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Originally Posted by Airwaves
If I recall, and it's been a week now since I heard the piece, the earth's radiational heat is reflected off clouds, contrails, etc and bounces back toward the ground helping to keep us warm, especially at night.
It's most noticable during the winter, the temperatures are lower on clear nights than on cloudy tonights during similar weather systems because the earth's heat is lost on a clear night with nothing to reflect it back.
You're local TV weatherdude calls it "radiational cooling". I was surprised to hear that there is enough air traffic over the US that would have had a radiational cooling effect when it wasn't there suddenly.
The type of clouds that would prevent the sun's rays from reaching and warming the earth to begin with would be something you'd find after catastrophic volcanic eruption or in the extreme, a nuclear winter.
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First the article orignally mentioned by Airwaves only stated that increased differences between night and day temperatures were noted in the 3 days post 9/11. The author (David J. Travis, University of Wisconsin) made no conclusions as to whether the net effect was overall warming or cooling. Second the effect of contrails (and cloud cover) is to both reflect sunlight back into space (thus decreasing the Earth's temp) and to block IR from the ground and subsequently reradiate it back (thus increasing the Earth's temp). I've yet to see any definitive article stating which effect dominates; the numbers presented for both effects are within 1-2 % of each other. One report estimated that contrails would effect the Earth's temperature by 2050 but that made some ambitious assumptions concerning the growth of air traffic.
http://www.agu.org/sci_soc/prrl/prrl9919.html
The effects of clouds (rather than contrails) is hotly debated right now. I don't know of anyone who claims that existing models for clouds are accurate.