Tornadoes

Tornadoes form evolving from cumulonimbus storm clouds when the atmospheric conditions are particularly humid. They begin with the downward shear of a part of the cloud, thus forming a funnel-shaped cloud that is the first sign of the birth of a tornado. The funnel cloud descends gradually towards the ground: if it reaches the Earth’s surface, it becomes a tornado. The dimensions of the funnel cloud are indicators of the strength of the tornado, and range from 15 m to a couple of kilometres in diameter. Along the surface of the funnel, air is rotating rapidly, with a spinning ascending motion. Tornadoes move over the Earth’s surface at a speed that can reach 120 km/hr, but the speed of the rotating winds within a tornado can reach 450 km/hr with ascending wind speeds of 290 km/hr. The pressure inside the vortex can be practically reduced to zero, and it is for this reason that tornadoes behave like enormous ‘vacuum cleaners’, that collect whatever they find along their path: houses, cars, trees, livestock, whatever happens to be on the course of a tornado gets uprooted and flung upwards. Just like in tropical cyclones, inside the vortex instead, the air is calm and practically motionless. There are several accounts of people who found themselves miraculously safe and unharmed inside the vortex of a tornado, or of livestock that was lifted up and then deposited unhurt on the roofs of buildings, or of electric and telephone cables completely coated with hay and of other ‘curious’ facts. As the destructive energy gets depleted, the funnel of the tornado slows down its run, the winds inside decrease in speed and the funnel changes shape taking on a flexuous, snake-like form, a clear indication that the energy of the tornado is running out. Tornadoes are surely the most destructive meteorological events, but generally the devastations take place on a reduced scale, in other words they bring about an almost total trail of destruction in their wake but over relatively restricted areas, unlike hurricanes. Generally, tornado damage paths range from 90 to 1,500 m. On average, a tornado dissipates within 15 minutes and covers around 15 kilometres, but some of the bigger clouds can travel even 400 km and last for several hours.

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