The terrorist attacks on New York and the Pentagon gave climate researchers an unusual opportunity to study the effects of contrails, the drifting streaks of heavy exhaust plumes left by aircraft.

Immediately after the attacks, the Federal Aviation Administration ordered the skies cleared except for military aircraft, and the suspension of flights lasted for three days. Because of that, hardly any contrails were over the United States.

A result of that, said Dr. David J. Travis, a lead author of the study and scientist at the University of Wisconsin-Whitewater, was an unusual widening in the average daily temperature range, the difference between the day high and evening low temperatures.

The study was published in the current issue of the journal Nature.

Dr. Travis and his colleagues theorize that the absence of contrails was responsible for the difference. In areas of high density air traffic, contrails contribute to thicker cloud coverings that reflect warmth from the sun, making days cooler, but trap heat radiated from the ground, making nights warmer.

This can harm natural ecosystems that are adapted to extreme temperatures.

The scientists based their conclusions on data gathered from more than 4,000 weather stations and compared them with other United States data collected from 1971 through 2000 for three periods, Sept. 8-11, 11-14 and 14-17.

For the 30-year period, the average daily difference between high and low temperatures of the nine-day period showed an overall decrease. But in the three-day period when flights were suspended, the average daily temperature range increased by 2 degrees. This increase is greater than any of the recordings for the same three days for the earlier 30 years.

Researchers studying climate change should consider contrails, especially in regions of heavy air traffic, Dr. Travis said. He said contrails "are expected to increase many times their present coverage by the middle of the century."