Tornado

• 1. Introduction

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  A tornado holds a record that has stood for nearly a century: on the 18th of March 1925, a single twister traveled across Missouri, Illinois, and Indiana for 219 miles without lifting from the ground, killing 695 people, the deadliest single tornado in United States history. At the other extreme, one tornado left a damage path only 7 feet long. Between those two poles lies one of the most varied and least understood phenomena in all of meteorology. What is a tornado, exactly? How does it form, grow, and die? Why does the United States have so many more than anywhere else on Earth? And what do scientists still not know, even after roughly 140 years of study?

• 2. What Makes a Tornado

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  For a spinning column of air to earn the name tornado, it must touch both the ground and the base of a cloud simultaneously. That contact with the cloud base is what separates a tornado from a dust devil, a gustnado, or any of the other swirling wind phenomena that share its profile. The word itself traces back to the Spanish tronada, meaning thunderstorm, rooted in the Latin tonare, to thunder. The familiar English spelling reflects a mixing of two Spanish words: tronada and tornado, the past participle of tornar, to twist or turn. That etymological tangle captures something true about the phenomenon itself: a tornado is not precisely defined, and meteorologists disagree, for example, on whether separate touchdowns of the same funnel count as separate tornadoes.

  Visually, a tornado is not necessarily a dark funnel. The intense low pressure at the center causes water vapor to condense into cloud droplets, producing the visible condensation funnel most people picture. But if the funnel picks up little or no debris, it can appear gray to white. Tornadoes over water can turn white or even blue. In the Great Plains, where the soil carries a reddish tint, tornadoes can take on that color entirely. Near sunset, the same storm can appear yellow, orange, or pink depending on the angle of the light. A tornado seen with the sun behind it appears almost black; the same tornado, viewed from the other direction, may look brilliant white. In dry environments, some tornadoes are nearly invisible, marked only by swirling debris at the base.

  Not all funnel clouds become tornadoes. Most tornadoes already produce strong winds at the surface while the visible funnel is still above the ground, which means the storm is technically a tornado even before condensation visually connects the funnel to the earth below.

• 3. Birth, Maturity, and Death

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  Most intense tornadoes grow from a class of thunderstorm known as a supercell. Supercells contain mesocyclones, areas of organized rotation typically 1 to 6 miles across, forming several miles up in the atmosphere. The process begins when increasing rainfall drags a mass of quickly descending air to the ground; this rear flank downdraft, or RFD, pulls the storm's rotating mesocyclone downward with it. As the mesocyclone lowers below the cloud base, warm air rising in the updraft meets cool air from the downdraft, producing a rotating wall cloud. The RFD then concentrates the mesocyclone's base into a smaller and smaller area, drawing the funnel downward until it reaches the surface.

  Within a few minutes of the RFD reaching the ground, the funnel typically begins causing damage. The tornado then enters what meteorologists call the mature stage. This can last from a few minutes to more than an hour, and during that window a tornado often causes its worst damage. In rare cases a mature tornado can exceed 1 mile across. Critically, a tornado draws its energy from the cloud above, not from the earth below; the flow inside the funnel is downward, delivering water vapor from the parent storm. This is the reverse of a hurricane, which draws energy from the warm ocean below.

  The end comes when the RFD wraps all the way around the tornado, cutting off its supply of warm, moist air. The vortex contracts into a thin, rope-like shape. Due to conservation of angular momentum, winds can actually increase at this point even as the tornado is dying. The dissipating stage typically lasts no more than a few minutes. In intense supercells, the cycle can restart: as one mesocyclone weakens, inflow may concentrate closer to the storm's center and feed a new mesocyclone, sometimes producing an entirely new tornado.

• 4. Size, Scale, and Records

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  In the United States, the average tornado is around 500 feet across and travels about 5 miles on the ground. But those averages hide an enormous range. On the 22nd of May 2004, a tornado near Hallam, Nebraska reached 2.5 miles wide at the ground. On the 31st of May 2013, a tornado near El Reno, Oklahoma measured approximately 2.6 miles wide, the widest on record.

  The Tri-State Tornado of the 18th of March 1925 holds the record for path length at 352 kilometers, and for the longest duration of any tornado on record. A 2007 reanalysis of its track suggested the storm may have begun 15 miles further west than previously believed, meaning the true path length could be even greater. The 2021 Western Kentucky tornado, in more recent years, stayed on the ground continuously for 165.6 miles.

  The fastest wind speed ever measured in a tornado was logged by Doppler radar during the 1999 Bridge Creek-Moore tornado: 486 plus or minus 32 kilometers per hour. Conventional anemometers cannot survive such winds or the flying debris, so mobile Doppler radar units provide the closest available measurements of tornado intensity. The 1974 Super Outbreak, which struck a large area of the central United States and extreme southern Ontario on the 3rd and the 4th of April 1974, featured 148 tornadoes in 18 hours, including seven of F5 intensity and twenty-three at F4. Sixteen tornadoes were on the ground simultaneously at the outbreak's peak, and more than 300 people died.

  Debris from a violent tornado can travel astonishing distances. A tornado that struck Great Bend, Kansas in November 1915 lofted objects more than 300 miles away; a cancelled check from the Great Bend bank turned up in a field outside Palmyra, Nebraska, 305 miles to the northeast.

• 5. Rating the Damage

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  The Fujita scale rates tornadoes by the damage they leave behind, from F0 tornadoes that damage trees but spare substantial structures, to F5 tornadoes that rip buildings off their foundations and can deform large skyscrapers. The Enhanced Fujita Scale, implemented in the United States in 2007, updated those categories using engineered wind estimates and improved damage descriptions while preserving the same numerical ratings, so a storm rated under the old system translates directly to the new one. A separate scale, the TORRO scale, runs from T0 for the weakest tornadoes to T11 for the most powerful known.

  In the United States, 80 percent of tornadoes fall into the EF0 and EF1 categories. Fewer than 1 percent reach violent status, classified as EF4 or stronger. The damage-based approach has a known limitation: a tornado whose strongest winds pass over open fields with no structures or vegetation leaves no scoreable damage, so its actual intensity can be substantially underestimated. Only a small portion of a violent tornado's path typically reaches violent intensity; most of the extreme damage comes from subvortices within the main circulation.

  The deadliest tornado in world history, as of 2025, was the Daultipur-Salturia Tornado in Bangladesh on the 26th of April 1989, which killed approximately 1,300 people. Bangladesh averages 179 tornado deaths per year, the highest in the world, driven by high population density, poor construction quality, and limited tornado safety knowledge. Bangladesh has recorded at least 24 tornadoes in its history that each killed more than 100 people, almost half the total for the rest of the world combined.

• 6. Where Tornadoes Form

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  The United States accounts for nearly four times as many tornadoes as all of Europe combined, excluding waterspouts. The geography of North America explains why. The continent stretches from tropical latitudes to arctic regions with no major east-west mountain range to block the flow between them. The Rocky Mountains block moisture and force drier air eastward at mid-levels of the atmosphere, while the Gulf of Mexico feeds abundant warm, moist air northward. The result is frequent collisions of warm and cold air masses that breed powerful, long-lived storms through much of the year.

  The Netherlands has the highest average density of tornadoes per unit area of any country, recording more than 20 per year relative to its small landmass. The United Kingdom records at least 34 tornadoes per year, and possibly as many as 50. Most are weak, but the Birmingham tornado of 2005 and the London tornado of 2006 both registered F2 on the Fujita scale and caused significant damage. Tornadoes also occur across South Africa, much of Europe outside the Alps, western and eastern Australia, New Zealand, Bangladesh and adjacent eastern India, Japan, the Philippines, and southeastern South America including Uruguay and Argentina.

  Worldwide, most tornadoes occur in the late afternoon, between 3 and 7 in the evening local time, with a peak near 5 p.m. Solar heating drives surface instability, and the atmosphere is most primed for supercell development by mid-to-late afternoon. The Gainesville Tornado of 1936, one of the deadliest tornadoes in recorded history, struck at 8:30 in the morning, demonstrating that dangerous tornadoes can form at any hour.

• 7. Detection and Spotters

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  Before the 1950s, the only way to know a tornado existed was to see it. News of a twister often reached a local weather office only after the storm had passed. The first public tornado warnings were issued in 1950. The first tornado watches and convective outlooks followed in 1952. In 1953, meteorologists confirmed that hook-shaped radar signatures were reliably associated with tornadoes, giving forecasters their first systematic tool for identifying storms in real time.

  Today, Doppler weather radar stations in the United States and a few other countries can detect rotation within storms from more than 100 miles away. But radar has a geometric limitation: the beam rises with distance due to the curvature of the Earth, so storms far from a station are observed only at higher altitudes, missing the critical low-level circulation. That gap is filled by human eyes on the ground. More than 230,000 trained Skywarn weather spotters operate across the United States, reporting to the National Weather Service. Canada runs a parallel volunteer network called Canwarn with more than 1,000 volunteers. In Europe, Skywarn Europe and the Tornado and Storm Research Organisation, which has maintained a spotter network in the United Kingdom since 1974, serve similar roles.

  Spotters typically watch the rear of a storm, looking for a rotating wall cloud, which usually precedes a tornado by five to thirty minutes. A funnel cloud dipping from a wall cloud almost always develops a surface swirl by the time it reaches halfway to the ground, indicating that a tornado is already in contact with the surface even before the condensation funnel visually bridges the gap. On the 24th of June 2003, near Manchester, South Dakota, a pressure probe measured a drop of 100 millibars as a tornado passed; the reading fell to 850 millibars at the vortex core before rising sharply again, tracing a sharp V-shape on the pressure record.

• 8. What Remains Unknown

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  Meteorology is a relatively young science, and the study of tornadoes is newer still. Despite roughly 140 years of research, including about 60 years of intensive study, forecasters still cannot reliably predict which supercell thunderstorms will produce a tornado and which will not. The precise roles of downdrafts, temperature contrasts, and moisture in pushing a rotating supercell across the threshold into tornadogenesis remain poorly understood. Occasional tornadoes still strike without any warning being issued.

  Field research programs including the VORTEX projects, which stands for Verification of the Origins of Rotation in Tornadoes Experiment, along with deployments of the TOtable Tornado Observatory and Doppler on Wheels mobile radar units, have narrowed many uncertainties. Tornadoes produce identifiable infrasonic signatures at frequencies below the range of human hearing, and because low-frequency sound travels great distances, researchers are working to develop detection systems based on these signals. Tornadoes also generate a detectable seismic signature, and work continues on isolating it reliably.

  There is mounting evidence from mobile Doppler radar and eyewitness accounts that most tornadoes contain a clear, calm center with extremely low pressure, similar to the eye of a tropical cyclone. Lightning is reported as the primary source of illumination for those who claim to have seen the interior of a tornado. Solar storms resembling tornadoes have been observed on the sun itself, though how closely they relate to tornadoes on Earth remains an open question for researchers at institutions including the National Severe Storms Laboratory and the National Center for Atmospheric Research.