Jet contrails, those white streaks of cloud that form behind the engines of airliners, may be contributing significantly to the cloud cover over regions served by busy air lanes, scientists say.
One figure-eight contrail created as part of a NASA-financed study was tracked by satellite for 10 hours last spring. It triggered formation of a cirrus cloud canopy that eventually spanned 20,000 square miles over the Gulf of Mexico.
"For 30 years, people have been speculating about whether contrails might be affecting the climate," said Dr. O. Brian Toon, a physicist at the University of Colorado at Boulder.
Now, thanks to new data from satellite photos and chase planes, researchers are finding some answers. "There's something there to be a little worried about," he said. "But we don't [yet] know how worried we need to be."
Toon and his colleagues in NASA's "Subsonic Aircraft: Contrail and Cloud Effects Special Study" (SUCCESS) reported their initial findings yesterday in Baltimore at the spring meeting of the American Geophysical Union.
Jet contrails form when small amounts of water vapor created by the combustion of fuel are released in engine exhaust. Within a second, the vapor cools and condenses, forming ice crystals that appear as long, thin clouds or condensation trails -- hence the name, contrails.
During the Cold War, U-2 spy plane pilots installed rear-view mirrors on the aircraft to monitor their exhaust. If the plane was producing a contrail when they crossed the border into the Soviet air space, they would turn around rather than risk detection.
Sometimes contrails evaporate quickly and disappear. But when the temperature and humidity of the atmosphere are right, the contrails persist for hours and become seeds that trigger the formation of feathery cirrus clouds.
During the day, these broad, bright cirrus clouds can reflect solar radiation from space, thereby cooling the land below.
At night, they can prevent the radiation of the Earth's warmth into space, raising the overnight low temperatures on the ground.
But in general, said Patrick Minnis, an atmospheric scientist at NASA's Langley Research Center, cirrus clouds create more warming than cooling.
Scientists say the average cirrus cloud cover over the United States has increased 5 percent since the 1960s.
Because that coincides with the growth of jet air travel, there has been speculation that contrails may be contributing to the growing cloudiness, and to changes in climate under busy air lanes, especially those in the United States and Europe.
National Aeronautics and Space Administration has budgeted $140 million over 10 years to help support a larger international study of contrails.
The study, including the SUCCESS mission, will assess the impact on the climate of both subsonic and future supersonic aircraft operations in the upper atmosphere, including their effects on cloud cover and atmospheric chemistry.
Minnis said previous contrail studies suggested the jet tracks account for just 0.5 percent to 2 percent of the cloud cover in the United States, too little to produce the observed increase in cloudiness since the 1960s.
The studies suffered, however, from the difficulty of distinguishing the smeared and expanded remains of old jet contrails from naturally formed cirrus clouds.
In a series of flights last spring, the SUCCESS project scientists flew a jet aircraft in distinctively shaped patterns to create contrails that could be identified and repeatedly photographed from space as they changed in shape and size over time.
One flight off the coast of California created a racetrack-shaped contrail more than 60 miles long.
GOES weather satellite photos show it gradually expanding as it drifted east over the coastline, widening into a white, 4,000-square-mile canopy before dissipating six hours later over the Sierra Nevada mountains.
Minnis said the data provide "solid evidence that contrails can grow far beyond [the size] of young contrails, and may be partly responsible for the increased cloudiness over the United States. They may have an important climatic effect."
Pub Date: 5/30/97
