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Strange shimmering cloud in the sky is a fallstreak over Australia

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A fallstreak hole seen over Victoria, Australia, surprised and bewildered viewers on the ground. This rare cloud formation occurs when water droplets in clouds freeze into ice crystals, then start to fall. A gap created in the clouds allows sunshine to pass through, which is refracted and reflected among ice crystals and creates a rainbow.

Fallstreak holes are sometimes known as cloud punch holes, due to the way they create a nearly-circular gap in the meteorological features. The rare meteorological event was seen about 1 p.m. local time on Nov. 1.

"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. In this case, when the water does start to freeze, it falls down to the surface ... so you're left with this cloud surrounding it, this clear area," Michael Efron, of the Bureau of Meteorology in Australia, said.

Clouds surrounding the fallstreak hole were located just over three miles above the surface of the planet. Fallstreak clouds are often triggered by airplanes flying through clouds under certain conditions. Researchers believe propellers or wings of aircraft may set up conditions favorable to fallstreak cloud formation. They can persist for as long as meteorological conditions continue.

The rainbow seen within the unusual display is the result of iridescence, similar to the process that creates colors seen in soap bubbles, some sea shells and butterfly wings.

"This type of [cloud] iridescence occurs with high level clouds and with diffraction, [meaning] that water droplets are gathering light," Efron told the press.

Skygazers who witnessed the event took to the Internet, to spread their stories and photos of the bizarre phenomenon. Some people referred to the natural formation as a "rapture cloud."

A similar feature appeared over the city of Perth, Australia, in October, shaped like a watery flying saucer. Meteorological organizations do not keep track of how often these features form, although they can be dramatic at times.

"We just looked up and there was this thing that looked like something out of Independence Day," Ben Stewart, who witnessed the earlier event while vacationing with his family on Phillip Island, said.

The events are seen in locations around the globe, wherever mid- to high-altitude clouds can form, and temperatures and lighting conditions are correct for the phenomenon to be seen from the ground.

Video of the fallstreak hole was made available by Jason Prekop on his YouTube page.

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