Wednesday, December 10, 2014


A weak disturbance moved through Heartland this evening and produced a little surprise for a few, sleet.  This system moved into very dry air and was forecast to die out before making it into our area. The image below is from the forecast sounding for Cape Girardeau, a profile of the atmosphere.  The red line indicates the air temperature as you move up the atmosphere, the green line shows the dew point.  The larger the difference between the two, the drier the air.  This sounding showed very dry air. The dark blue line is the freezing line.  You can see as you move up in the atmosphere the air temperatures warmed above freezing.  More on this later.

 As the precipitation moved into the area it encounters that warm level of above freezing temperatures. This of course causes the snow to melt.  Now with the new Dual Pol Doppler Radar we can see when the snow begins to melt.  The sleet shower was moving between Poplar Bluff and Dexter at the time of the image below.  Notice most of the colors here showing a dark red color.  This indicates the precipitation falling is all of the same type.  Since the radar beam was hitting the shower at an altitude of 7,500 feet here, it was below freezing and all snow.

The next image sampled the shower from an altitude around 5,800 feet. Here we have a mix of yellow and dark red.  The yellow colors indicate different types of precipitation in the shower.  Since the precipitation was all snow at 7,500 feet and seeing that warm nose of air from the sounding, we know snow is melting so this radar image shows rain and snow mixed. The warm nose was thick enough to allow for the snow flakes to melt  so when the precipitation fell out of the cloud, it was rain with a tiny bit if ice left in the center.

In addition to being warm, the air is very dry.  so as these drops of rain fall through the dry air they begin to evaporate.  As this happens a process called evaporational cooling takes place as the drops cool to the wet bulb temperatures.  Because the air was so dry the wet bulb was below freezing.  So the drops will refreeze on the way to the surface, even if the air temperatures is above freezing. But in this case the air temperature briefly drops below freezing again as well so there was no problem in seeing drops freeze into sleet. Weather is always fun!