The intensity of tornadoes is generally specified on the Enhanced Fujita Scale (see Fujita scale), based on the damage caused, although in some respects the TORRO scale has advantages, because it is based on wind speed. The record speed for a tornado (512 km h−1) was determined by Doppler radar in the extremely violent tornado that hit the outskirts of Oklahoma City on 3 May 1999. The pressure drop in the centre is estimated to reach 200–250 hPa, which causes the air to reach saturation, giving rise to the visible vortex. Tornadoes have typical durations of 15 minutes, but occasionally much more; diameters of 100–2 000 m; and path-lengths of 10–100 km. Multiple outbreaks frequently occur, one extreme case being on 3–4 April 1974, when 148 were recorded. Some record path-lengths may actually represent a series of tornadoes forming and decaying one after another. See also dry line; feeder band; gustnado; hook echo; suction vortex; undular bore.
Subjects: Meteorology and Climatology.