9 Scary Images of Shelf Clouds

Thunderstorms and politicians are a lot alike: Both drift with the wind, both are full of hot air and both shy away from high pressure. And, fair or not, many people judge both by their faces.

While politicians grin for votes, though, storms glower over their constituents. Some even grow strange "shelf clouds" along their leading edges, like the one pictured here over Enschede, Netherlands. These cloudy countenances stretch out ahead of a storm, sometimes foreshadowing danger and sometimes just grandstanding.

To see more scowling storms, and to find out what causes them, check out the following photo gallery of nine frightening shelf clouds.

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Miami Beach, Fla.

South Florida is no stranger to thunderstorms, but it's still hard to shrug off a sight like this. A graduate student from MIT shot this shelf cloud as it swept across Miami Beach on Dec. 4, 2010.

Shelf clouds are a type of arcus cloud, formed by colliding updrafts and downdrafts. As a storm vacuums up warm air from below, it also pumps out cooler air at the top, which can spill forward, slip below the warm updrafts and condense into a horizontal "shelf." While some arcus clouds float away on their own — they're known as "roll clouds" — shelf clouds like this one remain attached to their parent storms.

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Warsaw, Poland

This dramatic shelf cloud, seen over Poland's capital on July 5, 2009, certainly looks menacing. It's easy to see why shelf clouds are often confused with wall clouds (drooping formations that can spout tornadoes), but the two aren't as similar as they might seem.

While shelf clouds are capable of causing trouble, they mainly serve as harbingers of more severe weather on the way — and even then, they've been known to exaggerate the threat. Wall clouds, on the other hand, typically form near a thunderstorm's more turbulent backside, as do most tornadoes, and are more likely to cause damage on the ground.

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Little Chute, Wis.

As a storm cell swept across eastern Wisconsin on June 13, 2004, it was led by a haunting shelf cloud, seen here over the town of Little Chute.

The storm made for some stunning images from Greenville to Green Bay, but luckily it was less severe than the photos suggest. "Although its appearance is threatening, and almost always precedes gusty winds, the [shelf] cloud is not necessarily a precursor to severe weather," the National Weather Service explains. Still, shelf clouds can generate dangerous straight-line winds, including the rare "derecho" and "gustnado," and shouldn't be taken lightly.

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Rochelle, Ill.

Like a giant wave crashing ashore, this shelf cloud seemed to swallow the town of Rochelle, Ill., on June 18, 2010. It didn't, but the storm behind it did dump nearly half an inch of rain, according to Weather Underground.

This photo also offers a good example of how shelf clouds form: There's a clear division as the dense, rain-cooled shelf cuts underneath the warm, moist air in front of the storm. And if that wasn't intimidating enough, an eerie blue glow gives the scene an almost supernatural quality.

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Öland, Sweden

This thunderstorm may have spoiled a day at the beach, but it also provided an unforgettable sight for beach-goers as it drifted toward the island of Öland, Sweden, on July 18, 2005.

Heavy rainfall can be seen gushing into the Baltic Sea from the storm's underside, while cool air spills unevenly from the top, helping give this shelf cloud its bizarre swooping shape.

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Hampton, Minn.

A UFO isn't beaming up this neighborhood in Hampton, Minn.; that's a shelf cloud, spotted south of the Twin Cities on June 25, 2010.

And the otherworldly blue glow? That's a "glow discharge," according to Cornell atmospheric scientist Mark Wysocki. It can occur when a thunderstorm is close to the ground, creating a large "surface charge density" for a long, slow-burning lightning strike — similar to how an electrical charge illuminates suspended particles in a fluorescent light bulb.

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Yucatán, Mexico

July is often a stormy month for Mexico's Yucatán Peninsula, and this picturesque shelf cloud on July 15, 2005, was just a precursor to a far more dangerous storm three days later.

After nearly an inch of rain on July 15, the eastern Yucatán got double that on July 18, when Hurricane Emily made landfall as a Category 4 storm. Emily left a path of destruction from Grenada to Mexico, and remains the strongest Atlantic hurricane ever recorded in July.

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Saskatchewan, Canada

Shelf clouds don't only glow blue, or from within. This one shone red, for example, as it was struck by the rising sun over the prairies of Saskatchewan, Canada, in August 2001.

This photo shows a similar effect on a roll cloud.

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Wichita, Kan.

This shelf cloud was part of a thunderstorm system that formed over western Kansas on May 6, 2008, gaining strength as it found more low-level moisture to the east, according to a National Weather Service storm summary.

The system eventually became a "bow echo," in which several storms unify into a squall line that resembles an archer's bow in overhead radar images. Shelf clouds often develop along bow echos, which sometimes generate dangerous winds like derechos and gustnados.