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Roll clouds are a pretty rare phenomenon. Aside from Australia's Morning Glory cloud which appears on a fairly regular basis thanks to specific meteorological conditions, these unique cloud formations are pretty unpredictable. So you can imagine our surprise and excitement when we spotted one rolling passed our office in Durban, South Africa last week.

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As it made its way across the city, Durbanites took to social media to share their pics of the unusual tubular formation. These sideways cloud spirals are sometimes described as horizontal tornadoes, but you needn't rush to the storm shelter, unlike twisters, these clouds aren't destructive. The roll cloud didn't hang around for long, but we did manage to snap a few quick pics and admire the rare event. 

So what exactly is a roll cloud? Along with shelf clouds, roll clouds fall into the arcus cloud category. However, they differ from their category cousins in that they are completely detached from other cloud features and, as their namesake would suggest, they appear to be rolling on a horizontal axis.

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According to the Cloud Appreciation Society, "roll clouds can be formed by the sinking cold air associated with storms. Vertical columns of warmer air surge up the middle of storm clouds, while cooler air sinks downwards around them – helped along by all the falling rain and hail. In certain conditions, this sinking air can hit the ground in such a violent way that it causes a solitary wave of air to advance ahead of the storm. In the middle of this advancing wave of air, a roll cloud can occasionally appear, rotating as it travels."

The roll cloud was a precursor for a short-lived, but menacing storm that concluded an exciting day of weather here in (usually) sunny Durban. It certainly caused a bit of buzz in the Earth Touch office.

Surprisingly, ours wasn't the only roll cloud to hit the healines recently. Earlier this month, a couple in Timbercreek Canyon filmed this dusty vortex formation south of Amarillo, Texas.

Have you ever seen a roll cloud, let us know in the comments below and share your pics with us on Facebook or Twitter.