Anyone who has ever seen a streaky line of vapor, known as a contrail, behind a high-flying aircraft knows that airplanes can produce their own clouds. But in rarer cases aircraft can also punch round holes or carve long channels through existing, natural clouds.
Those hole-punch and canal clouds arise from the strong cooling effects of airflow past a plane's propeller or over a jetliner's wing, according to a new study. That cooling can spontaneously freeze water droplets in the cloud and stimulate precipitation, the study's authors say.
The phenomenon requires a very specific set of cloud conditions and so is unlikely to have significant large-scale effects, but it could have an impact on regional weather near airports. A team of researchers from the National Center for Atmospheric Research (NCAR) in Boulder, Colo., the NASA Langley Research Center in Hampton, Va., and the University of Wyoming in Laramie report the new findings on the inadvertent aircraft cloud seeding in the July 1 issue of Science.
"If you land in a humid area, especially in the tropical zone, you see the condensation over the wings," says lead study author Andrew Heymsfield, a cloud physicist at NCAR. That same cooling of air as it rushes over a jet's wing and decreases in pressure can drop the temperature of a cloud by 20 degrees Celsius. For non-jet-powered aircraft, the expansion of air behind a rotating propeller can generate even stronger cooling of up to 30 degrees Celsius.
Hole-punch and canal clouds form when the cloud layer is in a so-called supercooled state, in which the water droplets are below the point of freezing but have not yet crystallized into a solid. With a little encouragement—by providing a solid nucleus for ice to grow on, as in cloud seeding with silver iodide, or by cooling the cloud further, to about –40 degrees C, which forces the freezing process—the somewhat fragile supercooled state collapses, and water in the cloud begins to freeze.
The airplane-induced freezing works by the forced-cooling process. If a cloud's supercooled water is at temperatures of –20 degrees C, for instance, the passage of a propeller plane or jet can drop temperatures enough to reach the spontaneous freezing threshold of –40 degrees C. "The [clouds] don't have a strong predilection toward precipitation, but they are close enough that a nudge will push them into precipitating," says cloud physicist Patrick Chuang of the University of California, Santa Cruz, who did not contribute to the new study.
In a paper published last year in the Bulletin of the American Meteorological Society, Heymsfield and his colleagues documented inadvertent cloud seeding by turboprop planes during a December 2007 field experiment in an NCAR aircraft studying clouds over Colorado. "About six months after the field program the scientist who was running the radar came to me and said to me, 'You know, there's this crazy-looking feature in my radar during the experiment'," Heymsfield recounts. "It's this band of precipitation, which we don't usually see in the wintertime." The group identified two specific turboprop commuter planes as the cloud-busting culprits.
"We weren't content with that, so we went to really detailed high-resolution imagery" for the new study, Heymsfield says. He and his colleagues used a variety of radar readings, satellite data sets and Federal Aviation Administration flight records to locate holes in clouds caused by specific aircraft of all types—commercial jetliners, turboprop planes, private jets and single-engine propeller planes, to name a few. In one instance over Texas satellite imagery revealed 92 cloud features of both the hole-punch and canal variety, some lasting up to four hours, most of which were traced back to individual aircraft. "It was previously thought that only turboprop aircraft can inadvertently produce ice formation, but we documented that all manner of aircraft in fact do it," Heymsfield says.
The new study "offers a very convincing explanation" of how aircraft form holes in the clouds by inadvertently seeding them, says Ulrike Burkhardt of the Institute of Atmospheric Physics of the German Aerospace Center in Oberpfaffenhofen. "This aircraft effect has until now been largely overlooked."
So how much of an impact does the airplane effect have on precipitation? That depends on temperature, latitude and a number of other conditions. But in one case near Denver in 1992 the researchers found plane-carved channels in the cloud layer that corresponded to increased snowfall of about 2.3 centimeters per hour. The researchers note, however, that the circumstances required for airplane-triggered precipitation are rather rare. Reviewing archival data from two cloud-tracking satellites, the group found that the conditions necessary for cloud seeding by jet aircraft occur only 2 to 3 percent of the time over a sample of seven airports, including London's Heathrow, Paris's Charles de Gaulle and Chicago's O'Hare.
The global effects of the phenomenon indeed seem to be minimal, but it does raise a few questions. The National Climatic Data Center (NCDC), for instance, which collects data from around the U.S., uses airports as some of its monitoring stations. If airports affect their local weather, that could skew the NCDC data slightly. "That would potentially be the one place that I could see it being slightly relevant," Chuang says. "I know certainly one of the foundations of the database is all the airport observations, so it would be a good question to understand whether or not this really matters. My gut feeling is that it probably doesn't."
Heymsfield would also like to know if early studies on the efficacy of releasing cloud-seeding chemicals from aircraft may have mistaken the effects of the plane itself for the effects of the chemicals. "My thought is that some of those earlier observations need to be repeated because they may have been inducing ice in the clouds they were trying to seed," he says. "I think some of those early studies were affected by the research aircraft they were using."