A tornado is the result of extreme instability. It consists of a violent whirlwind, a tapering funnel of twisting cloud, dangling like an elephant's trunk from the base of a thunderstorm cloud, with winds stronger than 20 m/s at 10 m from the centre of the vortex. The word 'tornado' comes from the Spanish for 'thunder'. Its passage is accompanied by a peculiar whistling, and then a roar as the funnel approaches, and finally a screech of winds, obscuring the noise of crashing trees and buildings. The aftermath is an intermittent swath of destruction where the whirlwind's tip moves across the ground.
The funnel of a tornado extends waving from a low thick cloud, with a radius of a few hundred metres. It has a suction of possibly 200 hPa, sufficient to lift bricks. Also there are howling winds nearby, estimated as over 90 m/s in some cases, enough to propel timber through 15 mm steel or a spade 15 cm into a tree, and to demolish houses (Note 7.J). The funnel moves unsteadily along at 8-33 m/s.
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