Friday, January 25, 2013

Freezing Rain

On January 24, 2013, Salt Lake, Utah, Davis, Weber, and other counties in Northern Utah experienced freezing rain. Even a tenth of an inch of rain caused many problems. People had trouble walking to their car, let alone driving their car. Over 200 accidents were reported.

The following figure shows the basic meteorology at play when freezing rain occurs. We don’t usually get freezing rain in Utah because it requires cold surface temperatures and a warm, moist air mass above. Inversions, where the air near the ground is cold, is common in Utah. We don’t, however, experience warm winter storms very often. 

All month we have had a strong, shallow inversion in the valleys keeping the surface temperature in some places ten degrees below freezing or more. The other ingredient was also present. A warm, moist storm blew in from the southwest providing the precipitation. 

It is also worth noting that this storm did not clean out the polluted valleys. Because hot air floats on top of cold air, the air mass didn't dig into the valleys to replace the dirty air. All counties in northern Utah continued to have “Red” air quality days.

The next diagram is called a “skew-T.” This is the data collected from weather balloon launched from Salt Lake City Airport at 5:00 AM on the morning we woke up to freezing rain.

There are two black squiggly lines drawn up the graph. The right side black squiggly line represents the temperature for given heights. I drew in the sideways blue line to represent freezing temperatures. When the black temperature line is to the left of the freezing line, it is below freezing. When the black line is to the right of the freezing line, the temperature is above freezing. The blue and red highlighted part of the graph helps you see where temperatures are above or below freezing. A mountain is also depicted on the left to illustrate how tall the mountains are.

Warm air mass clouds can be made of ice crystals. The snow that falls from these clouds melt as it falls when temperatures are above freezing (the red highlighted area). The snow turns to rain. Near the ground is a shallow freezing layer. This is due to the temperature inversion caused by lack of solar heating in the day and losing radiation at night.

In this cold layer, you might wonder why it doesn't turn to hail. When water freezes it needs something to freeze onto. It has a hard time freezing to itself. Because the cold layer near the surface is thin, water doesn't freeze into hail. It becomes supercooled, meaning the liquid water is below freezing.

The instant a supercooled water droplet hits anything with a freezing temperature, such as a road, tree, or powerline, it will freeze to the surface creating an ice glaze.



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