Wednesday, November 29, 2006
Blue skies have been turning white this month as airplane traffic crisscrosses the sky.
The trails that jets leave behind grow, creating thin, cirrus-style clouds that aviation officials say result from water vapor from engine exhausts and in the air turning to ice crystals.
At least two Nashvillians aren't convinced. They subscribe to a concept that chemicals are being sprayed from planes.
"Why are there times that you don't see them at all?" said Scott Webb. "That's regardless of weather."
Lynn Lowrance, spokeswoman for the Metro Nashville Airport Authority, said it is, in fact, a matter of the weather.
The jet trails — called contrails — can disappear quickly, she said.
When the air is moist and it's colder, the trails can spread in a chain reaction as water vapor turns into ice particles. Feathery clouds result.
"They don't pose any health risk," Lowrance said.
Scott and Guy Avery, a local running coach, disagree, pointing to articles and talk on the Internet alleging that the government is spraying chemicals from planes, perhaps for experimentation.
"Chemtrails," as they're called on Web sites, have been a topic for several years, with unmarked military planes often accused of releasing them.
Representatives of Fort Campbell, which only has helicopters, the Tennessee Air National Guard, which has 10 marked propeller planes here, and a U.S. Air Force spokesman in Washington, D.C., said this week they do no such releases.
A NASA researcher said contrails are a cause for concern, but it's not related to chemicals.
A study has shown that the thin, cirrus clouds that plane exhausts can trigger are trapping heat next to the earth, said Pat Minnis, a NASA senior research scientist in Hampton, Va., and a Vanderbilt University graduate.
As air traffic increases, cirrus cloud coverage over the U.S. is rising by 1 percent a decade, Minnis said.
"A single plane can produce a rather large cloud," he said.
After 9/11, when all but a few planes were grounded, scientists had a chance to see that one lone military plane's contrail extended over Ohio and Pennsylvania, he said.
The icy cloud eventually covered about 6,170 square miles.
While NASA papers indicate that the impact on temperature of plane-produced clouds is significant, not everyone agrees, Minnis said.
"Whether or not it's a global climate problem, we'll see," he said.
More research is going on to try to determine that.
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