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How Airplanes Punch Holes in the Sky

By Michael D. Lemonick

Hole-punch clouds, such as this one, can result when aircraft fly through clouds
containing supercooled water droplets.
Photo credit: NCAR

How did this happen? The crazy-looking cloud formation in the photo above isn’t a still from a sci-fi movie. It’s not Photoshopped. It’s quite real.

It’s also totally artificial, which requires a bit of an explanation.

Since at least as early as the 1940s, meteorologists have been noticing formations like this, and it wasn’t long before they figured out that aircraft were probably involved somehow — perhaps by creating a pressure wave as they passed through, or by heating the clouds and evaporating them.

By the 1980s, says Andrew Heymsfield, a scientist with the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, the holes had become more common, and the explanation had become clearer: as planes punch through cloud decks that have particular characteristics, the air is compressed, then expands and cools (just like the coolant in an air conditioner). The cooling forces water droplets to freeze into tiny ice particles; these in turn act as seeds around which raindrops or snowflakes can form. The clouds then ‘rain out’ or ‘snow out’, leaving a hole in their wake. (Not everyone buys this theory, as a headline from The Sun makes clear.)

That’s the theory, anyway. But now Heymsfield has the smoking gun (here’s a press release from NCAR), so to speak. Back in 2007, he and some colleagues flew their research aircraft through a snow squall west of Denver. They checked later with ground radar, and learned that the band of snow was oddly shaped — about 20 miles long, but only about 2½ miles wide — and it had appeared and disappeared quite abruptly, leaving a couple of inches of snow in its wake. Then they checked the cameras on their plane, and discovered a hole in the clouds, something like the one above, except that it was sandwiched between solid cloud decks. You wouldn’t have seen it from the ground. 

You might not have seen it from a satellite either — such holes are often hidden entirely. But that’s not the case in this image from space, centered over the Texas-Louisiana border. Just about every one of the spots, big or small, is a hole punched by an ascending or descending plane. Some of the lines are also caused by planes traveling through the clouds at a constant altitude. “You can probably see around 50 of these artifacts in the image,” Heymsfield said.

This image, captured by the Moderate Resolution Imaging Spectroradiometer (MODIS)
on NASA’s Terra satellite shows shows up to 50 "hole-punch" features in clouds above
Oklahoma, Arkansas, Louisiana, and Texas. Photo credit: NASA.

The holes don’t just close up right away, either. “We’ve tracked some of them [by satellite] for up to five hours,” Heymsfield said.

If this phenomenon were just an example of gee-whiz science, the paper Heymsfield and four co-authors published about it in the Bulletin of the American Meteorological Society would be fascinating enough. But it turns out that it has military implications as well. Heymsfield has consulted with Boeing on how to fly jets so as not to leave marks in the sky. “Military aircraft,” he notes, “really don’t want to visible. On [the day the satellite image was taken] we saw a fantastic trail from a B-52.”

The study also emphasizes once again that one of our most natural instincts is simply wrong. The Earth is so enormous, and we’re so puny, that it seems impossible that human activity could alter the planet in any significant way. That’s one serious barrier to taking action on climate change in particular, and on environmental problems in general. 

Unfortunately, there’s plenty of evidence showing how wrong our instincts are. Oil spewing into the Gulf, another set of record warm temperatures, and endangered species wherever we look are just a few examples. Holes punched in clouds can now be added to the list of human modifications of the environment around us — even if the results do look like science fiction.

NCAR researcher Andrew Heymsfield discusses the aircraft-induced cloud modification study.
Video credit: NCAR.


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