What is a tornado?

A tornado is a rapidly rotating column of air that reaches between the base of a storm cloud and the Earth's surface. They form in very unsettled weather conditions as part of severe thunderstorms. Many conditions need to be present for a tornado to form but, when these conditions are met, a violently whirling mass of air, known as a vortex, forms beneath the storm cloud.

A funnel cloud usually develops as the vortex forms due to the reduced pressure in the vortex. Strong inflowing winds intensify, and the spin rate increases as the vortex stretches vertically. If it continues stretching and intensifying for long enough the vortex touches the ground, at which point it becomes classified as a tornado. The tornado then moves across the surface causing severe damage or destruction to objects in its path.

A tornado typically has the form of a twisting funnel-shaped cloud between the cloud base and the ground. Sometimes the vortex can appear as a slender rope-like form, particularly when the tornado is weakening; sometimes a tornado can be almost invisible, observable by the debris thrown up from the surface. Tornadoes typically spin anticlockwise in the Northern Hemisphere (cyclonically).

How big are tornadoes?

Tornado size and intensity vary greatly. Typically, a tornado is 20 to 100 metres wide at the surface, lasts for a few minutes and has a track of around a mile (1.6km). Wind speeds typically range from 75 to 100 mph (120 to 180 km/h). The largest tornadoes are very rare occurrences. They can be over 2 miles (3.2 km) wide, track for over 60 miles (100 km) and have wind speeds in excess of 300 mph (480 km/h).

A large, violent tornado passing through a populated area can lead to total destruction of buildings and property in its path and sometimes loss of life. Such large tornadoes are in the minority of occurrences. Most tornadoes, although they produce damaging winds, do not lead to the widespread devastation often associated with these weather events in the media. Tornado damage is localised; limited by the track of the tornado.

Where do tornadoes happen?

Tornadoes occur in many places around the world, but North America is the continent where they occur most often. The most violent tornadoes are rarely seen anywhere but the USA, Canada and Bangladesh. 'Tornado Alley', a region of Central USA, is particularly prone to violent tornado outbreaks and is susceptible to large, long-lived tornadoes. In the spring and summer, warm air from the Gulf of Mexico meets cool air from Canada in this region, and this leads to the formation of powerful storms known as supercells that, if the conditions are right, can spawn tornadoes.

Around 30 tornadoes a year are reported in the UK. These are typically small and short-lived, but can cause structural damage if they pass over built-up areas.

Impacts of tornadoes

Tornado forecasting is an evolving science, and much effort is put into improving the understanding and forecasting of tornadoes. The largest tornadoes can cause billions of dollars worth of damage, destroy thousands of homes and lead to loss of life.

Some areas prone to tornado outbreaks, such as the Central USA, have tornado warning procedures, shelters and educational programmes with the aim of minimising risk to life.

The UK has little need of tornado warnings, however the Met Office actively works with the National Oceanic and Atmospheric Administration (NOAA) in the USA to improve the science of tornado forecasts and to improve warnings.