Tornado basics

摘要

The distinguishing feature of tornadoes is that they extend from the base of a cloud down to the ground, unlike minor whirlwinds such as dust devils. The strongest tornadoes are spawned by supercells -violent thunderstorms with a strong, rotating updraught (visible in the picture opposite).rnTornadoes vary in size from a metre or so across to several kilometres, and last for anything from seconds to hours. The "Tri-State Tornado" of 1925 is the longest-lasting tornado on record at over 3 hours, during which it travelled 350 kilometres across Missouri, Illinois and Indiana. It killed 695 people.rnTornadoes occur all over the world but conditions in the US's "Tornado Alley" favour supercell formation. As a result, the US has 1270 tornadoes each year, more than any other country; Canada is a distant second with around 100. By contrast, there are only 300 tornadoes per year in the whole of Europe. However, by land area the Netherlands has more tornadoes than any other country, followed by the UK.