Thunderstorms produce a lot of weird things
, which are often easy to miss amid the urgency of tornadoes
. 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.
This eerie scene, captured in January 2009 at Uruguay's Las Olas beach, hints at how far roll clouds can stretch out. It also reveals another quirk: They work alone, often snaking through the sky without a storm in sight.
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.
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.
Roll clouds can occur almost anywhere, but they seem most at home in Australia's Gulf of Carpentaria, reportedly the only place on Earth where they can be reliably predicted.
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
Just because a roll cloud isn't linked to a storm doesn't mean it should be ignored. This one drifted over southwestern Ohio in 2006, for example, 5 to 10 miles
ahead of a severe storm system that was blowing in from Indiana. The spooky scene gave residents an early warning.
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.
As if this roll cloud didn't look dramatic enough on its own, its front edge was also illuminated by the morning sun, which was just beginning to rise over northern Missouri on June 10, 2005.
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
If they're big enough, some storms look even more impressive from above. This long, meandering roll cloud
was photographed over the Pacific Ocean on Oct. 5, 1985, from about 300 miles overhead.
An astronaut aboard the space shuttle
Atlantis captured the scene during the shuttle's first-ever mission, officially dubbed STS-51-J
Bowling Green, Ky.
Several roll clouds were spotted around central Kentucky on June 28, 2007, as a wave of scattered thunderstorms moved through the area. They signaled the arrival of a "gust front" — the cool rush of wind that often precedes a storm by several minutes.
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
Maldonado, Uruguay: Daniela Mirner Eberl/NASA
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