A TORNADO IS A RAPIDLY rotating column of air under a thundercloud. The term is believed to have come from the Spanish tronada, meaning thunderstorm. However, alternate etymologies have been traced from various Spanish and French words meaning “to turn.” Common names for a tornado include twister and funnel cloud. The biblical “whirlwind” was probably a tornado. Wind speeds within a tornado can reach up to 300 mi per hour (480 km per hour). Tornadoes are classified on the Fujita scale, named for Japanese-born American meteorologist T. Theodore Fujita. The Fujita scale runs from F0, a relatively weak tornado that damages only lightly built structures, to F5, a powerful tornado capable of leveling a small town. Tornadoes develop from supercell thunderstorms, which have a distinctive structure easily recognized by weather radars. Within the supercell storm, winds of different speed and direction create areas of wind shear that produce a rotating column of air known as the mesocyclone. This leads to the development of a wall cloud at the base of the supercell. The tornado proper descends from the wall cloud. Some very powerful tornadoes contain suction vortices, small sub-tornadoes that revolve around the funnel cloud’s central axis and leave a distinctive spiral path in the debris field. Almost all tornadoes in the Northern Hemisphere spin counterclockwise, a behavior believed to be an effect of the Earth’s rotation. Occasionally a clockwise tornado is observed, but it is generally weak and almost always a “sister” to one spinning the usual way. A more dangerous tornado rarity is the “invisible” tornado, which lacks a visible funnel. The cloud of dust and debris rising from its base may be mistaken for a harmless dust devil. However, dust devils form primarily on hot, sunny days, so an apparent dust devil under a cloudy sky should be considered a probable tornado.