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TORNADOES
A Brief Overview
George Pararas-Carayannis
Introduction
Tornadoes are destructive weather-related
disasters that strike suddenly and frequently.
They are violently rotating columns of air that
descend from the base of large cumulonimbus
clouds (thunderstorms) and make contact with
the ground. When they touch down in
populated areas they can cause a great deal of
destruction.
Supercell Formation
In recent years tornadoes have claimed
hundreds of thousands of lives and have
devastated entire communities. The National
Weather Service documents about 1000
touchdowns of tornadoes every year in the
United States. Additionally, there may be as
many as 1000 or more weaker tornadoes each
year that touch down in remote areas and thus
remain completely undocumented. Fortunately
only a small percentage of tornadoes that touch
down can be classified as violent.
Tornado Formation
The interaction and collision of
different air masses can result in
thunderstorms. The largest and most
violent of tornadoes are generated
by the "supercell" thunderstorms,
that are associated with strong
winds, heavy rains and hail.
Necessary for supercell
thunderstorm formation and tornado
generation, is warm, moist air close
to the ground - being forced upward
into colder, dry air at about 10,000
feet or higher in the atmosphere.
Air Mass Interactions in Tornado Formation (graphic, U.K. Meteorological Office)
Such conditions can be triggered by the rapid passage of a cold, or a low pressure front, which can
then generate strong vertical shear winds and updrafts which can often be visible as dark “wall
clouds”. "Shearing" winds that change direction and increase in speed with height seem to be
necessary for a thunderstorm to develop rotation and generate tornadoes. Wall clouds form in
extremely strong updraft regions of the storm, where the air is being pulled from below at speeds of
up to 100 mph. The winds thus generated can change directions, pick up additional speed with
altitude and develop into cyclonic patterns, with counterclockwise rotation. These tornadic supercell
thunderstorms suck additional warm, moist air from below and can form a "funnel cloud" - as rapid
cooling and condensation occur. Often such funnels will grow in a downward direction and make
make contact with the ground. Such tornado funnel touchdown can result in catastrophic damage if it
occurs in populated areas.
Usually, the Gulf Stream supplies some of
the warm, moist air that is needed for
tornado formation in the U.S., while the
Appalachian mountain region funnels cool
air toward the coasts. However, near the
southern U.S. coasts the effects of air
mass interactions are not as strong. Thus
tornadoes formed in Florida by
thunderstorms, although frequent, lack the
intensity of tornadoes that can form further
inland.
Supercell and Tornado formation
Conditions become ideal for the formation of more intense tornadoes in the Midwest region. Warmer
and moist air moving in from the Gulf of Mexico collides with the colder and dry air that funnels over
the Rocky mountains. As the colder air cascades down to the central plains, it expands and starts a
strong counterclockwise circulation, much like a hurricane does in the northern hemisphere - but in a
more compact and intense way. Depending on local conditions, such funneling effect may gain
momentum and develop into a tornado that may touch down with great intensity.
Tornado Touchdown
When the rotating funnel cloud makes contact with the ground, begins to suck debris and forms a
visible dust cloud, then it is officially classified as a tornado. Its rotating winds may reach speeds of
200 miles or greater. Its intensity, as explained below (measured by the Fujita scale) is determined
not by wind speed or funnel dimensions, but on the amount of damage the tornado causes when it
makes touchdown.
Tornado Occurrence, Frequency and Seasonal Variation
Occurrence and Frequency - Tornadoes occur worldwide and can form at any time of the year
when cumulonimbus clouds (thunderstorms) are present. As stated, about 1,000 tornadoes form
every year in the United States. Tornadoes can form anywhere but more frequently occur in the
plains region between Florida and the Rocky and Appalachian Mountains. As stated, most tornadoes
occur in Florida but are usually weaker and short lived. The more severe tornadoes occur across the
Central Plains, extending from Texas to Colorado to Oklahoma and South Dakota. About 40% of
them occur yearly the regions of Central United States along a corridor which often is referred to as
"Tornado Alley", which includes parts of Florida, Mississippi and Texas, but extends northward
through the states of Oklahoma, Kansas, Nebraska and South Dakota, and east to Iowa, southern
Ohio and northern Alabama. Also, dangerous tornadoes occur frequently in Indiana, Illinois, Kentucky
and Tennessee. Sometimes the term “Dixie Alley” is used to describe the path that tornadoes follow
in the southern states – more frequently along the lower Mississippi and upper Tennessee valleys.
Seasonal Variation - Tornadoes occur with higher frequency during certain times of the year referred to as "tornado season" - which also shifts geographically. More tornadoes strike the
southeastern states during the winter months. However, in early spring (between March to May),
most occur in the central States. As temperatures rise during the summer months, more tornadoes
are likely to develop further north in the region of central plains, where warm and cold air interactions
become more frequent.
Areas with the
greatest
incidence of
tornadoes with
F2 or greater
Intensity (After
Meyer et al.
2005)
Tornado
Intensity – The
Fujita Damage
Scale
The intensity of
a tornado is
usually
described by
the amount of
damage it
causes when it
touches down
in populated
areas. Estimating a tornado’s maximum wind speed from damage to structures is a very subjective
process and difficult to describe with the various available meteorological parameters. The size of a
tornado is not necessarily an indication of its intensity. Large tornadoes can be weak and small
tornadoes can be violent. Two percent of all tornadoes are classified as "violent" and account for
about 70% of all tornado-related fatalities in the United States.
In 1971, Professor Tetsuya Fujita of the University of Chicago and Allen Pearson, then director of
NSSFC (National Severe Storm Forecast Center - predecessor organization to the Storm Prediction
Center in Kansas City, Missouri), introduced a scale that would rate the intensity of the tornado, as
well as a measure of its path length and width. This is the Fujita Scale (also known as the FujitaPearson Scale) presently in use in estimating the strength of a tornado. The Fujita scale does not link
damage to wind speed, but it is simple enough to use as a measure of intensity.
The scale rates the intensity of a tornado, not by the appearance of its funnel but on the damage it
caused after it passed over man-made structures. Since the scale is based on a subjective
interpretation of damage, the designation of intensity may vary depending on how a survey is
conducted and the experience of surveyors in separating tornado damage from other wind damage.
Often, complex combinations of strong downbursts or other lateral winds may accompany tornado
generating thunderstorms - so their damge may be difficult to separate from that of the tornado. The
Fujita scale has applied retroactively to rate past tornadoes from 1950 onward in the United States,
and as well as in rating some prior events. Part of the Fujita scale is as follows:
Taken Thursday night, April 3,2008
South of Ft Stockton , TX . Lariat # 2 Sandridge Energy.
THE FUJITA TORNADO SCALE
F-Scale Number Intensity Phase
Wind Speed Type of Damage
Done
F0 Gale tornado, 40-72 mph.
Some damage to chimneys;
breaks branches off trees;
pushes over shallow-rooted trees; damages signboards.
F1 Moderate tornado, 73-112 mph. The lower limit is the beginning of hurricane wind speed; peels
surface off roofs; mobile homes pushed off foundations or overturned; moving autos pushed off the
roads; attached garages may be destroyed.
F2 Significant tornado, 113-157 mph. Considerable damage. Roofs torn off frame houses; mobile
homes demolished; boxcars pushed over; large trees snapped or uprooted; light object missiles
generated.
F3 Severe tornado, 158-206 mph. Roof and some walls torn off well constructed houses; trains
overturned; most trees in forest uprooted
F4 Devastating tornado, 207-260 mph. Well-constructed houses leveled; structures with weak
foundations blown off some distance; cars thrown and large missiles generated.
F5 Incredible Damage, 261-318 mph. Strong frame houses lifted off foundations and carried
considerable distances to disintegrate; automobile sized missiles fly through the air in excess of 100
meters; trees debarked; steel re-inforced concrete structures badly damaged.
F6 Inconceivable tornado, 319-379 mph. These winds are very unlikely. The small area of damage
they might produce would probably not be recognizable along with the mess produced by F4 and F5
wind that would surround the F6 winds. Missiles, such as cars and refrigerators would do serious
secondary damage that could not be directly identified as F6 damage. If this level is ever achieved,
evidence for it might only be found in some manner of ground swirl pattern, for it may never be
identifiable through engineering studies.
The Enhanced Fujita Scale (EF)
Essentially, the above given Fujita scale relates damage level associated with a range of wind speeds
– which are estimated but often cannot be verified with actual measurements. Frequently, the wind
speeds for strong tornadoes are greatly overestimated. To compensate for this problem an Enhanced
Fujita Scale (EF) was formulated and implemented in the U.S. as of 1 February 2007.
This Enhanced F-scale is also related to a set of wind estimates (not measurements) based on
damage, but it uses also three-second wind gusts estimated at the point of damage, based on a
judgment of 8 levels of damage to 28 indicators of different types of structures. The 3-second gust in
the EF scale is not the same wind as in standard surface observations. Standard measurements are
taken by weather stations in areas of open exposure, using a directly measured “one minute mile"
speed. Recently, new mobile doppler radar units mounted on trucks have been used and wind
speeds of up to 315 mph have been recorded within tornadoes.
Major Historical
Tornadoes
Documentation of
historical tornadoes
begun in the 1880s,
when John Finley of what was then the
U.S. Army Signal
Corps - was sent to
conduct surveys of
tornadoes in the
Midwest. Finley
began his work by
compiling a
historical record of
600 tornadoes from
1794 to 1881. Also,
he organized a
system of volunteer
tornado reporters.
Finley’s compilation
of historical
tornadoes in the
United States,
combined with more
recent events of the
20th Century were combined in a catalog published by Grazulis (1993), entitled “Significant
Tornadoes, 1680 – 1995” (see reference below). Deadly tornadoes occur nearly every year in the
U.S. Information for all the tornadoes and severe weather events -- including killer tornadoes -- is
provided in NOAA’s National Climatic Data Center (NCDC) publication “Storm Data”.
A photo of the April 11, 1965 (Palm Sunday) double tornado touchdowns at Elkhart Indiana (NOAA
archives)
Supecell Cloud - Web Photo