As airplanes fly through the clouds, they often punch holes through the ones that contain supercooled water, or water that has remained in liquid form below its freezing point. Now, researchers say that this phenomenon can lead to increased snowfall around the world's major airports.

The effect is similar to cloud seeding, which has been used in the past to influence the amount of precipitation falling from a cloud. But, new research shows that numerous private and commercial flights have been drilling holes and canals through clouds all along, influencing the snow and rainfall below them.

The study appears in the 1 July issue of the journal Science, which is published by AAAS, the nonprofit science society.

The inadvertent cloud-seeding effects described in the report are facilitated by the expansion and cooling of air behind a propeller aircraft's engine blades and over aircraft wings when supercooled cloud temperatures are about -10C and below.

The associated drops in temperature can be sufficient to spontaneously freeze the supercooled cloud droplets and form ice crystals, which then grow at the expense of the water droplets. The process then snowballs (literally) to produce a hole or a canal in the cloud layer that can continue expanding for hours, increasing precipitation in and below the cloud.

It is unlikely that this phenomenon affects the global climate. But, since many major airports have low cloud cover in wintertime, it may increase the need to de-ice planes more often in the future, researchers say.

Because airports in the Arctic and Antarctic house weather station records that climate modelers use, predictions for these areas may be skewed by the local weather conditions near the airport.

Andrew Heymsfield from the National Center for Atmospheric Research in Boulder, Colorado and colleagues from across the United States studied satellite images of these so-called hole-punch or canal clouds in detail, and then used weather-forecasting computer models to simulate the clouds' growth and evolution.

The researchers conclude that many different types of private and commercial propeller and jet aircraft can produce ice crystals and holes in supercooled clouds which then spread and create snow in and below them.

Such porous cloud covers, induced by aircraft, have been documented for decades-and they are sometimes mistaken for rocket launches or U.F.Os, like the ominous saucer-looking cloud that hovered over Moscow in 2009, prompting all sorts of speculation and conspiracy theories.

"Whether an airplane creates a hole or a canal in the clouds depends on its trajectory," Heymsfield said. "When they climb through a supercooled cloud layer, they can just produce a hole. But when they fly level through the cloud layer, they can produce long canals."

According to the new research, these supercooled cloud layers can be found within 62 miles (100 kilometers) of the world's major airports as much as five to six percent of the time. And the cloud-seeding effects that occur when aircraft pass through them are apparently more pronounced near the Earth's polar regions.

To draw such conclusions, Heymsfield and his team analyzed 20 satellite images of a cloud cover with holes in it that was suspended over the state of Texas on 29 January, 2007. Some of the holes in the cloud were visible for more than four hours and grew to lengths greater than 62 miles (100 kilometers) long.

The researchers then looked at archived aircraft flight information from the U.S. Federal Aviation Administration to find out what kind of aircraft had flown in that area-between 4.3 and 5 miles (7 and 8 kilometers) high-on that particular day.

They identified a number of different airplanes that are able to produce such holes and canals, from large passenger jets and military planes to small turboprops and private, single-engine jets.

"An aircraft propeller pushes air behind it, which generates thrust around the propeller tips," Heymsfield said. "This thrust, in turn, cools the air behind the propellers by up to 30 degrees Celsius, freezing cloud droplets and leaving a stream of small ice particles trailing behind the propellers."

Aircraft generate lift by moving air over the top of their wings, creating low air pressure above the wings than below them. "This also expands and cools the air above the wings by about 20 degrees Celsius, which is more pronounced at jet aircraft speeds," Heymsfield said. "And due to this cooling, a stream of ice can be produced behind the wings."

Up in the clouds, the air temperature often hovers around -10 degrees Celsius. But, if there is no dust or ice crystals-solid particles that can be used as nuclei to start the freezing process-water can remain supercooled, in its liquid form, until about -40 degrees Celsius.

This phenomenon forms the basis of cloud seeding operations that began in the 1960s: By inserting solid particles into clouds, researchers could initiate this freezing of supercooled water at temperatures warmer than -40 degrees Celsius.

"If you have supercooled water droplets and ice particles in the same cloud, the droplets will evaporate into the air and then condense onto the surface of the ice particles," Heymsfield said.

"This condensation creates latent heat and increases the surface temperature of the ice. This, in turn, generates some buoyancy in the cloud and creates upward motions in the cloud which may have been weak or non-existent before. The process can just cascade from there."

Aircraft coming and going from airports around the world can inadvertently seed clouds in this fashion to produce more snow in them, and if the clouds are at low levels, on the ground below, the researchers report.

By punching holes or driving canals through cloud covers, private and commercial flights alike could be responsible for increased snowfall around major airports.

For Heymsfield, the scientific investigation provided a thrill. "The most interesting part of this research to me is the physics-and the fact that the production and spread of the holes and canals can now be explained," he said.

"Now, I'm off to the Virgin Islands to investigate more tropical cloud covers there."

This report by Heymsfield et al. was funded by the National Center for Atmospheric Research, NASA grants, and the FAA Aviation Climate Change Research Initiative.

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