Areas around airports experience more rain and snow, say researchers, as planes trigger precipitation by flying through clouds.
Last year, data from the the National Center for Atmospheric Research (NCAR) indicated that planes were seeding clouds at takeoff or landing, causing narrow bands of rain to develop.
Now, NCAR scientists have confirmed that the weird-looking hole punch and canal clouds are forming when planes fly through certain types of mid-level cloud. This forces nearby air to rapidly expand and cool, so that water droplets freeze to ice and then turn to snow as they fall toward the ground. They leave behind them odd-shaped gaps.
The findings will be a disappointment - or, possibly, evidence of a conspiracy - for those people who have suggested that the hole-punch clouds are being caused by UFOs.
The research team used satellite images and weather forecasting computer models to examine how often this happens within 62 miles of six commercial airports: London Heathrow, Frankfurt, Paris Charles De Gaulle, Seattle-Tacoma, Chicago O'Hare, and Yellowknife in Canada's Northwest Territories, as well as Byrd Station in Antarctica.
They found that, depending on the airport and type of plane, the right atmospheric conditions typically exist up to six percent of the time - and rather more frequently in colder climates. The key variable is whether there are cloud layers in the vicinity that contain water droplets at temperatures far below freezing.
One set of radar readings indicated a snowfall rate of close to an inch an hour after several planes had passed through.
"It appears to be a rather widespread effect for aircraft to inadvertently cause some measureable amount of rain or snow as they fly through certain clouds," says NCAR scientist Andrew Heymsfield.
"This is not necessarily enough precipitation to affect global climate, but it is noticeable around major airports in the mid-latitudes."
Of the major, mid-latitude airports studied, they found that the Frankfurt, DeGaulle, and O'Hare airports most frequently experienced the right conditions for propeller aircraft to generate precipitation. In each case, the conditions existed more than five percent of the time over the course of a year.
And this results in a lot of holes: for example, one cloud layer over Texas in January 2007 contained 92 such gaps, some of which persisted for more than four hours and reached lengths of 60 miles or more.
The researchers also found that commercial jets, military aircraft, regional and private jets, turboprops and prop/piston planes can all be culprits.
"It appears that virtually any airplane that flies through clouds containing liquid water at temperatures much below freezing can cause this effect," Heymsfield says.