Those wispy streams of vapor that follow jetliners across the sky may not be as innocuous as they appear. A new NASA study claims man-made cirrus clouds formed by commercial jet engine exhaust may be responsible for the increased surface temperatures detected in the United States between 1975 and 1994. Climate data shows there has been a 1 percent per decade increase in cirrus cloud cover over the United States, which the NASA paper says is likely due to commercial air traffic. Cirrus clouds, whether natural or artificial, play an important climatological role because they trap heat in the atmosphere by reflecting infrared radiation emitted from the Earth's surface. The study, which appeared in the April 15 issue of the Journal of Climate, estimates that cirrus clouds from jet engine condensation trails, or contrails, increased the temperature of the lower atmosphere by anywhere from 0.36 to 0.54 degrees Fahrenheit per decade...