wired - Airplanes can punch holes in clouds and make it rain, new research shows. As propeller or jet airplanes pass through the right atmospheric conditions, they make liquid water droplets freeze and immediately drop as snow, leaving a circular fissure behind.

Odd clouds can sometimes elude explanation for decades, and these mysterious gaps in the sky, aptly called hole-punch clouds or channel clouds, have been puzzling sky gazers and scientists alike since the 1940s. A 1968 article in the magazine Weatherwise called them “A Meteorological Whodunit?”

As recently as October 2009, headlines touted a “Mystery UFO Halo Over Moscow.” Earlier studies had suggested a link between hole-punch clouds and airplanes, though the mechanism was unclear.

Previous research also suggests that propeller planes could make snow fall when they flew through supercooled clouds, where water droplets remain liquid despite subfreezing temperatures. But until recently, a direct connection between airplanes, hole-punch clouds and snowfall was missing. Now, a team of atmospheric scientists report observing all three in the June Bulletin of the American Meteorological Society.

Andrew Heymsfield of the National Center for Atmospheric Research and colleagues flew a research plane through the snow produced by a hole-punch cloud west of Denver International Airport in 2007. The plane was loaded with instruments for studying how ice forms in clouds. Radar from the ground picked up a strange echo in their wake, indicating oddly-shaped snowflakes.

“We didn’t know it, but we went right through this precipitation feature that was spotted from the ground,” Heymsfield said. Video from the flight showed a hole in a patch of altocumulus clouds (see below), and two inches of snowfall directly below the hole.

Heymsfield and colleagues examined flight records for the nearby airport and linked the hole and the snow to two commercial turboprop aircraft that had taken off about an hour before. The movement of supercooled water droplets over the planes’ propellers cooled them enough to make them freeze and plunge to Earth as snowflakes, Heymsfield said. Normally, snowflakes or raindrops need a speck of dust or another imperfection to form around. But supercooled water can freeze instantly, if it cools quickly to around minus 35 degrees Fahrenheit.

To confirm this idea, the researchers compared the snowflakes that fell on either side of the hole-punch cloud to the ice crystals that fell directly beneath it. The snowflakes that fell from the hole were more plate-like and simple than those that originated in the surrounding clouds. The more-complicated snowflakes had accumulated drops of liquid water as they fell, Heymsfield said. But in the middle of the hole, all the liquid had either frozen or evaporated.

“This phenomena removes the droplets, so these crystals as they fell were not collecting any droplets,” he said.

“My initial impression is that it’s reasonable,” commented Patrick Chuang, a cloud expert at the University of California, Santa Cruz, who was not involved in the new work.

The researchers then linked satellite images of hole-punch clouds to flight schedules to show that jet aircraft, not just propeller planes, can also punch holes and produce snow. The supercooled droplets freeze after passing over the jet planes’ wings, Heymsfield said.

“We know exactly which aircraft produced the holes,” he said. “Researchers had previously linked a lot of snow to propeller aircraft, but they hadn’t made the connection to jet aircraft.”

“There are a lot more jets flying around than there are propeller aircraft, so there’s much more opportunity to generate this effect,” he added.

Not all flights through banks of clouds will produce snow. About 7.8 percent of the Earth’s surface is covered with clouds at the right elevation for supercooled droplets to form. And because jet aircraft don’t generally cruise at those altitudes, they may only form hole-punch clouds when they take off or land.

The extra precipitation left by airplanes punching holes in clouds is probably not enough to change global climate patterns, Heymsfield added. But “regionally, it could certainly enhance snowfall.”

Cloud-Hole Phenomena Explained