1911 Encyclopædia Britannica/Tornado

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TORNADO (Span, tornado, a turning about, cf. “ turn ”), a local whirlwind of extreme violence, usually formed within a. thunderstorm. In appearance it consists of a funnel-shaped cloud, depending from the mass of storm-cloud above, and when fully developed tapering downwards to the earth. Besides its whirling motion, a tornado has an advancing movement of from 20 to 40 m. an hour—and along its own narrow path it carries destruction. Its duration is usually from half an hour to an hour. Tornadoes are most common in America, especially in the Mississippi Valley and the Southern states; in Europe and elsewhere they are comparatively rare. Owing to their association with thunderstorms they generally occur in warm weather. A tornado is the result of a condition of local instability in the atmosphere, originating high above the earth. A current of air is induced to ascend with a rapid spiral motion round a central core of low pressure. The moisture in the ascending air is condensed by cooling both as it ascends and as it expands into the low-pressure core. The cloud-funnel appears to grow downwards because the moisture in the air is condensed more rapidly than the air itself, following a spiral course, ascends.