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What’s the deal with the fog?

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slc fog

NORTHERN UTAH (News4Utah)-  Here’s an explanation from meteorologist Dan Pope on the fog northern Utah has been experiencing this week.

Over the years, pockets of “snow grains” form in low clouds/fog during inversions here along the Wasatch Front. Snow grains coalesce from super cooled water droplets found in the fog and low clouds. Since there has to be something to coalesce around, particular matter “PM” of a 2.5 or 10 micrometers are the more likely source. During inversions, pollution from vehicle and industrial sources increases.

In my more than 35 years of forecasting the weather here in Utah, I have seen several different ways that “snow grains” seem to form but the majority of the sources appear to be centered around the Salt Lake Airport, power generation along North Temple, and near the refineries north of Salt Lake City and in Davis County.

An explanation for these occurrences can be derived from what seems to be rising warm pockets of air, produced by steam (from power production) or refining of oil. Steam or rising air from any source can cause the supercooled air to coalesce around particulate matter and other sources.

In the past, seeding the clouds and fog over the Salt Lake Airport was a method used to condense fog into frozen particles, typically snow grains. There are over 1,000 aircraft takeoff and landings at the Salt Lake Airport each day and turbulence from these aircraft is likely one of the several ways supercooled droplets coalesce into snow grains.

It is my professional opinion that aircraft landing and departing the Salt Lake City Airport are likely the greatest source of snow grains which seem to fall from the sky along I-15, 21st South and other East/West running routs to the north and south of the Salt Lake Airport.  

Another significant source of snow grains is likely the Rocky Mountain power plant along North Temple and there is additional significant snow grain activity near the refineries.

Snow grains could also be the diurnal wind flow patterns which move up through the Salt Lake Valley via the northwest wind in the afternoon, and down the valley via the southeast wind in the early evening, nighttime and early morning hours, likely creating microclimates of convergence, where supercooled water vapor condenses into snow grains.

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