A tornado is a rapidly rotating column of air that extends from a thunderstorm or convective cloud toward the ground and is capable of producing extremely strong winds and destructive pressure and shear effects. The visible condensation funnel is not itself the entire tornado. A tornado is fundamentally a rotating wind circulation, and a visible funnel forms when pressure falls sufficiently for water vapour to condense into cloud droplets or when dust and debris reveal the near surface circulation.
Tornadoes can vary greatly in width, duration, intensity and appearance. Some are narrow and rope like, while others have broad wedge shaped structures. They occur most frequently in association with severe thunderstorms, especially supercells, but tornado like vortices can arise in several other convective environments.
Tornadoes are distinguished from hurricanes by their scale, formation mechanism and atmospheric setting. A tornado typically has a horizontal scale of hundreds of metres rather than hundreds of kilometres and normally lasts from several minutes to perhaps an hour, although exceptional events can persist longer. A hurricane is a large tropical cyclone whose circulation can extend hundreds of kilometres and persist for days or weeks. Tornado winds are concentrated into a much smaller area and can be extremely intense. Tornadoes commonly develop from thunderstorms over land, while tropical cyclones develop over warm tropical or subtropical oceans. The two phenomena can nevertheless interact because tropical cyclones can produce tornadoes in their outer rain bands after making landfall.
Tornado intensity is commonly described using the Enhanced Fujita scale in countries where that system is used. The scale classifies tornadoes from EF0 through EF5 based primarily on observed damage indicators and estimated wind speeds rather than direct measurements of wind speed in most cases. Because direct instruments rarely survive the strongest winds, investigators examine damage to buildings, vegetation, vehicles and other structures to estimate the intensity.
This means an intensity rating describes the wind potential inferred from damage and should not be interpreted as a precise measurement of maximum wind everywhere within the tornado. The scale also incorporates differences in construction quality, because the same wind speed can produce very different damage depending on structural strength and exposure.
Tornadoes can have several visual characteristics. A funnel cloud is a visible funnel extending downward from a cloud base, but a tornado is generally defined by the circulation reaching the ground. A tornado may be nearly invisible if humidity is low or may become highly visible when condensation, dust and debris fill the circulation. Multiple vortices can occur within a larger tornado circulation, producing several narrow intense subvortices that rotate around a common centre.
Tornadoes can also change shape rapidly as their internal flow evolves. Appearance alone is therefore not a reliable indicator of intensity. A wide tornado is not automatically stronger than a narrow one, and a highly visible funnel is not necessarily more intense than a less visible circulation.
Tornadoes can produce multiple forms of damage. Strong winds can overturn vehicles, strip roofs, break windows, uproot trees and damage or destroy buildings. Debris carried by the circulation can become dangerous projectiles. Tornadoes can also produce intense pressure and wind gradients over short distances, placing severe stress on structures. Damage patterns can help investigators reconstruct the path, direction and approximate intensity of a tornado.
The width of the damage path can vary substantially and may not correspond exactly to the visible condensation funnel. Terrain, vegetation, buildings and human development can affect what damage is observed, so damage surveys require careful interpretation rather than assuming that every visible effect was caused by one identical wind speed.
Most tornadoes develop within thunderstorms, particularly supercells that contain persistent rotating updrafts called mesocyclones. Rotation in the environment can be tilted from horizontal into vertical orientation by the storm's updraft, while stretching of rotating air can increase its angular velocity. The exact process by which a broad rotating circulation becomes a narrow tornado near the ground is complex and can involve interactions between the updraft, downdraft, cold outflow, boundary layer winds and near surface vorticity. Not all supercells produce tornadoes, and not all tornadoes require the same precise pathway. Tornadogenesis remains an active area of atmospheric research because small scale processes are difficult to observe directly.
Tornadoes occur on every continent except Antarctica, although their frequency and characteristics vary geographically. The central United States is particularly well known because it combines warm moist air from lower latitudes, dry or cooler air from other regions, strong vertical wind shear and frequent severe convective storms. Other tornado prone regions include parts of Canada, Argentina, Bangladesh, India, Australia, Europe and southern Africa. Tornado climatology depends on reporting practices as well as physical occurrence, so comparisons between countries must consider differences in population density, observation networks and historical records. Modern weather radar, satellite observations and automated reporting have improved detection of weaker and short lived tornadoes.
A common misconception is that tornadoes are giant suction devices that primarily destroy objects by sucking them upward. Their destructive effects are mainly caused by extreme winds, intense wind gradients and debris impacts, although pressure changes can contribute to structural damage. Another misconception is that a tornado must always have a visible funnel. The circulation can extend to the surface without a fully visible condensation funnel.
Tornado warnings are therefore based on atmospheric observations, radar signatures, trained observations and other evidence rather than visual appearance alone. Tornadoes are ultimately compact, rapidly rotating atmospheric circulations associated most often with severe convection, and their formation and intensity depend on complex interactions between storm dynamics and the surrounding environment.
