Tornado
Formation
The basic atmospheric
physics that gives rise to tornadoes is well understood by now. Thunderstorms
usually contain updrafts, large rising swells of warm, moist air. As the
updraft moves, it rotates; if the rotation grows sufficiently intense,
the storm can evolve into a tornado or funnel cloud (a tornado whose bottom
does not touch the ground). Supercomputer simulations depict this process
quite dramatically.
Most tornadoes form within an especially
intense weather system known as a supercell. Supercell thunderstorms occur
when the warm updraft punches through an overlying, stable layer and continues
upward into a zone of cool, dry air. The resulting instabilities produce
powerful vortex motions, the lifeblood of tornadoes (a pair of computer-generated
images depicts the difference between supercell and non-supercell storms).
Within the fiercest tornadoes, wind speeds can approach 300 miles per hour.
Air rushing in to fill the low-pressure void left by the tornado creates
additional fierce, potentially damaging winds. Staying out of danger is
no easy task when a tornado is anywhere near.
One of the most dangerous aspects
of tornadoes is their capriciousness. Sometimes an updraft gives rise to
a tornado; sometimes it does not. Scientists are still hard pressed topredict
exactly when and where a tornado will appear; that uncertainty makes it
difficult to raise the alarm in time tosave lives.
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