Tornadoes, or twisters as they are often called, are violent storms that can rip paths of destruction through towns and cities and kill large numbers of people. Their characteristic funnel shapes look very dramatic in photographs and videos, though it is difficult and dangerous to get close enough to take these shots.
Tornadoes tend to occur more frequently in parts of the world such as the mid-western United States where weather patterns create the general set of conditions they need to form.
Use the BBC News animated tornado guide to see how tornadoes form and learn more about the destruction they can cause.
Image: A tornado near Zurich, Kansas (credit: Charles Doswell, Visuals Unlimited/SPL)
Iain Stewart explains how the troposphere behaves like a fluid.
Dr Iain Stewart explains how the troposphere, the innermost layer of the Earth's atmosphere, behaves like a fluid.
Scientist Louis Wicker discusses his tornado computer model.
Scientist Louis Wicker discusses a computer model, which he designed to investigate what factors may start a twister.
Scientists demonstrate the deadly power of a tornado's winds.
Scientists demonstrate the power of a tornado's winds in a laboratory in Texas. This clip refers to the F Scale (Fujita Scale), a system used to rate tornadoes' intensity. Since 2007, a scale known as the Enhanced F Scale has been used to rate twisters in the US and some other countries.
A tornado is a violently rotating column of air that is in contact with both the surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a cumulus cloud. They are often referred to as twisters or cyclones, although the word cyclone is used in meteorology, in a wider sense, to name any closed low pressure circulation. Tornadoes come in many shapes and sizes, but they are typically in the form of a visible condensation funnel, whose narrow end touches the earth and is often encircled by a cloud of debris and dust. Most tornadoes have wind speeds less than 110 miles per hour (177 km/h), are about 250 feet (76 m) across, and travel a few miles (several kilometers) before dissipating. The most extreme tornadoes can attain wind speeds of more than 300 miles per hour (483 km/h), stretch more than two miles (3.2 km) across, and stay on the ground for dozens of miles (more than 100 km).
Various types of tornadoes include the landspout, multiple vortex tornado, and waterspout. Waterspouts are characterized by a spiraling funnel-shaped wind current, connecting to a large cumulus or cumulonimbus cloud. They are generally classified as non-supercellular tornadoes that develop over bodies of water, but there is disagreement over whether to classify them as true tornadoes. These spiraling columns of air frequently develop in tropical areas close to the equator, and are less common at high latitudes. Other tornado-like phenomena that exist in nature include the gustnado, dust devil, fire whirls, and steam devil; downbursts are frequently confused with tornadoes, though their action is dissimilar.
Tornadoes have been observed on every continent except Antarctica. However, the vast majority of tornadoes occur in the Tornado Alley region of the United States, although they can occur nearly anywhere in North America. They also occasionally occur in south-central and eastern Asia, northern and east-central South America, Southern Africa, northwestern and southeast Europe, western and southeastern Australia, and New Zealand. Tornadoes can be detected before or as they occur through the use of Pulse-Doppler radar by recognizing patterns in velocity and reflectivity data, such as hook echoes or debris balls, as well as through the efforts of storm spotters.
There are several scales for rating the strength of tornadoes. The Fujita scale rates tornadoes by damage caused and has been replaced in some countries by the updated Enhanced Fujita Scale. An F0 or EF0 tornado, the weakest category, damages trees, but not substantial structures. An F5 or EF5 tornado, the strongest category, rips buildings off their foundations and can deform large skyscrapers. The similar TORRO scale ranges from a T0 for extremely weak tornadoes to T11 for the most powerful known tornadoes. Doppler radar data, photogrammetry, and ground swirl patterns (cycloidal marks) may also be analyzed to determine intensity and assign a rating.