How tornadoes form
Tornadoes begin as a menacing tube-like protuberance, or tuba, hanging from the base of a thundercloud surrounded by what is known as a wall cloud. When warm humid air is lifted up by a cooler advancing air mass, a sharp change of wind direction occurs higher in the atmosphere. This can initiate spin. The resultant anticlockwise rotating tube of air gravitates to the surface because of a build-up of air at the top of the column. This causes higher pressure here and exerts a downward force at the very centre of the column. It becomes a tornado when the tube reaches the Earth’s surface. Condensation and debris in the funnel give tornadoes their dark, ominous appearance on land. They are often part of a thunderstorm, the word tornado coming from the Spanish to thunder. They are much more common across the UK than people realise, particularly in the autumn and early winter. In fact, there are the highest number per square mile than anywhere worldwide, although most are less powerful than their US cousins. However, a few can pack a punch, causing death and much destruction. One of the most damaging occurred in 1913, when a particularly vicious tornado tracked from South Wales to Shropshire, along with violent thunderstorms and incessant lightning. The path was no more than 330ft (100m) wide, but hundreds of houses were ruined. Six people were killed, and 200 families lost most of their possessions. A hayrick weighing several tons was carried through the air for a mile and part of a roof was blown several miles. An ash tree weighing several tons was hurled over a hill at Abercynon and an oak tree flung a quarter of a mile.