Wiki:
Supercell relationship Tornadoes often develop from a class of thunderstorms known as
supercells. Supercells contain mesocyclones, an area of organized
rotation a few miles up in the atmosphere, usually 1–6 miles (2–10 km)
across. Most intense tornadoes (EF3 to EF5 on the Enhanced Fujita Scale)
develop from supercells. In addition to tornadoes, very heavy rain,
frequent lightning, strong wind gusts, and hail are common in such
storms. Most tornadoes from supercells follow a recognizable life cycle. That
begins when increasing rainfall drags with it an area of quickly
descending air known as the rear flank downdraft (RFD). This downdraft
accelerates as it approaches the ground, and drags the supercell's
rotating mesocyclone towards the ground with it. Formation As the mesocyclone lowers below the cloud base, it begins to take in
cool, moist air from the downdraft region of the storm. This convergence
of warm air in the updraft, and this cool air, causes a rotating wall
cloud to form. The RFD also focuses the mesocyclone's base, causing it
to siphon air from a smaller and smaller area on the ground. As the
updraft intensifies, it creates an area of low pressure at the surface.
This pulls the focused mesocyclone down, in the form of a visible
condensation funnel. As the funnel descends, the RFD also reaches the
ground, creating a gust front that can cause severe damage a good
distance from the tornado. Usually, the funnel cloud begins causing
damage on the ground (becoming a tornado) within a few minutes of the
RFD reaching the ground. Maturity Initially, the tornado has a good source of warm, moist inflow
to power it, so it grows until it reaches the "mature stage". This can
last anywhere from a few minutes to more than an hour, and during that
time a tornado often causes the most damage, and in rare cases can be
more than one mile (1.6 km) across. Meanwhile, the RFD, now an area of
cool surface winds, begins to wrap around the tornado, cutting off the
inflow of warm air which feeds the tornado.[15] Dissipation As the RFD completely wraps around and chokes off the tornado's air
supply, the vortex begins to weaken, and become thin and rope-like. This
is the "dissipating stage"; often lasting no more than a few minutes,
after which the tornado fizzles. During this stage the shape of the
tornado becomes highly influenced by the winds of the parent storm, and
can be blown into fantastic patterns.
Even though the tornado is dissipating, it is still capable of causing
damage. The storm is contracting into a rope-like tube and, like the ice
skater who pulls her arms in to spin faster, winds can increase at this
point. As the tornado enters the dissipating stage, its associated
mesocyclone often weakens as well, as the rear flank downdraft cuts off
the inflow powering it. In particular, intense supercells tornadoes can
develop cyclically.
As the first mesocyclone and associated tornado dissipate, the storm's
inflow may be concentrated into a new area closer to the center of the
storm. If a new mesocyclone develops, the cycle may start again,
producing one or more new tornadoes. Occasionally, the old (occluded)
mesocyclone and the new mesocyclone produce a tornado at the same time. Although this is a widely accepted theory for how most tornadoes
form, live, and die, it does not explain the formation of smaller
tornadoes, such as landspouts, long-lived tornadoes, or tornadoes with
multiple vortices. These each have different mechanisms which influence
their development—however, most tornadoes follow a pattern similar to
this one.
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