Dear Tom,
After our recent subzero spell it was raining in Lindenhurst with a
temperature of 12 degrees. How can this be?
Bob Old, Lindenhurst Ill.
Dear Bob,
We get this question a lot in winter, but the answer bears repeating.
The form in which precipitation reaches the ground is not determined by the
surface temperature, but by the temperature profile from the ground to
cloud-level.
Most winter precipitation begins as snow, but if it encounters
a deep enough layer of above-freezing air as it falls to earth, the snow
melts and turns to rain. If the rain refreezes when it makes contact with
the below-freezing ground, the result is a glaze-producing freezing rain.
Freezing rain usually occurs at least a couple of times each winter in the Chicago area
but surface temperatures are usually in the 20s or very low 30s.
