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.
They also found that during the years 2000 through 2002, almost a third of all contrail outbreaks were in the Midwest. Next in line were the Northeast and Southeast, each with a fifth of the overall U.S. contrail action. Western states have contrail outbreaks as well, but not so many, Carleton told Discovery News.
Contrails form a layer of high clouds that seem to cool things off during the day. However, that effect is deceptive. It turns out that the clouds also reduce the temperature difference between night and day by trapping "long-wave radiation" -- which means heat -- and not letting it escape into outer space.
"They tend to heat the upper troposphere more than they warm the surface," explained Patrick Minnis of NASA's Langley Research Center. "The debate now is whether the warming is larger than the albedo."
In other words, do the white contrails reflect more sunlight energy than they trap? Most researchers lean towards the heat-trapping effect being greater. But the jury is still out.
The good news is that even if the contrails turn out to contribute to local heating and global warming, it's fairly easy to avoid making contrails by simply having plane avoid the air that is most likely to form the cloud -- or to fly in natural cirrus clouds that are already there, said Minnis. To make that possible, he and his team are already working on a contrail forecasting tool.
Related Links:
Larry O'Hanlon's blog: Earth Impacts
NASA's Langley Research Center
How Stuff Works: Why do those long, white clouds form behind jets flying high overhead?