Tornado intensity

Tornado intensity can be measured by in situ or remote sensing measurements, but since these are impractical for wide scale use, intensity is usually inferred via proxies, such as damage. The Fujita scale and the Enhanced Fujita scale rate tornadoes by the damage caused. The Enhanced Fujita Scale was an upgrade to the older Fujita scale, with engineered (by expert elicitation) wind estimates and better damage descriptions, but was designed so that a tornado rated on the Fujita scale would receive the same numerical rating. An EF0 tornado will probably damage trees but not substantial structures, whereas an EF5 tornado can rip buildings off their foundations leaving them bare and even 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 award a rating.

Tornadoes vary in intensity regardless of shape, size, and location, though strong tornadoes are typically larger than weak tornadoes. The association with track length and duration also varies, although longer track (and longer lived) tornadoes tend to be stronger. In the case of violent tornadoes, only a small portion of the path area is of violent intensity; most of the higher intensity is from subvortices. In the United States, 80% of tornadoes are EF0 and EF1 (T0 through T3) tornadoes. The rate of occurrence drops off quickly with increasing strength—less than 1% are violent tornadoes (EF4, T8 or stronger).

For many years, before the advent of Doppler radar, scientists had nothing more than educated guesses as to the speed of the winds in a tornado. The only evidence indicating the wind speeds found in the tornado was the damage left behind by tornadoes which struck populated areas. Some believed they reach 400 mph (640 km/h); others thought they might exceed 500 mph (800 km/h), and perhaps even be supersonic. One can still find these incorrect guesses in some old (until the 1960s) literature, such as the original Fujita Intensity Scale developed by Dr. Tetsuya Theodore Fujita in the early '70s. However, one can find accounts (e.g. [1]; be sure to scroll down) of some remarkable work done in this field by a U.S. Army soldier, Sergeant John Park Finley.

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