Shelf Clouds: Not Tornadoes, But Still Scary and Ominous (PHOTOS)

Jon Erdman
Published: April 15,2015


 

  A shelf cloud, also known as an arcus or arc cloud, may be the most frequently submitted cloud photo to our weather.com photo gallery and Facebook page. Based on their appearance, we certainly understand why.
Shelf clouds are typically seen at the leading edge of a thunderstorm or squall line of thunderstorms. While menacing in appearance, shelf clouds are not tornadoes or wall clouds.
(PHOTOS: Scary Clouds That Are Not Tornadoes)
What you're seeing in a shelf cloud is the boundary between a downdraft and updraft of a thunderstorm or line of thunderstorms.
Rain-chilled air descends in a thunderstorm's downdraft, then spreads laterally when reaching Earth's surface. Warmer, more moist air is lifted at the leading edge, or gust front, of this rain-cooled air. When this warm, moist air condenses, you see the shelf cloud.
As the shelf cloud passes, you feel an abrupt shift in wind direction and increased wind speed, followed within minutes by heavy rain or hail. Wind gusts once the shelf cloud has passed may be quite strong, causing downed trees, tree limbs and power outages.

A typical roll cloud. (Credit: swatwife via weather.com/photos)

A more rare variety of this type of cloud, a roll cloud, resembles a giant rolling pin in the sky. Indeed, the roll cloud may appear to be rolling about a horizontal axis in the sky.
Unlike a shelf cloud, the roll cloud is detached from its parent thunderstorm(s). Either the gust front surged well away from the parent thunderstorm(s), or the thunderstorm(s) may have fizzled, leaving this remnant roll cloud.
Advancing cold fronts have also triggered roll clouds on occasion.
The National Weather Service in Lubbock, Texas, documented a spectacular case of cold-frontal roll clouds in late September 2007.
Share your favorite shelfies (photos or video of shelf clouds) to our weather.com/photos gallery, our Facebook page, or via Twitter @weatherchannel.
(MORE PHOTOS: 10 Spectacular Clouds | Earth's Highest Clouds | Hole-Punch Clouds)