TORNADOES
What is a tornado?
A tornado is a short, powerful whirling windstorm that is
shaped like a "Y" and looks like a funnel. Tornadoes are
sometimes called twisters because of their twisting motion.
They usually move about 50-100 kilometres per hour and the
winds inside a tornado travel on average 150-300 kilometres per
hour. Tornadoes are very stormy weather that strike sporadically
and violently, causing more deaths in the United States annually
than any other natural phenomenon other than lightning.
Tornadoes generate the strongest of all surface winds on our
planet. The world's tornado hot spot is in the United States,
particularly the central and southeastern portions of the country;
this region is known as tornado alley. Tornado alley includes
Texas, Oklahoma, Kansas, Missouri, Nebraska, as well as Iowa.
What does a tornado look like?
The funnel of a tornado is usually cone shaped, and over
the life course of a tornado, the size, shape, and colour of the
funnel might change markedly, depending on the intensity of the
winds, the properties of the inflowing air, and the type of
ground over which it hovers. A tornado looks like a funnelshaped cloud coming down from a cumulonimbus cloud, which
is a large thundercloud. In terms of colour, tornadoes can vary
from a dirty white to a blue or grey when it consists mostly of
water droplets, or tornadoes can be red or brown depending on
the colour of the dirt or soil that they pick up from the ground.
How is a tornado formed?
Tornadoes are formed in the updrafts of a thunderstorm or
are associated with hurricanes when they pass over land. They
are tightly woven vortices of air, rarely more than several
hundred feet across. They rotate in a counterclockwise direction
in the Northern Hemisphere and a clockwise direction in the
Southern Hemishpere. Drawn by the greatly reduced
atmospheric pressure in the central core, air streams into the
base of the vortex from all directions. The air then turns
abruptly to spiral upward around the core, and finally merges
with the airflow in the parent cloud at the upper end of the
tornado. Radar experts look for patches of colour in an ''S'' or a
''6'' shape; this signifies the creation a funnel cloud. A funnel
cloud can only form if the pressure drop in the core reaches a
critical value that depends on the temperature and humidity of
the inflowing air. As air flows into the area of lower pressure, it
expands and cools, causing water vapour to condensate and
form water droplets. Sometimes, no condensation cloud forms,
and the only way the tornado can reveal itself is by the dust and
debris it carries over land or water spray over the ocean. In this
case, it becomes a waterspout, which often frequent the Florida
coast and the Bahamas.
Since tornadoes are born in the updrafts of a thunderstorm,
it is necessary to examine the key conditions that are required
for thunderstorms to form. Moisture in the lower to mid levels
of the atmosphere must be present - that is, air that will continue
rising once it begins its ascent from near ground level. There
must be a lifting force to cause the air to begin rising. The most
common lifting force is the heating of air near the ground.
As the air warms it becomes lighter and begins rising.
Advancing masses of cool air, which force warm air
upward, also trigger thunderstorms. When all the conditions are
present, humid air will rise high into the sky. This rising air is
called an updraft. As air rises, it cools and the moisture in it
begins condensing to form a cloud. Raindrops and balls of ice
known as hail form inside the thunderstorm. Positive and
negative electrical charges become separated, creating the
areas of different charges that lead to lightning and thunder.
The strongest thunderstorms normally form in warm,
humid air that is east or south of advancing cold air. Warm,
humid air blowing from the south only 3,000 to 5,000 feet above
the ground helps feed the violent thunderstorms that can form
tornadoes. Dry wind from the southwest around 10,000 feet
from the ground helps add energy to thunderstorms. Finally,
the dangerous kinds of thunderstorms known as super cells are
likely under the area where winds are speeding up. Even though
the jet stream is 25,000 feet or higher above the ground, it
helps pull air upward, increasing the violent nature of the
thunderstorm. The strongest tornadoes are often near the edge
of the updraft, not far from where air is descending from the
thunderstorms. This is why a burst of heavy rain or hail often
hits right before a tornado strikes. Damaging winds can hit
hundreds of yards from the actual tornado vortex. While not
all tornadoes actually reach the ground, often a large system
can create tornado conditions for several days in a row.
DISCUSSION QUESTIONS
1.
How can you measure how fast a tornado is travelling?
How fast a tornado is travelling could refer to two different
things - the first is how fast is it moving from one place to
another, and the second could be how fast are the winds
whirling around in the vortex. Finding how fast the tornado is
moving from one place to another is relatively simple. Using
radar, forecasters can easily calculate the speed of a tornado's
parent thunderstorm by clocking how fast it moved particular
distances. Finding out how fast the winds are circulating in the
vortex is harder and is rarely measured directly while the
tornado is still going on. Usually, meteorologists from the
nearest National Weather Service office examine tornado
damage. Based on what kind of damage the tornado did, they
estimate how strong the winds were on the following Fujita
scale, which classifies wind speed in tornadoes on a 1 to 5 rating
system.
The Fujita Scale
Strength
Description
Wind Speed
F0
F1
F2
F3
F4
F5
F6
Gale
Moderate
Significant
Severe
Devastating
Incredible
Inconceivable
40
73
113
158
207
261
316
to 72 mph
to 112 mph
to 157 mph
to 206 mph
to 260 mph
to 315 mph
+
mph
2.
There appears to be a great deal of information
available about what makes a tornado start, but what
makes them stop?
A tornado is defined as a strongly rotating column of air
attached to the base of a thunderstorm and extending to the
ground. Tornadoes spin up beneath thunderstorms, and
thunderstorms need a good supply of humid air to stay alive.
However, this moist flow of air does not last forever and can
easily be disrupted by other storms, or by the storm itself when
rain falls and spreads cool air ahead of the storm. Without the
updraft of warm humid air, the tornado's spiraling winds unravel
and weaken, and it falls apart. Movies of tornadoes in the past
two decades have been extensively studied and most researchers
agree on the tornado life cycle: formation stage, organization
stage, mature stage, shrinking stage and decaying stage.
The tornado begins to decay as the whirling column of air
is stretched into a very thin tube. This is often referred to as the
rope stage, when the tornado becomes very thin and begins
"roping out." The rotating updraft that feeds the super cell
warm, humid air is known as a mesocyclone - "meso" meaning
a relatively small or miniature storm system. Researchers have
found that as a super cell evolves, the mesocyclone would form
and mature, then begin to weaken. Tornadoes might spin up
beneath each intensifying mesocyclone, but beyond these
observations, however, there is not a lot understood about how
exactly tornadoes form, grow and die. Tornado researchers are
still trying to solve the tornado puzzle - how can a tornado be
so destructive, yet sometimes pick up certain objects, transport
them great distances and then set them down perfectly?
3.
Is there any scientific explanation for how a tornado
can destroy or damage about everything in its path, yet
leave standing and undamaged a home or two that was
also in its path?
First, tornadoes are relatively small; even the worst are
unlikely to be even a quarter-mile wide. Wind speeds drop off
fairly quickly outside the funnel. But, an even bigger reason for
the scattered patterns of damage is that large tornadoes often
have multiple vortices rotating around with the main vortex.
As a tornado with multiple vortices moves across a town, some
places are hit with winds that are the sum of the winds of the
twister's forward motion, the wind in the main vortex and the
winds in one of the secondary vortices. Other nearby places are
hit by winds that are the main vortex winds minus the storm's
forward motion and no vortex.
4.
Do most tornadoes move northeast? If this is true,
why is that?
Tornadoes very often move from the southwest toward
the northeast, but they can also come from other directions.
The reason that they go toward the northeast so often is because
tornadoes come from strong thunderstorms. These thunderstorms need warm, humid air. Very often, especially in the
central and eastern United States, the needed warm, humid air
is supplied by wind blowing from the southwest, from over the
Gulf of Mexico. This is why winds tend to carry these storms
and any tornadoes the thunderstorms are producing toward the
northeast. Tornadoes are steered by the jet stream, and they
generally travel in a northeasterly direction for 5 to 15 miles.
TORNADO DAMAGE
THE TRI-STATE TORNADO
Relating to the topic of tornado destruction comes the story
of the Tri-State Tornado. This tornado killed 695 people, injured
2,027, and caused $16.5 million dollars in damage (over $50
million dollars today), all in a time period from 1:00 p.m. to
4:30 p.m. Four towns were completely obliterated, and six more
were damaged. The tornado began in Missouri at around 1:00
p.m. The town of Annapolis, Missouri was destroyed first,
as the tornado whipped through the state at 72 mph.
The tornado entered the state of Illinois at the town of
Gorham, which was completely destroyed. Once in Illinois, the
tornado grew to a chilling one mile wide on the ground. Most
of the loss of life was suffered in Illinois because the funnel
appeared as a large amorphous cloud rather than a deadly
tornado. The next town to be wiped off the map was Griffin, as
the tornado crossed over the Wabash River and into Indiana.
Once in Indiana, the Tri-State picked up speed to 73 mph again.
The tornado finally dissipated northeast of Princeton, Indiana.
The tornado lasted three and a half hours and covered 219 miles.
Some scientists theorize that tornadoes which cause
heavy damage more often than not are tornadoes with multiple
vortices. Professor Fujita, of the University of Chicago was
one of the first scientists to study these multiple suction
vortices.
He was also responsible for studying the damage left in the
wake of tornadoes. As mentioned earlier, he developed a scale
which is widely used today to categorize tornadoes based on
their destructiveness. This scale is known as the Fujita Scale.