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Why Spring?
Thu May 06, 2004
Listen in RealAudio 
Have you ever wondered why all the really severe weather in the Midwest United States
seems to occur in the spring? Why not the summer? The answer has to do with a
clashing of the Titans. Hi, I'm Bryan Yeaton, and this is The Weather
Notebook.
In the summer, air masses rolling out of Canada have warmed sufficiently that, for the
most part, they are not all that different from the air they run into in the United States.
Same thing in the winter.
But in the spring, the air over the plains states is starting to warm as the suns creeps
northward on its trek into summer. This air can also be pretty humid, as influenced by
the Gulf of Mexico. But up in Canada, summer is still a distant dream -- winter retains
its cold hold on our northern neighbor. The bigger the difference between the two, the
bigger the clash.
Cold air is denser than warm air, so a charging cold front will burrow underneath the
warm air, and force it upwards -- sometimes at great velocity. As this air drives higher,
it cools and eventually reaches its dewpoint where it condenses into cloud. If the
upward push fizzles out, the clouds will appear as puffy, fair-weather cumulus clouds.
But if the energy keeps cranking, these friendly clouds will build through cumulus
congestus and cumulus castellanus, and into cumulonimbus -- the towering
thunderheads.
During tornado formation, powerful updrafts and downdrafts set spinning in a
horizontal column of air. Eventually, this column gets lifted upright and becomes the
mesocyclone. If conditions are right, a concentrated circulation reaches from the
middle of the "meso" to the ground -- the tornado.
There are other related atmospheric conditions and air masses that can produce
severe weather, but it seems that the most violent storms occur when giant weather
systems collide.
You can revisit any of our shows online at www.weathernotebook.org. We are
supported by the National Science Foundation and Subaru -- Driven By What's Inside.
We are produced by the Mount Washington Observatory.
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