They are spinning columns of air that reach the ground from cumulonimbus (thunderstorm) clouds.
Where do they occur and how do they form? – Most continents have regions with favourable conditions for tornado formation. The central and southern states of the USA have the most violent tornadoes in the world due to a unique combination of geographical and meteorological circumstances. This region, from Nebraska through Oklahoma to Texas, is known as ‘Tornado Alley’. Here cold dry air moving south and east from the Rocky Mountains meets warm moist air from the Gulf of Mexico, giving perfect conditions for severe ‘supercell’ thunderstorms to develop. These thunderstorms can often produce tornadoes, especially in late spring and summer.
Tornadoes form as air starts to spin due to winds at different heights blowing at different speeds, creating wind shear. This causes the air to start spinning horizontally. If this gets caught in a supercell updraft, the updraft tightens the spin, speeds it up and tilts it towards the ground.
As tornadoes develop, funnel shaped clouds extend from the base of the cloud. It is only when these funnel clouds touch the ground that we get a tornado. If the funnel cloud touches down at sea we get a waterspout.
What happened this time? – Air masses have been colliding over the last few days. Unusually cool dry air from the Rocky Mountains and the northern plains, hot dry air from the desert and warm, humid air from the Gulf of Mexico have collided in a region stretching from Texas to Chicago, forming severe supercell thunderstorms and, in some instances, large tornadoes.
How severe was the tornado that hit Oklahoma? – It is virtually impossible to measure the speed of winds in tornadoes. Their strength is estimated by the amount of damage they cause. They are categorised using the Enhanced Fujita (EF) Scale of zero (weak) to five (strong). The tornado that hit Oklahoma yesterday is estimated to have been EF5 with winds of up to 200 mph. It was up to two miles wide and on the ground for 45 minutes. The tornado that hit Oklahoma yesterday was of a similar strength to the worst ever tornado to hit the area on 3 May 1999.
What about tornadoes in the UK? – It is claimed that the UK gets more tornadoes per square kilometre than the USA, but not more tornadoes in total. On average, around 30 tornadoes are reported each year in the UK. However, these are generally much weaker than their American counterparts.
How will climate change affect tornadoes? – It is currently not possible to make a link between climate change and tornado activity. Climate change may have a number of effects on atmospheric conditions that may or may not favour tornado formation, the relatively short and unreliable record of tornado activity makes it difficult to determine a definite trend in this. Climate models are currently unable to resolve small-scale phenomena such as tornadoes, and no models exist which can use climate model data to predict future tornado activity.