DEFINITION OF TORNADOES

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 to huge

systems with diameters of a mile or more. 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 supercell 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 scale measures the rotational

speed (not the forward speed) and classifies the tornadoes

into F0–F5 categories.

Many tornadoes form in the mid-western states’ tornado

alley in the springtime when warm moist air from the Gulf of

Mexico is overrun by cold air from the north. 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. Many tornadoes are also spawned by supercell

thunderstorms that have large rotating updrafts. 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

non-severe thunderstorms, from squall lines, and even smaller

cumulus clouds. These types of tornadoes are usually shortlived

and less severe (F0–F1) than the supercell tornadoes.

Waterspouts are related phenomena and include tornadoes

that have migrated over bodies of water, and 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 THUNDERSTORMS.

Fujita Tornado Scale

Scale Category Wind Speed Damage Potential

FO weak 40–72 mph Minor; broken tree branches, damaged signs

F1 weak 73–112 mph Moderate; broken window, trees snapped

F2 strong 113–157 mph Considerable; large trees uprooted, mobile homes tipped, weak structures destroyed

F3 strong 158–206 mph Severe; trees leveled, walls torn from buildings, cars flipped

F4 violent 207–260 mph Devastating; frame homes destroyed

F5 violent 261–318 mph Incredible; strong structures damaged, cars thrown hundreds of yards

trace fossil See BIOGENIC SEDIMENT; ICHNOFOSSILS.

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