Shelf Clouds: 20 Storm Cloud Photos From Romania to Saturn's North Pole

The low, dark wall that slides across the sky ahead of a summer thunderstorm has a name: the shelf cloud. Most of the storm photographs below show one, from a city square in Romania and a beach on Cape Cod to the plains of Kansas and the edge of Warsaw, and the gallery ends far from Earth, over the north pole of Saturn.

What is a shelf cloud?

A shelf cloud is a low, horizontal, wedge-shaped cloud attached to the base of a thunderstorm. Meteorologists class it as an arcus cloud, a family that also includes the detached, tube-shaped roll cloud.

It forms at the gust front. Rain falling through a thunderstorm cools the air, and that cold, heavy air sinks and spreads out along the ground ahead of the storm. As it pushes forward it undercuts the warm, moist air in its path and lifts it. The rising air cools, its moisture condenses, and a cloud appears along the boundary. Wind shear gives the leading edge its layered, rolling look.

Shelf clouds across North America

The Great Plains and the Midwest are classic shelf-cloud country, because lines of thunderstorms cross them all summer. Todd Shoemake photographed one developing over rural eastern Colorado. Near Great Bend in central Kansas, a shelf cloud marked the front of a bow-echo line of storms on 6 May 2008, a curved line whose strongest winds blow out of its centre; the picture by KSN-TV was published by the National Weather Service office in Wichita.

On the evening of 13 June 2004, a storm crossed Waupaca and Outagamie counties into Brown County in Wisconsin. Brian Severa photographed its shelf cloud from Little Chute, and the NWS Green Bay office collected images from several spotters, noting the gust front on radar along the storm's leading edge. Jerry Huddleston caught another over Hampton, Minnesota, on 25 June 2010.

On the coasts, sea and sky make the scale obvious. Kunal Mukherjee photographed a shelf cloud over Miami on 4 December 2010. Anthony Quintano recorded one over Race Point Beach at the tip of Cape Cod on 23 June 2012 and another over Wildwood, New Jersey, on 22 August 2013. Further north, Jeff Kerr shot a shelf cloud from the Trans-Canada Highway in Saskatchewan in August 2001, lit from the east by the rising sun as the storm rolled in from the west. NASA chose it as its Astronomy Picture of the Day on 22 January 2008.

Storm clouds over Europe

Europe gets its share too, mostly in summer when heat and humidity build ahead of cold fronts. The best known shelf-cloud photograph on the internet is probably John Kerstholt's view over Enschede in the eastern Netherlands, taken on 17 July 2004. It became a featured picture on English Wikipedia and still illustrates the arcus cloud article. Dariusz Wierzbicki photographed a shelf cloud approaching Warsaw, and Arnold Paul a towering cumulonimbus over the Baltic Sea coast.

Reading the sky: shelf cloud, wall cloud or roll cloud

• Shelf cloud: attached to the storm, on its leading edge. It brings a sudden gust of cooler wind and is usually followed quickly by heavy rain.
• Wall cloud: a lowered, often isolated block of cloud under the rain-free base of a storm, usually towards its rear. This is the feature storm spotters watch for rotation and tornadoes.
• Roll cloud: a horizontal tube completely detached from the storm. The most famous example is the Morning Glory over the Gulf of Carpentaria in Queensland, Australia, which can appear on up to four days in ten in October.

A shelf cloud signals strong gusts close by, even when the storm behind it stays below severe limits. When one approaches, get off the beach, the water or the open slope and move indoors before the rain arrives. A ragged leading edge with scud rising into it points to an especially strong squall.

Saturn's north pole: a storm the size of two Earths

The final picture is a false-colour image of Saturn's north pole. NASA's Cassini spacecraft, which orbited Saturn from 2004 to 2017, photographed a giant hurricane-like vortex there, sitting inside a six-sided jet stream known as the hexagon. According to the European Space Agency, the hexagon is about 30,000 km across and its winds blow at around 320 km/h. The eye of the central storm is up to 50 times larger than the eye of a hurricane on Earth.

Related reading

• Twenty Cloud Photographs: Shapes, Light and Atmospheric Features
• Richard Clarkson’s Cloud: Turning Storm Light into an Object
• Eight Cloud Forms: A Gallery of Structure and Light
• Lenticular Clouds: How a Still Shape Can Contain Moving Air
• Storm clouds that look like a rolling ocean over Nebraska
• Summer hailstorm on a beach in Novosibirsk

Sources

• Wikipedia: Arcus cloud
• NWS Green Bay: Shelf cloud of 13 June 2004
• NWS Wichita: Great Bend storms, May 2008
• NASA APOD: Shelf Cloud Over Saskatchewan
• ESA: A cosmic hurricane
• Photographs: Imgur, Todd Shoemake, Kunal Mukherjee, Jeff Kerr, Anthony Quintano, KSN-TV, Sensenmann, Jerry Huddleston, Arnold Paul, Bill & Vicki Tracey, Brian Severa, Dariusz Wierzbicki, John Kerstholt