Last Thursday night’s 10 p.m. news on WDAY-TV featured a time-lapse video made from our Forum high-definition tower camera of a small, non-severe thundershower located on the southwest edge of the Fargo area.
A few people remarked on social media that the time-lapse indicated rotation, and asked if there had been any danger of a tornado. There was no danger. The rotation, however, was real.
Just about any shower or thundershower based on a column of rising air has rotation. It is a basic principle of fluid dynamics that any air movement in the vertical will rotate. Think about a bathtub of water. When the drain is opened, a small vortex usually will appear. Yet there is no danger because the velocity is weak. Weak rotation in a shower or thundershower is not dangerous at all unless the air is rising rapidly, in which case the rotation is also quite rapid. Then a tornado, which is a smaller spin-up with even higher wind speed, can do serious wind damage.
