The active tornado season in the US – which peaks in late spring to early summer and again in late autumn – is causing havoc across many mid-Western states this year. The areas most at risk lie along ‘Tornado Alley’ positioned between the Rocky Mountains and the Appalachian Mountains. According to the US National Weather Service twisters have caused the deaths of around 100 people in the US so far this year, the worst toll in a decade. .
The image to the right shows shows a tornado in Oklahoma in May 1999.
The US Midwest is prone to tornadoes and thunderstorms as a result of the contrasting air masses found in that area. Cold air from the Rockies meets with warm and moist air from the Gulf of Mexico, creating instability and as a result an increase in activity.
The image to the left shows shows tornado activity in the USA. The darker colours indicate a higher number of tornadoes, with red indicating more than 25 tornadoes per 3,700 square miles (based on NOAA Storm Prediction Center statistics).
A tornado develops from a supercell, or large cumulonimbus cloud, which contain mesocyclones or areas of rotation within the cloud. Large downdraughts in the cloud drag the mesocyclone towards the ground, evident by a large condensation funnel which descends from the base of the cloud and extends to the ground. This vortex is classified as a tornado when it is in contact with both the ground and the cloud base. The image to the left shows the birth of a tornado.
On average the US experiences about 1,200 tornadoes per year but tornadoes can develop almost anywhere in the world (except in the polar regions). The UK has around 35-40 tornadoes a year and experiences more than any other European country.
Many tornadoes last for only a few minutes, but in 1950 a tornado swept across southern England with a path that was at least 66 miles long and lasted for more than two and half hours. It swept through towns and villages near London moving north towards the Cambridgeshire fens, leaving destruction in its wake. Find out more
For more information on tornadoes you can visit the The TORnado and storm Research Organisation (TORRO)site