April Weather Brings More Contrails

April 8, 2008 -- March winds bring April jet contrails crisscrossing the sky, say meteorologists who have figured out where, when and why the sun-blocking, regionally climate-warming outbreaks happen across the United States.

By studying years of satellite views of contrail outbreaks and sorting out the conditions and the locales where they are most likely, the rhyme and reason of the human-made, high-altitude cirrus clouds are beginning to come into focus.

One of the least surprising discoveries is that the outbreaks tend to favor places with lots of air travel. But that alone does not explain why some days there are no contrails and on others, particularly days in April and October, the skies can be infested with the white lines.

"They tend to occur in clusters," said contrail researcher Andrew Carleton of Pennsylvania State University. "Rather than getting just one or two, you get a lot of them." Carleton and his colleagues David Travis, Kara Master and Sajith Vezhapparambu published their discoveries about contrail outbreaks in a recent issue of the Journal of Applied Meteorology and Climatology.

By studying the satellite data and matching it to other meteorological factors, the team confirmed that colder-than-normal air, plus more humidity way up in the upper troposphere, where jets fly, is key to getting contrails that persist rather than just evaporate.

These two conditions tend to be more common during the transitional seasons of spring and fall, he explained.