Tornadoes don't just pop into existence -- they develop out of
thunderstorms, where there's already a steady, upward flow of warm,
low-pressure air to get things started. It's kind of like when a rock
concert erupts into a riot. Conditions were already volatile; they
merely escalated into something even more dangerous.
Thunderstorms themselves form like many other
clouds: A warm, moist air mass rises and cools, causing the
water
vapor to condense into clouds. However, if the updraft continues, this
cloud mass will continue to grow and rise 40,000 feet (12,192 m) or more
up into the
troposphere, the bottommost layer of the
atmosphere that we live in. A typical thunderstorm cloud can accumulate
an enormous amount of energy. If the conditions are right, this energy
creates a huge updraft into the cloud, but where does the energy come
from?
Clouds are formed when water vapor condenses in the air.
This change in physical state releases heat, and heat is a form of
energy. A good deal of a thunderstorm's energy is a result of the
condensation that forms the cloud. Every gram of water condensed results
in about 600 calories of heat -- and another 80 calories of heat per
gram of water results from freezing in the upper atmosphere. This energy
increases the updraft temperature, as well as the kinetic energy of
upward and downward air movement. The average thunderstorm releases
around 10,000,000 kilowatt-hours of energy -- the equivalent of a
20-kiloton nuclear warhead [source:
Britannica].
In
supercell thunderstorms,
the updrafts are particularly strong. If they are strong enough, a
vortex of air can develop just like a vortex of water forms in a sink.
This precursor to the tornado is called a
mesocyclone,
and is typically 2 to 6 miles (3 to 10 kilometers) wide. One a
mesocyclone forms, there's a roughly 50 percent chance that the storm
will escalate into a tornado in around 30 minutes.
Some tornadoes consist of a single vortex, but other times multiple
suction vortices
revolve around a tornado's center. These storms-within-a-storm may be
smaller, with a diameter of around 30 feet (9 meters), but they
experience extremely powerful rotation speeds.
The tornado reaches
down out of a thundercloud as a huge, swirling rope of air. Wind speeds
in the range of 200 to 300 mph (322 to 483 kph) aren't uncommon. If the
vortex touches ground, the speed of the whirling wind (as well as the
updraft and the pressure differences) can
cause tremendous damage,
tearing apart homes and flinging potentially lethal debris.
The
tornado follows a path that is controlled by the route of its parent
thundercloud, and it will often appear to hop. The hops occur when the
vortex is disturbed. You've probably seen that it is easy to disturb a
vortex in the tub, but then it will reform. The same thing can happen to
a tornado's vortex, causing it to collapse and reform along its path.
Smaller
tornadoes may only thrive for a matter of minutes, covering less than a
mile of ground. Larger storms, however, can remain on the ground for
hours, covering more than 90 miles (150 km) and inflicting near
continuous damage along the way.
At this point, you might be
wondering just how tornadoes eventually dissipate. Scientists still
debate exactly how these deadly storms die, but one of the prime
suspects is none other than the parent thunderstorm: the rotating
mesocyclone. Tornadoes need instability and rotation. Disrupt the
airflow, take away its moisture or destroy its unstable balance of hot
and cold air, and it can't function. Often, a tornado will die because
the cold
outflow of air from falling precipitation upsets the balance.
