The grounding of all commercial aircraft over the U.S. in the days following last September 11 gave meteorologists an unprecedented chance to study how high-flying aircraft can influence climate.

High-altitude jets often leave in their wake cloud-like trails of condensation called, appropriately, "contrails". They're made of ice crystals that form around aircraft exhaust particles. Naturally-occurring clouds at this altitude are known to slightly cool the earth during the day by reflecting a little sunlight. But at night these clouds slightly warm the lower atmosphere by absorbing some of the heat given off by the earth. Basically, such clouds reduce the difference between the daytime high and the nighttime low. Scientists have suspected that contrails do the same.

By comparing temperature data from last September with that of previous years, meteorologists were able to confirm this suspicion. They found that during the three-day hiatus of air traffic, daytime highs were a little above average, and nighttime lows were a little below average. The largest deviations - on the order of a couple degrees - occurred in areas of the Midwest and the Northeast that are usually heavily blanketed with contrails in mid-September. This is just the latest evidence that human activity can, indeed, affect climate.