Thunderstorms produce a lot of weird things, which are often easy to miss amid the urgency of tornadoes, floods and lightning. But before a storm arrives, or sometimes out of the blue, rare "roll clouds" like this one command attention as they float ominously overhead.
"It was amazing," photographer Rob Sharrock told ZUMA Press in 2010, after shooting a roll cloud over Warrnambool, Australia. "I just looked up in the sky and said, 'Bloody hell, what on Earth is that?' It seemed to go on for miles."
Discover what causes these stormy spindles by checking out the following nine rockin' roll clouds from around the world.
Roll clouds are a type of "arcus cloud," formed when updrafts and downdrafts churn the front edge of a thunderstorm (or cold front) into a sideways cylinder. But unlike shelf clouds, the other arcus variety, roll clouds are detached from their parent storms — sort of like if a car's front axle broke off and rolled away.
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.
Known as "kangólgi" to Aboriginal people and "morning glory" clouds to local anglophiles, they most often form in the morning, especially from September to November. Their exact origins are hazy, though, since they aren't associated with thunderstorms like many other roll clouds are.
Morning glory clouds regularly attract glider pilots to the Gulf of Carpentaria — including Mick Petroff, who shot this photo in 2009 near Burketown, Queensland.
White Oak, Ohio
The National Weather Service notes that when an arcus cloud outruns its thunderstorm like this, it may be "a sign the storm is losing its potential to produce severe damaging winds." Still, that's not always the case, and it's often a good idea to take such foreboding skies seriously.
Photographer Dan Bush snapped this photo from a moving truck as he chased the cloud, which he estimated to be moving from west to east at about 35 or 40 mph. See more images here.
Bowling Green, Ky.
In this case, the detached gust fronts were a false alarm, with no severe weather in tow. They still offered some memorable images, though, including this one shot by local resident Scott Davidson over Bowling Green.
This sinister-looking roll cloud found a willing audience as it passed over the National Weather Service offices in Lubbock, Texas, on the morning of Sept. 25, 2007. It was one of several bands of roll clouds that moved through the Lubbock area between 6 and 8 a.m. that day, as a cold front pushed south across West Texas.
This loosely organized roll cloud may not be as smooth or tubular as some of its relatives, but thanks to the lightning bolt behind it — and the quick shutter finger of photographer Joe Thomissen — it still makes for a pretty impressive scene. Thomissen snapped this shot as a storm system drifted over southeastern Belgium on June 28, 2011.
Click for photo credits
Racine, Wis.: Brian Carlock/Flickr
Australia: Mick Petroff/NASA
White Oak, Ohio: John Graff/National Weather Service
Albany, Mo.: Dan Bush/NASA
Pacific Ocean: Image Science & Analysis Laboratory/NASA Johnson Space Center
Bowling Green, Ky.: Scott Davidson/National Weather Service
Lubbock, Texas: National Weather Service
Kanne, Belgium: Joe Thomissen/Flickr