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Retrograde Storms
Mon Sep 13, 2004
Listen in RealAudio 
Some things we all take for granted. The sun comes up in the East. In the United
States, people drive on the right-hand side of the road. And weather systems move
from west to east. Hi, I'm Bryan Yeaton, and this is The Weather Notebook.
Weather systems move from west to east because they follow the flow of the jet
stream, a river of air that flows west to east across the mid latitudes, around 8-12
miles up. But sometimes, the jet stream shifts into reverse.
Every so often, a low-pressure trough -- a kink or dip in the jet stream -- will move
toward the west even as the air inside it continues flowing to the east. A meteorologist
named Carl-Gustav Rossby discovered years ago that troughs naturally tend to move
more slowly than the air inside them. An average trough moves at half the speed of the
jet stream. The smaller and more tightly packed troughs are, the faster they tend to
move. Conversely, the larger and more distinct troughs often move slowly or even drift
backwards, or in the North American mid-latitudes, to the west.
You can find a different kind of backward motion if you look at small-scale weather.
Sometimes a thunderstorm will develop on its west flank so quickly that it appears to
be moving that way, even though the whole complex might be drifting east. The storms
that produce Nor’Easters are actually still moving from west to east. Picture a traffic
jam that works its way backward even as the cars are inching forward. Whether it's
cars or clouds, you know things will go the right direction eventually -- you just don't
know when.
The Weather Notebook is underwritten by Subaru of America, with support provided by
the National Science Foundation.
Today's Links
How the jet stream influences weather - USA Today:
http://www.usatoday.com/weather/wjet.htm
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