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Tornadoes are a rapidly circulating column of air with a central zone of intense low pressure that reaches the ground. Most tornadoes extend from the bottom of severe thunderstorms or supercells as funnel-shaped clouds that kick up massive amounts of dust and debris as they rip across the surface. The exact shape of tornadoes is quite variable, from thin rope-like funnels, to classic cylindrical shapes, to powerful and massive columns that have almost the same diameter on the ground as at the base of the cloud. Many tornadoes evolve from an immature upward swirling mass of dust to progressively larger funnels that may shrink and tilt as their strength diminishes. Funnel clouds are essentially tornadoes that have not reached the ground. More tornadoes occur in the United States than anywhere else in the world, with most of these occurring in a region known as "tornado alley," extending from Texas through Oklahoma, Nebraska, Kansas, Iowa, Missouri, and Arkansas.

Most tornadoes rotate counterclockwise (as viewed from above) and have diameters from a few hundred feet (100 m) to a mile or more (2 km). Wind speeds in tornadoes range from about 40 miles per hour to more than 300 miles per hour (65-480 km/hr) and may move forward at a few miles per hour to more than 70 miles per hour (2-115 km/hr). Most tornadoes last for only a few minutes, but some last longer, with some reports of massive storms lasting for hours and leaving trails of destruction hundreds of miles long. some super-cell thunderstorms produce families or outbreaks of tornadoes, with half a dozen or more individual funnel clouds produced over the course of a couple of hours from a single storm.

The strength of winds and potential damage of tornadoes is measured by the Fujita scale, proposed by the tornado expert Dr. Theodore Fujita. The

Tornado crossing road near Manchester, South Dakota, June 24, 2003 (Mike Berger/Photo Researchers, Inc.)

scale measures the rotational speed (not the forward speed) and classifies the tornadoes into F0-F5 categories.

Many tornadoes form in a region of the midwestern states known as tornado alley. This most commonly occurs in the springtime when cold air




Wind Speed

Damage Potential



40-72 mph

Minor; broken tree branches, damaged signs



73-112 mph

Moderate; broken windows, trees snapped



113-157 mph

Considerable; large trees uprooted, mobile homes tipped, weak structures destroyed



158-206 mph

Severe; trees leveled, walls torn from buildings, cars flipped



207-260 mph

Devastating; frame homes destroyed



261-318 mph

Incredible; strong structures damaged, cars thrown hundreds of yards

from the north overruns warm moist air from the Gulf of Mexico. Tornadic supercell thunderstorms form in front of the cold front as the warm moist air is forced upward in front of the cold air in this region. Supercell thunderstorms that have large rotating updrafts also spawn many tornadoes. Spinning roll clouds and vortexes may form as these storms roll across the plains, and if these horizontally spinning clouds are sucked into the storm by an updraft, the circulation may be rotated to form a tornadic condition that may evolve into a tornado. Before the supercell spawns a tornado, rotating clouds may be visible, and then a wall cloud may descend from the rotating vortex. Funnel clouds are often hidden behind the wall cloud, so these types of clouds should be eyed with caution.

Some tornadoes have formed from smaller and even nonsevere thunderstorms, from squall lines, and even smaller cumulus clouds. These types of tornadoes are usually short-lived, and less severe (F0-F1) than the supercell tornadoes. Waterspouts are related phenomena and include tornadoes that have migrated over bodies of water; they may also form in fair weather over warm shallow coastal waters. These weak (F0) funnel clouds form in updrafts, usually when cumulus clouds are beginning to form above the coastal region. Their formation is aided by converging surface air, such as when sea breezes and other systems meet.

See also atmosphere; clouds; energy in the Earth system; hurricanes; meteorology; precipitation; thermodynamics.


Ahrens, C. Donald. Meteorology Today: An Introduction to Weather, Climate, and the Environment. 7th ed. Pacific Grove, Calif.: Thomson Brooks/Cole, 2003. National Weather Service, National Oceanic and Atmospheric Administration, home page. Available online. URL: Last modified September 15, 2008. Data updated continuously. Schaefer, Vincent, and John Day. A Field Guide to the Atmosphere: The Peterson Field Guide Series. Boston: Houghton Mifflin, 1981.

transform plate margin processes Processes that occur where two plates are sliding past each other along a transform plate boundary, either in the oceans or on the continents, are known as transform plate margin processes. Famous examples of transform plate boundaries on land include the San Andreas fault in California, the Dead Sea Transform in the Middle East, the East Anatolian transform in Turkey, and the Alpine fault in New Zealand. Transform boundaries in the oceans are numerous, includ ing the many transform faults that separate segments of the mid-ocean ridge system. Some of the larger transform faults in the oceans include the Romanche in the Atlantic, the Cayman fault zone on the northern edge of the Caribbean plate, and the Eltanin, Galápagos, Pioneer, and Mendocino fault zones in the Pacific Ocean.

Three main types of transform faults are those that connect segments of divergent boundaries (ridge-ridge transforms), offsets in convergent boundaries, and those that transform the motion between convergent and divergent boundaries. Ridge-ridge transforms connect spreading centers and develop with this geometry because it minimizes the ridge segment lengths and therefore minimizes the dynamic resistance to spreading. Ideal transforms have purely strike-slip motions and maintain a constant distance from the pole of rotation for the plate.

Transform segments in subduction boundaries are largely inherited configurations formed in an earlier tectonic regime. In collisional boundaries the inability of either plate to be subducted yields a long-lived boundary instability, often formed to compensate the relative motion of minor plates in complex collisional zones, such as that between Africa and Eurasia.

The evolution of the San Andreas-Fairweather fault system best represents the development of a divergent-convergent boundary. When North America overrode the East Pacific rise, the relative velocity structure was such that a transform resulted, with a migrating triple junction that lengthened the transform boundary.

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  • Florian
    Can crustal plates be involved with tornadoes?
    2 years ago
  • Olive McNabb
    Is tornado is a tectonic hazard?
    2 years ago
  • Allison
    How is a tornado formed tectonics?
    2 years ago
  • marigold clayhanger
    What do most tornado on earth happen on the tectonic plate?
    1 year ago
  • katherine
    What severe weather does plate tectonics?
    1 year ago

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