Do jet contrails cause climate change? (Ask E).

--Isadore Klevitch, Palatine, IL

Condensation trails, which are also known as contrails, are formed when airplanes fly at high altitudes, where the air is extremely cold. The phenomenon is similar to exhaling in cold weather and "seeing your breath." Although contrails look harmless, they may actually affect the climate as they spread into high, thin cirrus clouds after they were formed. NASA researcher Patrick Minnis has found that this increased cloud coverage tends to warm the Earth because it traps the heat of incoming sunlight that cannot be reflected back into space. Some studies suggest that contrails may even trigger rainfall.

Contrails were difficult to study before September 11, 2001, because they normally cross each other by the hundreds. But with planes grounded for three days following the terrorist attacks, Minnis observed that six military aircraft caused about 7,700 square miles of cirrus clouds to form over Virginia and Pennsylvania in otherwise clear skies.

During that same time, David Travis, a University of Wisconsin atmospheric scientist, studied daily temperature ranges across the country and found that the average range increased, with the largest changes in the Northeast, the Midwest and the Pacific Northwest, where air traffic is usually heaviest. Although this complicates global warming arguments, it offers further evidence that contrails do affect climate in some way.

Contrails typically cover up to five percent of the sky over Europe and the U.S. But as air travel grows in popularity, Minnis warns they could increase by a factor of six by 2050. CONTACT: NASA Research Center, www.larc.nasa.gov.