Tornadoes
Tornadoes are the most powerful
weather phenomenon known. A tornado
is an intense system of low pressure with violent updrafts and
converging winds. Though tornadoes have been intensely studied for
years, the mechanism that actually creates them still eludes us.
Tornadoes have been documented in most all the regions of the Earth,
though they are most prevalent in the United States. Figure
WS.24 Supercell thunderstorm are especially capable
of spawning tornadoes. Tornadoes are spawned from severe
thunderstorms. Wind shear, where winds are traveling at
different speeds and from different directions aloft cause rotation of
air about a horizontal axis within the thunderstorm. The rotating
circulation is tilted into the vertical by the updrafts of air in a
severe thunderstorm. As the rotating air increases in height and
shrinks in size a mesocyclone is formed.
For whatever reason, a tornado funnel is spawned within the
mesocyclone. [ The funnel can remain aloft,
twisting and turning without wreaking much havoc below, but is most
destructive when it touches the ground. A tornado can vary in diameter
from a few hundred feet to greater than a mile. Tornadoes typically
move across the surface at speeds ranging from 22 - 33 mph (10 - 15
meters per second). [ The central U.S. contains a unique
mix of topography and weather factors that combine to create these
ferocious weather systems. The most favorable situation for these
storms to develop is during the months of April through June when there
is the most contrast between air masses in the central United States.
The region of highest concentration is that of "tornado alley", a
region that stretches from eastern Nebraska through central Kansas and
Oklahoma in to the panhandle of Texas. The tornado season varies with
latitude, with the southeastern U.S. season from January through
March and the north central states during July through
September. On April 3, 1974 148 tornadoes struck 13 states
leaving a swath of death and destruction across the U.S.[
Figure WS.25 Tornado risk in the United States Tornadoes are categorized on the
basis of their destruction by the Fujita
scale. The scale relates tornado destruction to wind speed,
though the relationship has not been scientifically proven. For
instance an F1 tornado with winds of 73-112 mph causes moderate damage.
An F1 may peel the
surface off roofs; mobile homes may be pushed off foundations or
overturned; moving autos can be blown off roads. [ Get inside a tornado courtesy of
National Geographic |
Watch "Super Twisters" 