Tornadoes

The main difference between tornadoes and other tropical storms is that they form over land. Also known as ‘twisters’, these are tall whirling funnels of air up to 200 m high. Tornadoes form from large thunderclouds in very hot and humid weather. Sometimes these clouds can be so large that pressure in them can vary, and they therefore have their own internal winds. If these winds start to rotate, they may result in the formation of a whirling funnel. This extends downwards from the base of the thundercloud, and can often look like a piece of hosepipe or an elephants trunk. Twisters have also been compared to the funnel that forms when water is sucked down a plug-hole. Have a look next time you take the plug out of a sink full of water.

Inside this funnel, air is sucked upwards and it spins at enormous speeds – up to 400 km/hr at the centre. These are the fastest winds on Earth and we can see how they cause considerable damage to anything they touch.

Tornadoes start off white or grey in colour, although they quickly become darker as they pick up dirt and debris from the ground. The funnel acts a bit like a large vacuum cleaner, with the low pressure at the centre causing very strong upward currents which suck up or destroy everything in their path. Tornadoes have been known to carry all sorts of objects into the air before hurling them back to the ground nearby (sometimes unharmed). The picture on the right shows a car that was picked up by a tornado. They have even been known to pluck all the feathers from a chicken!

Tornadoes usually only last for about 15 minutes, but in that time they can travel hundreds of kilometres. Because of their relatively small size and short life times, they are very difficult to predict.

Tornadoes occur most often in the United States – particularly in the mid-western states such as Nebraska and Kentucky. Storms of this sort can be found elsewhere however, although these are usually much smaller and more short-lived. If a tornado develops over the sea, it is called a ‘waterspout’. These throw mist, spray and water into the air as they move across the surface of the sea, but they are never as severe as tornado funnel clouds on land. Average wind speeds are about 80 kph.

In deserts, whirling winds can produce sandstorms known as ‘dust devils’. However these are formed by columns of hot air spinning upwards from the ground, and are usually much weaker than tornadoes. Similar movements of air cal also occur over snow and water.

Key fact

A tornado makes a deafening roar as it passes by.

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