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A violently rotating column of air extending from a cumulonimbus cloud to the Earth, ranging in width from a few meters to more than a kilometer and whirling at speeds between 64 km (40 mi) and 509 km (316 mi) per hour or higher with comparable updrafts in the center of the vortex. The vortex may contain several smaller vortices rotating within it. Tornadoes typically take the form of a twisting, funnel-shaped cloud extending downward from storm clouds, often reaching the ground, and dissolving into thin, ropelike clouds as the tornado dissipates. Tornadoes may travel from a few dozen meters to hundreds of kilometers along the ground. Tornadoes usually form in the tail end of violent thunderstorms, with weaker funnels sometimes forming in groups along a leading squall line of an advancing cold front or in areas near a hurricane. The strongest tornadoes, which may last several hours and travel hundreds of kilometers, can cause massive destruction in a relatively narrow strip along their path. The causes of tornado formation are not well understood.