Planes leave mark in mid-valley skies

By Hasso Hering
Albany democrat-herald

On an otherwise cloudless day, the aerial tracks left by passenger jets can turn the sky a wispy gray.

And there are those who wonder whether — or how much — this affects the weather below and whether it might even contribute to global warming.

Dozens of contrails were visible above the mid-valley Thursday afternoon, a warm spring day. Some of them seemed to have spread out into regular cirrus clouds.

Jim Todd, head of the planetarium at the Oregon Museum of Science and Industry in Portland, called attention to the same phenomenon one day in April.

“The contrails were quite noticeable in the morning sky. Even now there are some noticeable trails,” he wrote in an e-mail, going on to explain: “Contrails are clouds formed when water vapor condenses and freezes around small particles (aerosols) that exist in aircraft exhaust. Some of that water vapor comes from the air around the plane; and some is added by the exhaust of the aircraft.”

High clouds tend to let sunlight through and trap warmth below, raising the question of whether more and more contrails from increasing jet traffic increase the average temperature and thus contribute to global warming.

Some say this is happening. According to a NASA study quoted on livescience.com, cirrus clouds can raise surface temperatures enough to account for a warming trend in the United States between 1975 and 1994. The study found that cirrus clouds over the United States increased about 1 percent per decade.

NOVA, the science program on public television, reported on the issue in April.

In the mid-valley, state climatologist George Taylor said it’s likely that contrails affect the temperature down below.

“The question is how much, and we really don’t know,” he said. “It’s pretty subtle and hard to measure.”

According an article posted on the NOVA Web site, scientists had an unprecented chance to study the effect of contrails in the few days after Sept. 11, 2001, when all commercial air traffic was grounded.

A study by scientists at the University of Wisconsin-Whitewater compared temperatures across the United States during that time with average readings during normal times. They found that the days were slightly warmer and the nights cooler during the days of no air traffic.

They thought there had been a net warming but acknowledged there was still a lot of argument, according to the NOVA report.

For more on contrails and their effect on the weather, check http://www.livescience.com/environment/050126_contrail_climate.html