TORNADO (Span., tornada, 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 in-stability 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 .