12. The Persistent Rotation in the Cloud Base
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Persistent rotation in the cloud base of a thunderstorm is one of the most revealing indicators that a tornado might be developing. For those who know what to look for, this rotation is a vital early warning indicator since it is sometimes obvious well before a funnel cloud or tornado forms. Usually seen in the rear of a supercell thunderstorm—the kind most prone to generate tornadoes—the revolving cloud base is sometimes referred to as a mesocyclone. Though in many situations it is fairly clear, showing up as a big, revolving cylinder hanging from the base of the storm, this rotation can occasionally be subtle and challenging for an inexperienced eye to perceive. Although the rotation starts slowly, as the storm gets stronger it can up speed. Though nearly all major tornadoes originate from storms displaying this characteristic, not all rotating cloud bases may cause tornadoes. This spin suggests a significant updraft and the wind shear required for tornado development in the storm. Should you notice continuous rotation in the cloud base of a storm, particularly in conjunction with other warning indicators such as a wall cloud or strong winds, you should remain vigilant and ready to seek cover rapidly. A tornado striking in a few minutes could have started here. Although specialised radar technology is used by weather forecasters and storm chasers to identify this rotation, even to the unaided eye this rotation clearly indicates approaching danger. If you see this phenomena, always give your safety top priority and follow official warnings.
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