Thunderstorms produce a lot of weird phenomena, many of which are easy to miss amid the urgency of lightning strikes, tornadoes or flash floods. But before a storm arrives, and sometimes just out of the blue, rare atmospheric quirks known as "roll clouds" command attention as they float ominously overhead.
Roll clouds are often mistaken for tornadoes, especially when they hang low like this one did over downtown Racine, Wisconsin, in June 2007. But despite a superficial resemblance, roll clouds and funnel clouds don't have much in common.
For starters, roll clouds are generally harmless. While a tornado's vertical vortex can wreak havoc on the ground — destroying entire cities in extreme cases — roll clouds tumble along slowly and horizontally. They also form at the front of thunderstorms instead of the back, where most twisters are born, and they aren't even attached to the storms that spawned them.
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