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Twister science
By Andrew Bridges / December 9, 2012
“Tornadoes devastate South, killing at least 251”
What could be more f orcef ul than this real news headline?
T he tornadoes that inspired it.
Tornadoes are nature’s most violent storms, with winds that can exceed 480
kilometers per hour (300 miles per hour). T hey don’t just toss cars (and sometimes
cows). Tragically, tornadoes also kill about 60 people a year in the United States. But
that’s just
an average.
T he
tornadoes
behind the
headline
were
eventually
blamed f or
the deaths
of 316
people. All
were killed
on a single
A to rnad o p as s e s thro ug h a fie ld in G o s he n Co unty, Wyo ., as s c ie ntis ts q uic kly d e p lo y an ins trume nt p ac k the y ho p e
day, April
will b e inte rc e p te d b y the twis te r. Cre d it: Ryan Mc G innis
27, 2011,
when a
massive storm system spun of f hundreds of tornadoes.
Wait, hundreds?
Yes, tornadoes are common, especially in the United States, which records about 1,300 twisters a year. Most
strike across a swath of the Great Plains nicknamed “Tornado Alley.” Tornadoes also have been reported on
every continent except Antarctica.
T he violence of tornadoes drives meteorologists — scientists who study atmospheric events, including
weather — to learn more about how, why and when tornadoes f orm f rom severe thunderstorms called
supercells.
“T here is some secret ingredient that causes some of these thunderstorms to make tornadoes, but we don’t
know what that secret ingredient is,” says Joshua Wurman. A meteorologist, he is f ounder of the Center f or
Severe Weather Research in Boulder, Colo.
What he and other researchers are learning could improve tornado f orecasts, provide more accurate advance
warnings and, most importantly, save lives.
Recipe for a tornado
A tornado is a violently rotating column of air extending f rom the ground to a thunderstorm above. Tornadoes
can leave a path of damage up to 1.6 kilometers (1 mile) wide. T hey can travel more than 160 kilometers (100
miles) over land and last anywhere f rom minutes to more than an hour.
All tornadoes start with a thunderstorm but require other ingredients too. One is instability. Air is unstable when
it is warmer closer to the ground than it is higher up. T hat warm air will rise, just like a hot-air balloon does.
If that air contains water vapor, the vapor may condense — or transf orm into droplets — at the cooler
temperatures higher up. T hese droplets may eventually f all as rain or hail. T he conversion of water f rom a gas
to a liquid also releases heat. T hat heat creates strong upward currents of air, called updraf ts.
Tornadoes
also need
wind shear.
Wind shear
occurs
when winds
at varying
distances
above the
ground
blow in
dif f erent
directions
or at
dif f erent
speeds. As
the winds
blow, a
horizontal,
invisible
tube of
rotating air
begins to
f orm in the
Lo o king thro ug h the hail-d amag e d wind s hie ld o f a ve hic le us e d d uring VO RTEX2. The he avy hail hit while p ro je c t
s c ie ntis ts we re c has ing a to rnad o ne ar LaG rang e , Wyo . Cre d it: Se an Waug h, NO AA/NSSL/VO RTEX2
atmosphere. T hat tube rotates parallel to the ground — picture a giant spinning f ootball or rolling pin.
A strong updraf t can eventually lif t that rotating tube of air, until it is essentially on end (perpendicular to the
ground). Soon, the whole updraf t starts to rotate, f orming the special kind of thunderstorm known as a
supercell. Sometimes, that spinning tightens into a tornado.
(Non-supercell tornadoes f orm when ground-level winds blowing f rom dif f erent directions set a vertical tube of
air spinning. An updraf t then stretches that tube, creating a smaller and less violent tornado. When this occurs
over water, it is called a waterspout.)
T his picture of tornado f ormation has come into f ocus over decades of study.
“I actually think we have a good grasp of how tornadoes f orm … we know a lot more than what we don’t know,”
says Paul Markowski, an atmospheric physicist at Pennsylvania State University.
Missing ingredients
Still unknown is why just one in f ive supercells becomes a tornado.
“To be blunt, we cannot tell the dif f erence between those that will f orm tornadoes and those that will not,”
Wurman says.
All supercells rotate, usually counterclockwise in the northern hemisphere. But a mean-looking, rotating
supercell might churn along f or an hour bef ore it suddenly makes a tornado.
“We know that the storm has a high potential f or tornadoes but we have almost no way of saying precisely
when in its lif e it will intensif y its rotation,” says Markowski.
And what causes the supercell’s vertical, rotating column of air to tighten enough to make a tornado?
Scientists understand the recipe f or creating a storm, says Jef f Trapp, a Purdue University tornado expert.
T he big unknown, he says, is what conditions it takes to squeeze a broadly rotating storm cell — what
meteorologists call a mesocyclone —into a tornado.
T he chase is on
Meteorology requires scientists to make observations and measurements under conditions that are of ten
dangerous. It is not an indoor science.
“We cannot make a tornado in the laboratory, though we do try to do that using computers,” says Wurman. But
to truly understand these storms, “We have to go out in the real world, which is why we tornado chase.”
Meteorologists chase tornadoes across the Great Plains each year, typically during the peak season, f rom
March to June. During 1994 and 1995, and again in 2009 to 2010, dozens of those scientists combined their
ef f orts. T heir giant f ield projects were called VORT EX — f or the Verification of the Origins of Rotation in
Tornadoes Experiment. T he second experiment involved more than 100 scientists and students, some as
young as 18.
Indeed, that
Indeed, that
most
recent
VORT EX
project was
the largestever study
of tornado
f ormation.
For weeks,
a roaming
herd of
scientists
used a
f lotilla of
vehicles
and
instruments
to study
supercells
and the
tornadoes
they can
spawn.
VO RTEX2 p ro je c t p artic ip ants Mallie To th (le ft) and Mare ike Sc hus te r d e p lo y ins trume nt p ac ks d uring a to rnad o
inte rc e p t in Minne s o ta o n J une 17, 2010. Cre d it: G ino De G rand is
Trucks carried mobile radars to measure wind speeds and directions. Remote-controlled airplanes and balloons
measured air temperature, humidity, pressure, wind speed and direction. Back on the ground, miniaturized, carmounted weather stations did the same. And scientists dropped special probes into the paths of oncoming
tornadoes to measure their core winds.
“It’s f air to say tornadoes have never been studied bef ore in this much detail, with this much instrumentation,”
says meteorologist Karen Kosiba. A VORT EX2 participant, she works with Wurman at the Center f or Severe
Weather Research.
During
During
VORT EX2,
the
scientists
measured
and
observed
25
supercells
that made
tornadoes
— and 25
that did
not. So f ar,
the
Kare n Ko s ib a and J o s hua Wurman ins id e a Do p p le r o n Whe e ls (DO W) mo b ile rad ar truc k d uring VO RTEX2. Cre d it:
Tim Mars hall
researchers have completed analyses of three of the tornado-producing supercells. T hey plan to complete indepth studies of the remaining 47 supercells in coming years. However, their early results already are helping
conf irm hypotheses, or testable ideas and explanations, about tornado f ormation.
For example, until about 20 years ago, meteorologists thought a warm updraf t lif ting a rotating tube of air until
it was perpendicular to the ground was enough to make a tornado, says Don Burgess. He’s a meteorologist at
the University of Oklahoma in Norman and one of VORT EX2’s main scientists. T hanks to data collected f rom
that project and its predecessor (the original VORT EX project), meteorologists today know that tornado
f ormation isn’t so simple.
Lif ting a
Lif ting a
rotating
tube of air
will not by
itself
generate a
tornado.
Why?
Because by
the time the
rotating
tube of air
is upright,
or vertical,
it is high
above the
ground —
not in
contact
with it. So
what
conditions
will pull that
rotation
down to
Re s e arc h me te o ro lo g is t Do n Burg e s s p o ints to a mo b ile rad ar us e d d uring VO RTEX2. “Eve rything was mo b ile and
the
p o rtab le . We d id n’t have a ho me b as e — yo u g o whe re the s to rms are ,” Burg e s s s ays o f the e xp e rime nt. Cre d it:
Sus an Co b b , NO AA/O AR/NSSL/VO RTEX2
ground?
And what
squeezes and tightens that vertical column of swirling air into a tornado? Figure skaters pull in their arms to
twirl f aster. What do supercells do?
Early VORT EX2 f indings suggest that the answers may come f rom the warm, moist air pulled in f rom a location
just ahead and to the right of a supercell, says Burgess.
Ahead of a thunderstorm, a downward rush of cool, dry air f lows out over the ground. T his is called a “gust
f ront.” (You can f eel it as a sudden blast of chilly air as a thunderstorm approaches.) T his gust f ront def lects
— or lif ts — the warm, moist ground-level air in f ront of the supercell, VORT EX2 data suggest.
T his hoisting of the warm, humid air to about 1 kilometer (0.6 miles) draws it into a swirling, counterclockwise
f low pattern. T he air speeds up as it twists, says Burgess, f ormerly of the National Oceanic and
Atmospheric Administration’s National Severe Storms Laboratory.
About half way around the storm, this air is then pushed back toward the ground by a downward current of air.
T his current is called a rear-f lank downdraf t. New VORT EX2 analyses suggest that this downdraf t f urther
squeezes and compresses the rotating column of air within the supercell, accelerating its twist. T he downdraf t
also shoves that air toward the ground — potentially all of the way to the surf ace, Burgess notes.
Right there, he says, “is where a tornado is born.”
However, the downdraf t has to be just right f or the spinning air to f orm a tornado, notes Markowski, who
helped plan VORT EX2. If it is too strong or too weak — no tornado.
At least that’s the picture as it now stands. Will this theory hold as meteorologists pore over their
measurements of the other supercells studied during VORT EX2? It may take 10 years to f ind out, as they
complete their analyses of all 50 storms.
Too close for comfort
Of course, meteorologists are f ocused on the ground f or other reasons too. It’s where people live — and
where tornadoes inf lict their damage.
Surprisingly, meteorologists know very little about tornado winds at ground level. Radar is of limited help
because of interf erence f rom hills, trees and houses. T his technology just cannot give a f ull, accurate picture.
T he only way to measure how powerf ul and chaotic those winds are is to embed instruments into a twister,
explains independent storm chaser Tim Samaras. “T hat is the piece of the puzzle I bring to the scientif ic table.”
“Chasing” tornadoes really means getting ahead of them. When the Bennett, Colo.-based Samaras succeeds in
getting right in f ront of a tornado, he drops a steel-encased probe of his own design into the twister’s path.
T hen he f lees, leaving the squat, conical probe to ride out the storm. Instruments inside the device record
pressure, humidity, temperature, wind speed and direction.
One of his
probes
clocked
tornado
winds at
160
kilometers
per hour
(100 miles
per hour)
just
centimeters
(inches)
above the
ground.
Many more
Tim Samaras ho ld s a “turtle ” p ro b e that c an re c o rd the e nviro nme ntal c o nd itio ns ins id e a to rnad o . Cre d it: Pat Po rte r
measurements, however, are needed to complete the picture.
“You would think it would be obvious, but we don’t know how strong the winds are inside a tornado,” says
Wurman, ref erring especially to the violent, ground-level blowing that causes so much destruction. Nor do
meteorologists know which is worse: brief blasts of the f astest winds or the sustained brunt of slower winds.
Unf ortunately, hits are less common than misses f or storm chasers. For example, Samaras has chased
countless tornadoes, but intercepted just 20 or 30. And he has successf ully deployed probes right in a
tornado’s path only a dozen times.
“T he joke is: If you’re trying to get hit by a tornado, it’s really hard to do,” quips Kosiba of the Center f or
Severe Weather Research.
You’ve been warned
If you don’t want to get hit by a tornado, you can rely on the improved accuracy of tornado warnings.
Unf ortunately, getting people to pay attention to those warnings remains another twister challenge.
In the 1980s, a warning preceded just one in f our tornadoes. Today, the National Weather Service puts out an
advance warning f or three out of every f our tornadoes.
“And if we look only at the 100 or 200 strongest tornadoes each year, 90 to 95 percent of them are warned in
advance,” said Harold Brooks. He’s a research meteorologist at the National Severe Storms Laboratory in
Norman, Okla.
Of course, there are plenty of f alse alarms: T hree in every f our warnings aren’t f ollowed by a tornado.
“If you want to warn about most tornadoes, you have to accept a lot of f alse alarms,” Brooks notes.
As of 2011,
the average
advance
time f or a
tornado
warning
was nearly
15 minutes,
according
to the
National
Weather
Service.
Extending
the warning
period by
even a f ew
minutes
could save
more lives
— if people
acted. It
pains
A to rnad o s p ins ac ro s s s o uth-c e ntral Kans as in this May 29, 2004 imag e take n b y s to rm c has e r Tim Samaras . Cre d it:
Tim Samaras
meteorologists when those in the path of a tornado do not heed their warnings.
“We try to do the best research in the world in terms of how this phenomenon f orms and then try to predict it,”
says Trapp, the Purdue tornado expert. But if researchers get the public the right inf ormation — but f ail to get
the public to take the right actions — “then we have f ailed in our mission,” he says.
Earlier this year, the National Weather Service began using much more dramatic language in some tornado
warnings. For example, it issued the especially strong caution below f or Wichita, Kan., at 10:27 p.m. on April 14.
T HIS IS A LIFE T HREAT ENING SIT UAT ION. YOU COULD BE KILLED IF NOT UNDERGROUND OR IN A
T ORNADO SHELT ER. COMPLET E DEST RUCT ION OF ENT IRE NEIGHBORHOODS IS LIKELY.
Did the new and stronger language work? It is impossible to judge f rom just one example. However, no one
died.
Power Words
tornado A violently rotating column of air extending f rom the ground to a thunderstorm above.
meteorology T he study of weather and climate events.
supercell A rotating thunderstorm that can produce a violent tornado.
water vapor Water in its gas phase.
waterspout A tornado that f orms over water.
updraft An upward current of air. A downdraft is a downward current of air.
wind shear T he ef f ect of winds at dif f erent levels above the ground blowing in dif f erent directions or at
dif f erent speeds.
physicist A scientist who studies energy, matter and the relationship between the two.
mesocyclone T he broad rotational movement inside a supercell thunderstorm.
hypothesis (plural: hypotheses) A proposed idea or explanation that can be tested by making f urther
observations or doing more experiments.
gust front T he downward rush of cool, dry air that f lows out over the ground ahead of a thunderstorm.
deflect To change something’s course by striking it.
flank T he side of something large — f or example, a supercell thunderstorm.
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