A cloud that appeared in the sky over southeast Australia caused a stir on social media Tuesday after photos of the bizarre formation were posted all over Twitter and Instagram. The formation, which Internet users quickly labeled the “rapture” cloud, appeared a day earlier like a giant footprint over the seaside town of Wonthaggi, about 132 kilometers (82 miles) from Melbourne, in the state of Victoria.
Some explanations for the rare sight included aliens, paranormal activity and even an act of God. But believers in the supernatural might be disappointed to know that there was a perfectly logical explanation for the phenomenon, according to scientists. "It looks like a fallstreak cloud," Michael Efron, a weather forecaster with Australia’s Bureau of Meteorology, told Fairfax Media. Fallstreak clouds, also known as hold punch clouds, are large, circular or elliptical gaps that appear in cloud cover. "They form when the water temperature in the cloud is below freezing, but the water has not yet frozen due to a lack of ice nucleation particles,” Efron said.
— Simon Crerar (@simoncrerar) November 4, 2014
— M McKenzie-Murray (@feed_the_chooks) November 3, 2014
Scientists believe fallstreak clouds appear after airplanes passing through the sky cool the air around them and create ice. “Once the ice crystals are introduced, the water droplets quickly freeze, grow and start to fall,” according to the National Oceanic and Atmospheric Administration. “A hole is left behind, which will start to expand outward as neighboring droplets start to freeze."
Fallstreak clouds can appear anywhere in the world. Residents of the U.S. state of Iowa saw a similar cloud formation over the weekend. Another fallstreak cloud appeared over California in May.
Sightings of fallstreak clouds can lead some observers to say they witnessed a UFO or, in other cases, a government cover-up. After several people saw the fallstreak cloud over northern California this year, some said it was a sign that the U.S. government had manipulated the weather, according to Roadtrippers.