Jet contrails--the line-shaped clouds produced by high-flying planes--have long been suspected of affecting weather on the ground. Now a unique experiment has tested the idea for the first time. By taking advantage of the U.S. air traffic shutdown during 3 days after the 11 September terrorist attacks, a research team has shown that jet contrails do indeed influence the weather.
Contrails are "condensation trails" made up of water vapor and particulates emitted from jet engines. Although contrails evaporate quickly in dry air, in humid air they grow into extensive cirrus clouds identical to naturally occurring clouds. Because clouds reflect solar radiation, they keep the ground cooler during the day. At night clouds trap heat, keeping the surface warmer. Researchers suspected that contrails would do the same in heavily traveled areas, but they never had direct evidence.
To fill that gap, climatologists David Travis of the University of Wisconsin, Whitewater, and Andrew Carleton of Pennsylvania State University, University Park, compared ground temperature data from 4000 weather stations in 48 states collected between 1971 and 2000 with similar data collected during the 3-day aircraft shutdown last September. The difference between daytime maximum and nighttime minimum temperatures was 1.1°C greater in the absence of contrails, the group reports in the 8 August issue of Nature. This difference was even greater in areas where contrail coverage is normally thickest--the Midwest, Northeast, and Northwest.
The results make sense, says climatologist and contrail expert Patrick Minnis of NASA's Langley Research Center in Hampton, Virginia. The increasing amount of jet traffic could explain why the daily temperature range has been decreasing over many parts of the globe during the last 3 decades, he says. "If contrails are significantly affecting the diurnal temperature range, then air traffic is having a measurable climate impact and therefore should be included in model estimates of climate in the future."