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Originally posted by EyeDontKnow
That article says they've observed persistent contrails for 30 years.
So that means 30 years PRIOR to the 1997 article...
edit on 28-11-2011 by EyeDontKnow because: 1997 edit
RICHARD SEMONIN (Illinois Institute of Natural Resources): In the absence of natural clouds, given the correct atmospheric condition, jet aircraft in high frequency can almost completely cover the atmosphere, visible atmosphere, with clouds.
O’NEIL: Semonin says, unlike most changes in the atmosphere caused by man, this one is beneficial. Clouds help farmers in the Midwest by blocking the sun. Temperature extremes can damage plants and speed up the evaporation of soil moisture. In the Winter, city people benefit because clouds act as a blanket, preventing warm air from escaping into the atmosphere. No one is trying to make clouds now using jet engines, but this study suggests that jet travel is inadvertently making our days more cloudy and some day, weather researchers may be able to use jets on purpose to change our weather. Roger O’Neil, NBC News, Champaign, Illinois.