...the William Brown Building at the University of Utah is stuck between the cold air in the valley and the warmer air above. Look at the quick temperature fluctuations in the evening.

Also, a picture from the University of Utah looking south-west. Lots of gunk in the valley.

Looks like we're on a clearing trend. Should be breathing a bit better for the next few days

Welcome back to the prolonged inversions and poor air quality

Air quality along the Wasatch Front has been getting worse the last few days. Salt Lake City is the winter home of temperature inversions and bad air. And this is just be beginning of the season. Below are shown measurements of PM 2.5 (Particulate Matter smaller 2.5 micrometers in size).

The air is not super unhealthy yet, but the pollution concentration is increasing. You may have noticed the decreased visibility last time you tried looking at the mountains across the valley. This weekend a storm should blow all this gunk away before it gets much worse, but be prepared for more days like this and try to drive less during these inversion periods if you can.

November 19, 2014 @ 2:50 PM
Photo: kbkb

November 20, 2014 @3:30 PM
Source: WBB webcam

Above is another look at the smog today. The sun is beginning to set. The concentration of the smog actually looks worse due to the fact that we are looking in the direction of the sun. When particles scatter light, they always scatter with constructive interference in the forward direction, toward your eyes if you are looking toward the sun. This optical phenomenon makes the haze look brighter, making it seem there is more stuff in the air. But still, the air is dirty.

source: atmospheric radiation course notes

Clean Air

(c) BKB

Finally, some clean air. We've been stuck in an inversion the last few weeks causing horrible pollution. The north half of Utah had the worst air quality in the nation for a week! Here is a picture of me in Big Cottonwood Canyon looking down on the valley inversion...

The time the cold front passed Salt Lake can be found by looking at the temperature change from the weather station at the Gateway Mall in Salt Lake City. The red line represents the temperature. Around 10:00 AM the temperature reached 44 degrees. But look at what happened quickly after...the temperature took a ten degree dive in about 30 minutes!

(image from Mesowest.utah.edu)

 You can also identify the passing cold front by looking at the wind directions. Before a cold front passes, winds will blow from the south. After the cold front passes winds come from the north and northwest. There is a definite change in wind direction at 10:00 AM.

(image from Mesowest.utah.edu)

The following image shows the PM 2.5 concentrations in the Salt Lake Valley. This graph is hard to read, but the back circles is what you want to look at. In the right third of the picture you can see the black circles getting lower. This means our air is getting cleaner! As this storm continues to push through we can expect the air quality to improve

(image from airquality.utah.gov)

This next graph shows Provo's air quality from the following week. Utah County's air quality has by far been the worse in Utah. With south winds increasing before the storm and movement of the storm through Utah County the air quality has improved and will continue to get better

(image from airquality.utah.gov)

So, as the storm first blew through Salt Lake, this is the stuff that fell...

(c) BKB

This isn't just normal snow. It is called graupel. Graupel looks kind of like hail, but it is formed differently. It looks and feels like foam pellets you might find in packaging material. It started as a common snowflake, but as it fell to the ground, it encountered supercooled water droplets which immediately froze to the snowflake. This is called rimming. This kind of stuff is usually perfect for making a snowball because it is relatively warm and wet snow.

It took a few more hours, but the cold front eventually reached Utah County also changing the wind direction from south to northwest, caused a 10 degree temperature drop, and brought graupel and snow. You better get the snow shovel ready for Monday morning! ;)