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Cirrus Warming
Tue Jul 06, 2004
Listen in RealAudio 
Cirrus clouds are the thin, wispy clouds found up in the higher levels of the
troposphere, the layer where most of our weather happens. They form naturally in
nature, but are also produced from contrails—the condensation cloud formed by
aircraft engine exhaust. This latter type may account for a warming trend observed in
the United States over the last 25 years.
Hi, I'm Bryan Yeaton, and this is The Weather Notebook.
These contrail-born cirrus clouds are increasing— one percent per decade over the
United States— most likely due to air traffic.
Now, researchers at NASA's Langley Research Center in Virginia have estimated the
effect cirrus clouds would have on surface and lower atmospheric temperatures in the
U.S. After taking into account 25 years of observations— of the clouds, plus
temperature, and humidity—they found that these high clouds caused surface
temperatures to rise about half a degree Fahrenheit per decade.
That's roughly the same increase in air temperatures as has been observed in the
U.S. from 1975 to 1994. The connection between cirrus cloud cover and warming is
greatest during winter and spring, the same seasons when contrails are most
frequent.
Such warming doesn't replace the greenhouse effect, a likely cause of global warming.
In fact, during the same period, warming occurred in many other areas where cirrus
cloud coverage decreased or remained steady. But the NASA study does demonstrate
that multiple factors can influence regional temperature trends, and that human activity
can have a visible impact on cloud cover and, therefore, on climate. And it shows that
contrail activity should be taken into account in future climate-modeling studies.
Writer David Appell sent us today’s story. The Weather Notebook is produced by the
Mount Washington Observatory, with support from the National Science Foundation,
and Subaru: Driven By What’s Inside.
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