Contrails forecast on the horizon. (Atmospheric Science).

Studies of the contrails generated by jets flying high over Alaska may lead to improved techniques for predicting the formation of these artificial clouds, which some scientists suggest have a warming effect on Earth's climate.

The skies over Fairbanks, Alaska, are busy because the city lies beneath air routes between North America and the Far East, as well as the flight paths between nearby Anchorage and dries in Europe. Between March 2000 and July 2002, more than 2,500 jets passed within 80 kilometers of Fairbanks International Airport, says Martha Shulski, an atmospheric scientist at the University of Alaska in Fairbanks.

Of those aircraft, about 10 percent flew past during daylight hours with good visibility and within 4 hours of when weather instruments were lofted from the airport. The scientists observed contrails in 223 instances, and the clouds' lifespans varied from a few seconds to several hours, says Shulski. In 20 cases, researchers spotted an aircraft that didn't produce a contrail.

About 97 percent of the contrails that lasted more than 10 minutes formed in air with a relative humidity greater than 25 percent. On the other hand, most of the aircraft that didn't produce contrails were flying through air with a relative humidity less than 25 percent.

Shulski and her colleagues have used their observations to develop a mathematical model for predicting whether or not contrails will form behind an aircraft. The new model is correct 92 percent of the time but doesn't do a good job at predicting how long contrails will persist, a factor needed for scientists to estimate the clouds' effect on global climate. Future analyses will investigate the effect of wind speed and wind shear on the spread and lifetime of contrails.--S.P.