A “hole in the sky” was spotted in three different countries all in just a single week.

These rare clouds known as hole-punch clouds or fallstreak holes create stunning visuals. 

People in Austin, Texas; Athens, Greece; and Manitoba, Canada, all got to catch the sight of one in mid-October.

Look up on the right day, and you might spot an odd sight: a oval-shaped gap cut into a sheet of clouds, with wispy streaks trailing down from the center like a jellyfish.

What are hole-punch clouds and how do they form?

Hole-punch clouds look alien, but they’re actually man made: a result of simple physics meeting a flying aircraft.

The holes form in mid-level clouds, usually altocumulus or cirrocumulus, made up of tiny water droplets that stay liquid even in subfreezing temperatures. 

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(MORE: Strange holes in Georgia's clouds: the science behind fallstreak formations)

These supercooled droplets need something to kick-start freezing, either a speck of dust or other particle to latch onto, or a drop in temperature below about minus 40 degrees Fahrenheit. Left alone, they can stay liquid far longer than their temperature would suggest.

That’s where aircraft come in. As a plane’s wings, propellers or landing gear cut through the cloud, air moving over and around them expands and cools rapidly, a process known as adiabatic cooling. 

That brief chill is enough to flash some of the supercooled droplets into ice crystals. Once a few crystals form, they pull in surrounding water vapor and grow quickly, a chain reaction that strips moisture from nearby droplets, which evaporate and clear out, leaving a widening gap in the cloud deck. 

Meteorologists call this larger process the Bergeron process, and the resulting clearing is labeled a cavum.

(MORE: New cloud types added for the first time in 30 years)

The ice crystals that triggered the hole keep growing until they’re heavy enough to fall, creating the trailing streaks of virga visible beneath the gap. A hole-punch cloud can expand well beyond the width of the aircraft that caused it, with some reported to grow tens of miles across within an hour.

Busy flight paths — like Austin, Winnipeg and Phoenix — are the best places to see one.

Content writer Ada Wood enjoys exploring the stories that science and climate teach us about our natural world and how it influences the way we live in it.