Many people think of spring as the season for tornadoes, and generally they are right. The strongest tornadoes usually reach their peak of occurrence in May. About the third week of May, Oklahoma and the Texas Panhandle is, on an average, the place and time where the world's most violent weather phenomenon is at its fiercest and most common. Back in February, we saw — tragically — that Florida and the Gulf Coast have an early tornado season, though usually not as early as in this year of a strong El Niño. Likewise, June and July bring the threat of tornadoes farther north — to eastern Colorado, for instance, where smaller tornadoes are numerous in early June, and even to the Northeast U.S., where a second peak of frequency for the year comes in July and early August.
Most of us have heard that tornadoes in the Northeast are usually far less powerful than their cousins in the Plains. But what's interesting is that scientists and storm-chasers have learned so much about severe weather in the past few decades that they can now give separate terms to a number of different varieties of wind vortex, including some of the weaker tornadoes. This isn't just a matter of classifying tornadoes from F0 (weakest) to F5 (strongest) on the famous Fujita scale, which relies on the type of damage caused. Different tornadoes and other whirlwinds have different means of formation.
Two kinds of vortex which are visibly different than tornadoes have been known and named for a long time: waterspouts and dust devils. You might think that a waterspout is just a tornado over water. Actually, the true waterspout forms over the water and is typically much weaker than a tornado. In cases where the vortex forms from a severe thunderstorm over land and moves onward to pass over water, it can have a ferocity far exceeding the true waterspout. As for dust devils, these whirls occasionally can be big and strong enough to be dangerous, but their genesis is from localized heating. Their source of energy is limited. The source of most strong tornadoes is the energy of a major weather system concentrated by a super-cell thunderstorm and a special rotating region of these storms called a mesocyclone. The mesocyclone might be typically five miles in diameter and produce a tornado up to a mile wide.
Waterspouts and dust devils look very different and occur in very different environments than tornadoes. But some of the new subclasses of tornado would not immediately he differentiated by the layperson. Some of these terms for them are storm chaser slang, but they make good sense. A landspout is a small weak tornado which typically doesn't arise from a mesocyclone or a supercell but instead from less severe thunderstorms and other convective clouds. Most of the many tornadoes in eastern Colorado in June are landspouts. A gustnado is a weak tornado which is formed from the gust front, the line of winds which races out ahead of a thunderstorm.
A cold-air funnel is another generally weak tornado or funnel cloud, which has yet another means of production — relatively cool, comparatively stable conditions. The only major vortices I've ever seen were a group of cold-air funnels in North Dakota. A meteorologist friend and I rode bicycles to within about a mile of the nearest of these funnels before it dissipated. We weren't taking much of a risk, though in rare cases such a vortex might generate winds in roughly the 70 to 100 mph range.
By the way, a funnel cloud is a condensation funnel which is not in contact with the ground. If its end ever touches the ground, it is then classified as a tornado. Not every tornado has a visible funnel, however. In some cases, an observer may see only a whirl of debris down on the surface, but the phenomenon can still be classified as a tornado. It can still be part of a violent storm, however, and may or may not be something fun to chase after on your bicycle.
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