A suspicious cloud formation develops during a thunderstorm, prompting concerns of an impending twister. But not all funnel-shaped clouds are tornadoes. A keen eye and patience is needed to spot the genuine from the fake. Here in this article, we will explain the difference between a real tornado, and clouds that are often mistaken for a tornado.
While many clouds may begin as funnel clouds, they are not the same as a tornado. 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. Once a funnel cloud makes contact with the ground as one continuous column, it is then reported as a tornado. Again, the funnel cloud must exhibit rotation. The funnel must look like it is spinning in the air.
If it is not a tornado, then what is it? Both severe and non-severe thunderstorms can produce “faux” tornadoes. The most frequent tornado look-a-like 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. 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.
Another cloud that can appear as a funnel is the wall cloud. A wall cloud extends 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. Roll clouds are relatively rare, low-level, horizontal clouds that resemble rotating tubes. Roll clouds develop along a thunderstorm’s outflow boundary. Wind shear 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.
There are two more clouds that can mimic a tornado, these are called a gustnado, and a shelf cloud. 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. Shelf clouds often form at the leading edge of a gust front or outflow boundary from a thunderstorm, or strong winds flowing down and outward from a storm. The outer part of a shelf cloud is often smoother with a notable rising motion exhibited by a tiered look (hence, the name shelf cloud!). Underneath, a turbulent, unsettled appearance is often the case. A shelf cloud should be seen as a harbinger of strong winds, so take caution.
The next time the sky is dark an ominous looking, remember which clouds are faux tornadoes, and which ones just may be a tornado. Remember, tornadoes rotate, these other clouds do not (with the exception of a gustnado).