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Posted at 10:39 AM ET, 06/08/2012

Is that a tornado? Wall, scud, shelf and other scary looking clouds


A well-formed wall cloud and tornado near College Park, Maryland, September 24, 2001. The tornado remained on the ground for 17.5 miles as it moved north-northeast through College Park, Beltsville and Laurel, Maryland. Source: Washington Weather
The severe weather event last week that dropped a near-record daily number of tornadoes across Maryland was pretty significant as far as local weather history goes, even if most of those tornadoes were rather short lived and quite weak.

At the same time, a number of images were passed around as tornadoes that were clearly not tornadoes. In that light, we thought it would be good to examine a few of the most common scary looking clouds that sometimes are mistakenly called tornadoes.

Funnel vs. tornado


Photo of a funnel cloud, via NOAA Central Library.

To start, there is a very specific definition of a tornado.

The circulation must reach the ground. If not, and it’s rotating, it’s likely either a funnel cloud or a wall cloud (see below). Simply, a funnel cloud is a tornado that does not reach the ground. Often, you’ll see funnel clouds make it about halfway from the base to the ground, though it can extend much further or much less.

Sometimes it’s not always apparent if the circulation — if there is one — is reaching the ground as trees (see here) can block the view, or a condensation funnel may not be apparent even if there is noted circulation at the surface.

Example: A likely funnel cloud in Brambleton, Va from June 1. It’s not possible to tell if this touched the ground. The National Weather Service has not reported a tornado in this area.

Wall cloud


A wall cloud is seen on a storm that eventually produced tornadoes in Kansas on May 25, 2012. Photo by CWG photographer Ian Livingston.
Wall clouds are often associated with tornadic thunderstorms but not always, and many wall clouds do not rotate. However, the presence of one is more often than not a sign of a mature severe thunderstorm. This low-hanging cloud brings the base closer to the ground which aids tornado development. When rotation is present, funnels or tornadoes can drop from them. In itself, it’s not terribly dangerous, but if seen, it should not be ignored.

Example of likely wall cloud with possible funnel forming from June 1 outbreak in Takoma, Washington, D.C.

Tail cloud


A wall cloud with tail cloud extending from it. NOAA Photo Library, NOAA Central Library; NSSL.
Tail clouds look like a funnel or tornado in many ways. They are often bent on an axis that is not quite horizontal and not quite vertical. While they are not themselves dangerous, like a wall cloud, tail clouds are often indicative of severe weather and potential tornadic activity. Tail clouds by definition connect into a wall cloud. It’s possible that the wall cloud would not be easily visible at the time of viewing a tail cloud though.

This video from June 1 in Greenbelt reported as a funnel or tornado closely matches what a tail cloud and wall cloud look like together.

Beaver tail


Photo of a beaver tail (top line) and tail cloud (lower) on a supercell thunderstorm by Chris Escandor, via NWS Rapid City, South Dakota.
A beaver tail is somewhat similar in appearance to a tail cloud, though in an idealized environment it is much longer and attached to the main storm base (typically a supercell) rather than the wall cloud. This is an area where the updraft and downdraft meet. The clash of air masses causes a boundary to form with clouds condensing along this line. This is also one type of feeder band sending moisture into the system.

Shelf clouds / roll clouds


A shelf cloud disonnected from the main cloud base (roll cloud) passes through Minnesota. Photo by Jon Zenzel, via NASA’s Earth Science Division.
Even though they are often easy to identify, many false reports of tornadoes are associated with shelf clouds. These clouds are caused by cold outflow rushing out ahead of a storm. Though they often resemble waves, obstructions can create the appearance that they are not horizontally aligned to the storm. This can also happen at the edge of the shelf cloud where it may look like one side of it is a tornado-shaped object.

Scud clouds


A scud cloud over southwestern Virginia. Photo by Ross Spoon, via Kevin Myatt’s Weather Journal.
Scud clouds are frequently noted as funnels or tornadoes, but they are completely harmless (other than often being associated with severe weather). Scud are not attached to large-scale storm rotation. They are caused by cold air outflow interacting with warmth ahead of the storm. This causes air to rise and condense. While these clouds often hang low under the base, they don’t indicate anything other than wind blowing out of the storm.

Many of the photos circulated during the June 1 event were likely scud clouds. Example from Elkridge, Maryland.

Rain or hail shafts


A thin ropelike rainshaft is seen on a supercell in Kansas that eventually produced tornadoes on May 25, 2012. Photo by CWG photographer Ian Livingston. Note: This rain shaft was seemingly initially reported as a tornado on this storm and helped guide ground reports on the first tornado warning for it.
This one seems simple enough to avoid confusing as a tornado as it’s not even a cloud, but some storms have such a small or dense central rain shaft that it may take on the appearance of a twister. The visual effect is due to the area of rain appearing darker than the surrounding air. Similar illusions can occur from hail shafts embedded within rain, though more often than not hail shafts appear lighter than the surrounding environment because of ice’s reflectivity.

Related

Frightening Cloud Appears at H.S. Graduation

See more examples of non tornado illusions

Scary looking cloud club

By  |  10:39 AM ET, 06/08/2012

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