"In the past, some studies have compared the climate of areas with little overhead air traffic with those under heavily used flight paths," says Dr. Andrew M. Carleton, professor of geography. "Other studies looked at cloud cover before the advent of heavy jet traffic in the 1960s and afterwards, but these studies really provide circumstantial evidence."

Carleton, and Dr. David Travis, climatologist at University of Wisconsin-Whitewater and Ryan Lauritsen, then an undergraduate at UW-W, looked at high and low temperatures recorded throughout the country during the three days of commercial air shutdown and at satellite photos taken during that time. The satellite photos show where contrails were occurring, mostly absent over the U.S. between Sept 11-14, but still occurring in Canada and northern Mexico.

"We show that there was an anomalous increase in the average diurnal temperature range for the period Sept. 11-14, 2001," the researchers reported in today's (Aug. 8) issue of the journal Nature. "Because persisting contrails can reduce the transfer of both incoming solar and outgoing infrared radiation and so reduce the daily temperature range, we attribute at least a portion of this anomaly to the absence of contrails."

The diurnal temperature range is the difference between the nighttime low temperature and the daytime high temperature, usually for a given day.

"Because the shutdown occurred before noon on Sept. 11, the low temperature had already been reached, so we looked at 24-hour periods beginning with the high on Sept. 11 to the low on Sept. 14," said Carleton.

The change in the temperature difference was plus 1.1 degree Celsius, equal to plus 2 degrees Fahrenheit, above the 30-year long-term mean diurnal temperature range. The researchers compared the temperature ranges on these three days to those of the three days directly before Sept. 11 and the three days after Sept. 14, finding that the days before and after were similar, but that the three days in question differed by 1.8 degrees Celsius or 3.2 degrees Fahrenheit.

"Sept. 11-14, 2001, had the biggest diurnal temperature range of any three-day period in the past 30 years," said Carleton. Contrails form when water vapor an

Story Source:

Materials provided by Penn State. Note: Content may be edited for style and length.