Heymsfield's research team looked back on a December 1992 radar image taken over the former Stapleton International Airport in Denver, and noticed curved bands of snow in the atmosphere, different from the straight lines formed naturally. The snow-band paths and their proximity to runways were clues, Heymsfield says, that airplane activity could influence snow production.
Although 32 degrees Farenheit is commonly called water's "freezing" point, water in the atmosphere can remain "supercooled" in liquid form at much lower temperatures. That's because water droplets, even on the ground, typically need a solid particle or a "nucleus" to latch onto as they condense into ice. Without a nucleus - and suitable particles are much less common high in the atmosphere - water freezes at only -40 degrees or below.
Airplanes pushing through supercooled clouds can cool the air around their wings and propellers by as much as 86 degrees - enough to start freezing supercooled droplets. According to Heymsfield, the resulting ice crystals act as nuclei that draw in more water droplets to condense and freeze together. Once started, a chain reaction creates expanding ice crystals which, if heavy enough, eventually fall from the sky as snow or rain.
In their computer model, Heymsfield and his team showed how this form of precipitation can leave behind holes in the cloud. Their simulations also predicted that the holes would expand over time, as ice formation releases heat (the reverse process by which melting ice requires heat input) and causes updrafts in the cloud.
The model predictions matched satellite images taken on Jan. 29, 2007, over Texas, where researchers measured 92 holes along flight corridors trafficked by a variety of airplane types, makes, and models. Some of the holes through clouds grew to more than 60 miles in length over a four-hour period.
