Ask A Scientist Weather Archive

name         Rhonda L. E.
status       other
age          20s

Question -   I grew up in Billings, Montana, and recently moved to 
the east (Western Pennsylvania).  Something that intrigued me is how 
often it rains here during the winter.  I never once saw an ice storm 
until I moved here.  Why is it that we never got these ice storms in the west?
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The north central and northwest do have ice storms sometimes, but in the
northern central and western states it is too darn cold and we get snow. This
year the Carolinas got some really nasty ice storms. It gets cold enough to
form sleet (ice) mixed with rain, but not cold enough to snow.

Vince Calder
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Rhonda,

Ice storms occur when warmer air aloft drops precipitation as rain which 
can freeze if lower air
masses and surfaces are at or below freezing.

Air masses like this are typical in the east.  As you noted, the higher 
altitudes of places like Billings in winter are visited by generally cold 
air masses both at high and low levels.  I am sure you have noted the 
occurrence this year (I live in Pennsylvania also) of regular snows 
exceeding occurrences of freezing rain due to a dip in the jet stream 
which carried arctic air into our state and southward even as far as 
Miami. The colder air reduces the likelihood of seeing the freezing rain 
you mentioned.

Thanks for using NEWTON!

Ric Rupnik
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Rhonda,

Ice storms are common in the southern great plains up through southern
New York and throughout the middle south. What is required is a
boundary between cold air to the north and humid air to the south,
such as an east-west lying cold front or stationary front. These
fronts are quite common in the winter in the eastern part of the
country, with the humid air coming from the Gulf of Mexico and
the cold air from Canada.  One of these fronts is presently
moving eastward through the eastern Midwest. Rain falls through
cold air pushing under the warm, humid air aloft, cooling the raindrops,
which then freeze on surfaces that are at or just below freezing
temperatures within the cold air zone.

Warm humid air from the south rarely gets to Montana during the
winter, so the required cold/warm boundary doesn't form there.
So Montana gets mostly snow during the winter.

My wife's family lives in the Pittsburgh area, I went to college in
central Pennsylvania, and I have field sites in Oklahoma and Kansas;
all of these places experience one or more ice storms per year. The
southern plains often have several such storms during the winter.

David R. Cook
Atmospheric Research Section
Environmental Research Division
Argonne National Laboratory
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