A roll cloud can reach 1,000 km in length. This one, captured by Ken Temple over Lake Michigan is not that long, but it's still huge [source, full video: buff.ly/2JjOyOs, buff.ly/3pIBuGE]
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I saw one of these once at the beach one beautiful entirely blue sky day. Arriving from the horizon like a very slow train. When it was over us it unrolled showing a dark inside filled with lighting. Not long after that it covered the entire sky and one of the worst storms ensued
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Remember the movie Mist ? …. That cloud scares the crap out of me. When I was out in central australia I saw one of those rolling clouds.
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In Australia up in the Gulf of Carpentaria their called "Morning Glory " .
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