iWitness contributor ErinMarr sent this picture of a scud cloud near Shepard, Mont. (iWitness/ErinMarr)
Sky Watching and Storm Chasing: Faux Tornadoes
The sky is a dark, ominous, blackish-green. The sirens are blaring. The wind is screaming, and hail is dropping all around.
Suddenly, you look up and see THAT in the sky. But is it really a tornado?
It's happened to amateurs and professionals alike: a suspicious cloud formation develops during a thunderstorm, prompting concerns of an impending twister. But not all funnel-shaped clouds are indeed tornadoes. A keen eye and patience is needed to spot the genuine from the fake.
(MORE: Sky Watching - Mammatus Clouds)
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Is It Rotating?
By definition, a tornado is a violently rotating column of air in contact with the ground and extending between a convective cloud (like a wall cloud) and the surface of the earth. Rotation is the key property when observing suspicious, tornado-like clouds.
Is It Extending from The Cloud and In Contact With the Ground?
While many tornadoes begin as funnel clouds, they are are not classified the same as tornadoes. Once the funnel makes contact with the ground as one continuous column, then it is reported as a tornado. Still, meteorologists are interested in funnel cloud reports since the possibility exists that the funnel could extend to the ground into a tornado, and subsequently produce tornado damage. Again, the funnel cloud must exhibit rotation - the funnel must look like it is spinning in the air.
If It's Not a Tornado, Then What Is It?
Both severe and non-severe thunderstorms can produce a number of "faux tornadoes". The most frequent tornado look-alike is the scud cloud. Scud clouds are fragments of clouds that are unattached to and below a layer of higher clouds, like cumulonimbus clouds. Scud clouds are usually generated along the outflow, or gust front of a thunderstorm, where rain-cooled air clashes with warmer air ahead of the thunderstorm. When viewed at particular angles, the lighting within the thunderstorm and the perspective of the scud against the cumulonimbus cloud may give the appearance of a funnel cloud or a tornado. The key is to look for rotation - scud clouds do not rotate.
Wall clouds are ominous features that extend down from the base of cumulonimbus clouds - there is no precipitation beneath them, and they form where the thunderstorm's updraft, or inflow, enters the cloud. Wind shear (rapid change of wind direction and speed) from the ground to the top of the cumulonimbus cloud causes the wall cloud to rotate. Funnel clouds and tornadoes extend down from the wall cloud.
Tail clouds are tubular clouds that can extend from a wall cloud. Tail clouds are an extension of the air flowing into the updraft and wall cloud. Depending on the viewer's perspective, the tail cloud may resemble a tornado - closer inspection will reveal that it is not.
Gustnadoes are weak, short-lived vortices that appear as temporary dust whirls ahead of a thunderstorm. They usually develop along the thunderstorm's gust front, where the rain-cooled air slams into warmer air ahead of the thunderstorm. While the spinning vortex of a gustnado appears to extend from the ground to the sky, the column is not connected to nor has it developed from the cloud - therefore, the vortex is not a tornado.
Roll clouds are relatively rare, low-level, horizontal clouds that resemble rotating tubes. Roll clouds develop along a thunderstorm's outflow boundary. Wind shear (rapid change of wind direction and speed) from the ground to the top of the cumulonimbus cloud causes the roll cloud to rotate. Although they are associated with thunderstorms, they are completely detached from the cumulonimbus cloud and do not make contact with the ground.
Do you have a picture of a "faux tornado"? Share it on our iWitness Page!
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