Tornado

Meteorological agencies generally define a tornado as a rotating column of air extending from the base of a cumuliform cloud to the ground. The circulation may be marked by a condensation funnel, but a visible funnel is not required: dust and debris can reveal a tornado at the surface even when the air itself is not visible. A funnel cloud that has not reached the ground is not classified as a tornado. Tornadoes vary greatly in width, duration and path length. Some form beneath rotating supercell thunderstorms; landspouts, waterspouts and cold-air funnels are among other recognized types.

Many of the most damaging tornadoes are associated with supercells—thunderstorms with a persistent rotating updraft. In an environment where wind changes speed or direction with height, wind shear can create horizontal rotation; a strong thunderstorm updraft can tilt and stretch that rotation into a vertical, storm-scale circulation. Interactions among the updraft, downdrafts and near-ground air can help concentrate rotation. These processes are not a complete recipe: meteorologists continue to study why some rotating storms produce tornadoes while others do not, and different tornado types form in different ways.

Tornado intensity is difficult to measure directly because the strongest winds are short-lived and occur over a narrow, often inaccessible path. In the United States, the National Weather Service assigns Enhanced Fujita (EF) ratings from EF0 to EF5 after surveying damage. Investigators compare damage to indicators such as buildings or trees and use the observed degree of damage to estimate a likely wind-speed range. These are damage-based estimates, not direct readings from a wind instrument. Other regions use other assessment systems, so ratings should be interpreted in the context of the scale applied.

Tornadoes occur on several continents, not only in the United States. Their frequency and intensity vary with region and season, and local records depend in part on how storms are observed and reported. A tornado's destructive effects are concentrated along its track: intense winds can damage buildings and infrastructure and carry debris, creating hazards beyond the visible funnel. Forecasting and warning systems use observations such as weather radar to identify dangerous storms, but tornado development remains difficult to predict precisely. The National Severe Storms Laboratory notes that the processes leading to tornado formation are still an active research subject.