Pleasant in the Petunia City; Polar in the Pretzel City

Something very interesting occurred today across northern Illinois.  A strong southwesterly wind ahead of an approaching cold front ushered in warmer than average temperatures.  Depending on where you lived, your high was either 4 degrees or up to 16 degrees above average!  There are a few factors that led to the 42° reading in Freeport and the 54° reading in Dixon.  This is pretty amazing, considering these two cities are only about 35 miles apart!

One important factor was the warm air advection from the south.  But what really propelled the temperature difference was the snowpack.  There was little snow on the ground in Dixon and points south, so it was able to melt off yesterday and early today.  The bright white snow, said to have a high albedo, easily reflects the sun’s incoming radiation and warmth.  That was not an issue today in the Petunia City!  With a dry ground, the surface was able to warm up and retain the heat; thus, Dixon climbed into the mid-50s this afternoon.

On the opposite side of the spectrum, Freeport’s snowpack was three to four times more than Dixon’s.  Some melting did occur, but because of the depth of the snow, it couldn’t all evaporate.  With a cold layer of snow blanketing Freeport and the sun’s radiation reflected back into the atmosphere, the afternoon high temperature was kept in check.  The thermometer only reached the low-40s in the Pretzel City!  -JA


Posted under snow, sunlight, weather, weather geek, Wind

This post was written by qni_it on February 26, 2012

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