Scientists believe that contrails turn into larger cloudbanks that substantially alter the atmosphere’s heat balance. They may even an important role in shaping our weather.

Scientists have long suspected this may be true, but haven?t been able to test it until 911, when the FAA grounded commercial flights nationwide for three days following the terrorist air attacks. They have now discovered that the American climate was noticeably different during those three days.

A team of climatologists say that temperatures in the United States fluctuated by 1.2 degrees Celsius more when airplanes were grounded than when normal flight patterns prevailed. That is, planes in the sky decrease the variability between day and nighttime temperatures. More air travel, the researchers means less meteorological difference between noon and midnight. This research provides one of the strongest indicators that air travel itself changes our climate.

“We actually found a much greater change in temperature range for parts of the country that normally get the greatest contrail coverage,” says David J. Travis, of the University of Wisconsin in Whitewater. Large contrails, he says, only form when the cruising-altitude atmosphere is both sufficiently moist and sufficiently cool (somewhere in the range of minus 40 to minus 65 degrees Celsius). The skies above the Southwest are typically too dry, and the skies above the deep South are too hot for extended contrail coverage.

These factors, plus the varying density of air traffic over different parts of the country, combine to make the skies in the Midwest and Northeast — and, to a lesser extent, the Pacific Northwest — particularly filled with contrails. Insuch contrail-heavy parts of the country, Travis’ team found that during September 11-13, the difference between day and night temperatures increased even more. The Midwest and Northeast experienced a “contrail effect” of 3 degrees Celsius, more than twice the national average.

Travis says that the new data does not suggest that contrails make global warming either better or worse. Other research has suggested that contrails have a global-warming effect. But Travis’ data addresses variability beween day and night temperatures, not an overall warming or cooling trend. “It complicates the debate,” he says.

Conclusions are more certain on the regional scale. As air traffic increases over some regions of the world, the increased density of contrails will likely bring even smaller differences between daytime and nighttime temperatures, and that will alter the local environment.

Cranberry bogs and citrus orchards, for instance, require a combination of cool nights and warm days for optimum yield. And in the spring, sugar maples don’t produce sap if daily temperatures don’t fluctuate enough. Some insects are particularly sensitive to changes in day and night temperature variations. And changes in insect populations can in turn have some unexpected aftershocks.

Patrick Minnis of NASA’s Langley Research Center in Hampton, Virginia, says Travis’ results confirm his own studies on contrails. “Having this data set made the relationship a little more definitive,” Minnis says.

Instead of studying the lack of airborne jets during the FAA’s three-day moratorium, Minnis looked at the few aircraft that were in the skies — military jets and transport planes.

In a usually packed air corridor around Washington, D.C., Minnis followed satellite images of a lone contrail flying over the mid-Atlantic states on September 12. The three days of grounded air travel provided him a unique opportunity to model the evolution of a single contrail where normally scores or hundreds would be found.

He saw six contrails, each no wider than an airplane wing, evolve in a matter of hours into cloud banks that covered 20,000 square kilometers. “This is a once-in-a-lifetime opportunity to measure these contrail effects,” Travis says. “Or, at least, we can only hope it’s once in a lifetime.”

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