Large, mysterious holes that occasionally appear in clouds have kept scientists guessing since the 1940s. Now, serendipity has led a group of researchers to an explanation for the phenomenon.

A new study by the National Center for Atmospheric Research in Boulder shows that “hole-punch clouds” are formed when airplanes cool water droplets in the clouds to create snow or rain.

“Basically what the aircraft can do is they produce ice crystals in clouds or areas of clouds which otherwise might not have them,” said Andy Heymsfield, an NCAR scientist who worked on the study, which will be published in this month’s Bulletin of the American Meteorological Society.

When a propeller plane goes through a mid-level cloud, the thrust causes the air to expand behind the propellers, allowing it to cool to 14 degrees Fahrenheit. The process works similarly in jet planes, but the air expands over the wings and is cooled to 5 degrees, Heymsfield said.

Snowflakes created by the planes use up all the water vapor in that isolated area and create the hole-punch shape in the cloud.

The process was difficult to discover because weather conditions below the clouds can prevent the snow or rain from reaching the ground.

But during a 2007 research flight, Patrick Kennedy, a research associate in Colorado State University’s atmospheric science department, noticed something unusual on his radar.

“This echo was long and narrow and had sharply defined edges,” Kennedy said. “At the time I thought it was some sort of ordinary snowband, although this was so sharply defined it seemed a little eye-catching.”

Kennedy studied the images, and researchers eventually reached their conclusion by comparing the data to airplane departure times and routes from Denver International Airport.

“What struck me as the most remarkable thing is that .. you have this phenomenon you don’t understand and by stroke of luck you have a fully instrumented airplane fly right through it and make its measurements,” Kennedy said.