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September 11th
09/11/2002
Listen in RealAudio 
A year ago today, satellite photos showed a trail of smoke streaming from what
used to be the World Trade Center. The disaster may have had a brief impact
on our nation's weather--but it wasn't because of the smoke plume. It was
from the absence of other plumes that we're used to seeing all the time.
Air traffic across the country was grounded for several days after September 11.
Suddenly, we didn't have those familiar white streaks that crisscross the
sky. These are contrails-basically, high-level cirrus clouds. They're made
of up ice crystals that form on exhaust particles spewed out as a plane flies
by.
Where there's a lot of air traffic, such as the corridor between Chicago
and the East Coast, contrails can cover more than half the sky. Clouds in
general tend to insulate: they keep nights warmer and days cooler. But
nobody's been able to show this effect for contrails until now. The period
after September 11 was the first time in half a century that US skies were
virtually free of contrails.
David Thomas, a climatologist at the University of Wisconsin-Whitewater, has studied contrails
for almost 20 years. He thinks our weather changed for a few days last fall when the
contrails vanished. Thomas found that the nationwide spread between highs and lows expanded to
about 2 degrees more than average during the flight ban. The change was most dramatic just
where Thomas expected--in places like the Midwest and Northeast that normally have the most
air traffic. The manmade clouds might even mask a bit of global warming, but a lot more
research is needed before we can run-or fly--with that one.
Today's Links
More
http://www.psu.edu/ur/2002/jetcontrails.html
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