<P> There are several scales for rating the strength of tornadoes . The Fujita scale rates tornadoes by damage caused and has been replaced in some countries by the updated Enhanced Fujita Scale . An F0 or EF0 tornado, the weakest category, damages trees, but not substantial structures . An F5 or EF5 tornado, the strongest category, rips buildings off their foundations and can deform large skyscrapers . The similar TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes . Doppler radar data, photogrammetry, and ground swirl patterns (cycloidal marks) may also be analyzed to determine intensity and assign a rating . </P> <P> The word tornado is an altered form of the Spanish word tronada, which means "thunderstorm". This in turn was taken from the Latin tonare, meaning "to thunder". It most likely reached its present form through a combination of the Spanish tronada and tornar ("to turn"); however, this may be a folk etymology . A tornado is also commonly referred to as a "twister", and is also sometimes referred to by the old - fashioned colloquial term cyclone . The term "cyclone" is used as a synonym for "tornado" in the often - aired 1939 film The Wizard of Oz . The term "twister" is also used in that film, along with being the title of the 1996 tornado - related film Twister . </P> <P> A tornado is "a violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud". For a vortex to be classified as a tornado, it must be in contact with both the ground and the cloud base . Scientists have not yet created a complete definition of the word; for example, there is disagreement as to whether separate touchdowns of the same funnel constitute separate tornadoes . Tornado refers to the vortex of wind, not the condensation cloud . </P> <P> A tornado is not necessarily visible; however, the intense low pressure caused by the high wind speeds (as described by Bernoulli's principle) and rapid rotation (due to cyclostrophic balance) usually cause water vapor in the air to condense into cloud droplets due to adiabatic cooling . This results in the formation of a visible funnel cloud or condensation funnel . </P>

When is a tornado classified as a tornado
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