Off the coast of California, contrails form an extensive cover of artificial clouds.
Photograph courtesy of Orbimage/NASA
The three-day ban on commercial air traffic over the United States after the September 11 attacks created an opportunity for a normally impossible environmental experiment. Two atmospheric scientists seized the moment to settle a long-standing controversy: Do jet contrails influence weather?

David Travis of the University of Wisconsin at Whitewater and Andrew Carleton of Pennsylvania State University compared the average daily temperature range—the difference between daily highs and lows—for the three flight-free days with the average temperature range on equivalent dates of the past 30 years. With no airplanes in the sky, the daily temperature range across the United States was about 2 degrees Fahrenheit greater than normal.

Contrails, which are essentially artificial clouds, form when moisture in the air condenses around particles in jet exhaust. Like ordinary clouds, contrails block incoming sunlight and trap heat radiated from Earth's surface. This process reduces daytime highs and increases nighttime lows, narrowing the temperature range. Multiple contrails can cluster together and obscure an area as large as Iowa, Illinois, and Missouri combined, magnifying the effect. "Increased jet contrails could substantially affect regional climate," Travis says. Although they may not alter the overall climate, contrails could still have environmental consequences, he warns. For instance, warmer nights might improve conditions for insect pests, allowing them to increase their numbers.