Roll clouds are often mistaken for tornadoes, especially when they hang low like this one did over downtown Racine, Wis., 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.
The long, sideways shape is usually enough to identify a roll cloud, but if you still aren't sure which one is looming above you, it might be wise to just assume it's a tornado and take cover.