Tornadoes can occur in many parts of the world, but are most frequently found the United States east of the Rocky Mountains during the spring and summer months. In an average year, 800 tornadoes are reported in the US, resulting in 80 deaths and over 1,500 injuries.
The most violent tornadoes are capable of tremendous destruction with wind speeds of 250mph or more. Damage can occur over an area one-mile wide and 50 miles long. Once a tornado in Broken Bow, Oklahoma, carried a motel sign 30 miles and dropped it in Arkansas.
It is estimated that this April alone there were something like 600 tornadoes in the US. Scientists are not sure why this year has been such a record tornado year but one suggestion is that there has been a particularly strong jetstream blowing over the North American continent. This is likely to establish the strong shear forces that set the air spinning and cause tornadoes to form.
* Scientists believe that the conditions needed for a tornado to form start well before thunderstorms develops. An invisible column of air in the lower atmosphere begins to spin horizontally in response to a change in wind direction and an increase in wind speeds.
When a thunderstorm does develop, warm humid air collides with colder air. This causes the air to rise, leading to an updraft that lifts part of this horizontally-spinning column of air. As a result of it rising unevenly, the horizontal column of spinning air transforms into a vertical vortex pointing down to the ground. Warmer air rises rapidly above colder air, forming an updraft that begins to move in a swirling motion which reaches along the length of the column to cause the classic twisting of a tornado.
The downward moving column of air in a tornado – the funnel cloud – is caused as water vapour condenses into its outer sheath due to falling pressure and temperature. Eventually the funnel can reach the ground and with it comes the high-speed winds created by the sudden clash of warm and cold air.
* There is no evidence so far that the record number of tornados have anything to do with climate change. However, some experts believe they may become more frequent.