Justin Hobson (Justin1569 at English Wikipedia)

A tornado is a violently rotating column of air that is in contact with both the surface of the Earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. It is often referred to as a twister, whirlwind or cyclone, although the word cyclone is used in meteorology to name a weather system with a low-pressure area in the center around which, from an observer looking down toward the surface of the Earth, winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern. Tornadoes come in many shapes and sizes, and they are often visible in the form of a condensation funnel originating from the base of a cumulonimbus cloud, with a cloud of rotating debris and dust beneath it. Most tornadoes have wind speeds less than 180 km/h (110 mph), are about 80 m (250 feet) across and travel several kilometres (a few miles) before dissipating. The most extreme tornadoes can attain wind speeds of more than 480 km/h (300 mph), are more than 3 km (2 miles) in diameter, and stay on the ground for more than 100 km (dozens of miles).

Various types of tornadoes include the multiple vortex tornado, landspout, and waterspout. Waterspouts are characterized by a spiralling funnel-shaped wind current, connecting to a large cumulus or cumulonimbus cloud. They are generally classified as non-supracellular tornadoes that develop over bodies of water, but there is disagreement over whether to classify them as true tornadoes. These spiralling columns of air frequently develop in tropical areas close to the equator and are less common at high latitudes. Other tornado-like phenomena that exist in nature include the gustnado, dust devil, fire whirl, and steam devil.

Tornadoes occur most frequently in North America (particularly in central and southeastern regions of the United States colloquially known as Tornado Alley; the US and Canada have by far the most tornadoes of any countries in the world). Tornados also occur in South Africa, much of Europe (except Spain, most of the Alps, Balkans, and northern Scandinavia), western and eastern Australia, New Zealand, Bangladesh and adjacent eastern India, Japan, Philippines, and southeastern South America (Uruguay and Argentina). Tornadoes can be detected before or as they occur through the use of pulse-Doppler radar by recognizing patterns in velocity and reflectivity data, such as hook echoes or debris balls, as well as through the efforts of storm spotters.

Une tornade (de l'espagnol tornado, dérivé du verbe tornar, tourner) est un tourbillon de vents extrêmement violents, prenant naissance à la base d'un nuage d'orage (cumulonimbus) lorsque les conditions de cisaillement des vents sont favorables dans la basse atmosphère. De très faibles tornades peuvent également se développer sous des nuages d'averses (cumulus congestus).

Ce phénomène météorologique a un pouvoir destructeur supérieur à celui d'un cyclone tropical au mètre carré, mais est de durée et d'étendue limitées : il concerne un corridor de quelques centaines de mètres de large sur quelques kilomètres de long. Certaines tornades ont engendré les vents les plus forts signalés à la surface du globe. Elles tuent chaque année de 300 à 400 personnes (selon une estimation de l'Organisation météorologique mondiale), dont 150 aux États-Unis.