Release Date 21 May 2013

Experts from the University of Reading's renowned Department of Meteorology provide stats, facts and comment on tornados, in the light of the disastrous impact of the Oklahoma tornado.

What is a tornado, why do they occur in Tornado Alley and do they ever impact on the UK?

Dr Andrew Barrett - frequency and strength of tornadoes in ‘Tornado Alley'

"Tornadoes are quite common in the Great Plains in May, averaging about 3-4 per day (often in clusters) - but not usually as strong as this one, and not usually in urban areas. Only 2% of tornadoes in the USA reach EF4 status (scale EF0 to EF5), with winds over 165mph.

 "Tornados can occur in many places across the globe, but tornado alley of the Central Plains of the US is most famous. Central Plains in US is ‘Tornado Alley' because it sees frequent collisions of warm, moist air from the south and cold air from further north and has no major east-west mountain range to block air flow between these two air masses."

"Moore, Oklahoma has been hit by significant tornadoes four times in the last 15 years (1999, 2003, 2010 and yesterday) including the most intense storm ever with winds of 317 mph. The size of the tornado was not unprecedented, but at larger end of scales (widths vary from: 100m - 3 km)

Dr Suzanne Gray - thunderstorm supercells

"Tornadoes form in severe thunderstorms e.g. warm humid surface air overlaid by cold dry air aloft - this makes the atmosphere unstable and air tends to rise. The contrast of warm/cold air also creates strong changes in winds with height which can cause air to rotate. This tornado was linked to a thunderstorm supercell  (large, organised areas of intense thunderstorms ). Supercells tend to exist in regions of strong wind shear (where winds change dramatically with height) and this can cause updrafts to rotate. Rotating clouds at the base of a storm is the first sign of a tornado forming."

Dr Suzanne Gray - climate change and tornadoes

"Tornados are too small scale for current climate models to simulate, so it is not possible to say very much about how strength and occurrence might alter under climate change. But climate change means warmer temperatures and more moisture and that is providing more energy for the types of storms that produce tornadoes in a warmer climate."

Dr Pete Inness -tornadoes in the UK

"The geography of the US High Plains is unique in creating the perfect environment for tornado formation. In the UK we simply don't have the right set of circumstances to generate the intense storms in which big tornados form.

"According to the UK Tornado and Storm Research Organisation (TORRO) the UK experiences 30-40 tornados per year, although these are all far weaker and shorter lived than their US equivalents and most cause little or no damage to property. Recent occurrences include a small tornado in Oxfordshire in May 2012 which was tracked using Doppler radar by researchers at the University of Reading.

"In July 2005 a tornado hit Birmingham where damage to trees, houses and cars was widespread across an area to the south-east of the city centre. This was one of the few UK tornadoes to cause significant damage (estimated at 40 million pounds) and 19 people were injured."

What is a tornado?

A rotating column of air usually accompanied by a funnel-shaped downward extension of a thunder cloud and having winds whirling destructively at speeds of up to 300 miles per hour.

• Speeds vary from 18ms^-1 to >120ms^-1
• Categorised according to fijtia (EF) scale from EF0 (weakest) to EF5 (strongest)
• Width of tornados can be: 100m - 3 km
• Lifetime: few seconds - (rarely) an hour or more. Many last 10s of mins.
• Depth: several 100m to 10 km.
• Most rotate cyclonically; a few rotate anticyclonically