A mesocyclone (or supercell)
is a highly developed and dangerous thunderstorm. As its central updraft spirals
skyward at 60 miles per hour, reaching heights of more than 45,000 feet, a bulging
wall cloud starts protruding from the storm's base. The wall cloud's core may
begin to rotate rapidly, leading it to lower a spinning funnel, which usually
evolves into a tornado.
Tornadoes
can strike anywhere in the United States, which has more tornadoes than any
other country. (A tornado is any rapidly rotating column of air that touches
the ground. In the air, it's called a funnel cloud.) Tornadoes are most apt to
occur during the late-afternoon and evening hours in the spring and fall, when
the air is least stable.
"First
there has to be a favorable area for thunderstorms to form - an unstable air
mass in which the wind direction turns with height," says NSSL
meteorologist Dr. Edward Brandes. "If the air is then lifted by a passing
cold front or rises because of daytime heating, a mesocyclone - a severe
thunderstorm with a powerful rotating updraft -may develop." With few exceptions, tornadoes
are born mesocyclones. Once they spot a mesocyclone, forecasters know they're
dealing with a fierce storm that produces 3/4-inch (1 3/4-centimeter) hail and
winds blowing at a minimum of 60 miles (96.5 kilometers) per hour. Says NSSL meteorologist
Doug Forsyth: "It also means there's a fifty-fifty chance of a
tornado."
Meteorologists
aren't sure why some mesocyclones produce tornadoes and others don't. And
though they've learned from video recordings and direct observations that
tornadoes almost always form in the right rear quadrant of a severe storm, they
don't know why. But they've learned a great deal since the early 1970s, when they
first began chasing tornadoes across the plains with scientific instruments.
For one
thing, meteorologists have learned that tornado wind speeds don't exceed 300
miles (480 kilometers) per hour. And they've discovered that although a
tornado's atmospheric pressure may be as much as 2.9 inches of mercury (100
millibars) lower than the surrounding air pressure, that isn't enough to make a
building explode. So it isn't necessary to open your windows when a tornado
approaches.
But
there's still much that meteorologists don't understand about tornadoes. What
triggers them? Is lightning a factor? Why are most tornadoes relatively weak
while a few are monsters? Why do some have longer damage paths? Do wind speeds
change during a tornado's lifetime? Does a mile-wide tornado have stronger winds
than a smaller tornado?
To
answer these questions, tornado researchers are using Doppler-radar devices,
their most important tools. There are several sizes of Doppler radars,
including large stationary dishes and smaller portable ones. All emit
microwaves that are reflected back to a receiver by moving particles, such as
water droplets or hail.
Conventional
radars can detect thunderstorms, but they can't tell meteorologists whether a
storm's winds are rotating. Doppler devices can identify a mesocyclone.
Dopplers can also spot wind shear-sudden gusts near the ground. And vertical
wind shears may indicate a likelihood of tornado development two to four hours
before it occurs. (Grolier New Book of
Knowledge Encyclopedia)