Tornadoes
What Is a Tornado?
What Causes Tornadoes
Wind Speeds In Tornadoes and The F-Scale
Forecasting Tornadoes – Watches and Warnings
Tornado Safety Rules
What Is a Tornado?
A tornado is a violently rotating column of air in contact with the
ground. Tornadoes, just like about anything else, come in many sizes,
shapes and colors. They can range in width from a few yards to more
than a mile. They can last from a few seconds to several hours. They
can move forward at speeds of up to 70 miles per hour, or stand nearly
still.
Most people recognize the funnel-shaped cloud of a typical tornado,
but the funnel can vary considerably in appearance. Some are long and
slender while others are wider than they are tall. Some of the worst
tornadoes in history, such as the tri-state tornado of March 18, 1925
which killed 740 people in Missouri, Illinois and Indiana, were never
recognized as tornadoes since they were so large or were wrapped in
rain that most people reported an area of black skies to the west or
a wall of dark clouds.
Some tornadoes do not have a visible funnel; their presence is indicated
only by a swirl of dust near the ground. Other tornadoes may consist
of several funnels sometimes rotating around each other. Colors range
from jet-black to brown or reddish color when dust is lifted into the
funnel; to almost white when sunlight falls on the funnel and dark
clouds are in the background.
In addition to having various sizes, shapes and colors, some tornadoes
have distinctive sounds. Those who have been close enough to hear a
tornado usually describe the sound as a loud roar like that of a jet
aircraft or a freight train. Not all tornadoes have this roar, but
at night this may be the only clue that a tornado is approaching.
What Causes Tornadoes
Tornadoes form under a certain set of weather conditions in which
three very different types of air come together in a certain way. Near
the ground lies a layer of warm and humid air along with strong south
winds. Colder air and strong west or southwest winds lie in the upper
atmosphere. Temperature and moisture differences between the surface
and the upper levels create instability, while the change in wind with
height is known a wind shear. This shear is linked to the eventual
development of rotation from which a tornado may form.
A third layer of very warm dry air becomes established between the
warm moist air at low levels and the cool dry air aloft. This very
warm layer acts as a cap and allows the atmosphere to warm further,
making the air even more unstable. Things start to happen when a storm
system aloft moves east and begins to lift the various layers. Through
this lifting process the cap is removed, thereby setting the stage
for explosive thunderstorm development as strong updrafts develop.
Complex interactions between the updraft and the surrounding winds
both at storm level and near the surface may cause the updraft to begin
rotating, which gives birth to the tornado.
The plains of the Central United States are uniquely suited to bring
all of these conditions together, and so have become known as tornado
alley. The main factors are the Rocky Mountains to the west, the Gulf
of Mexico to the south, and a terrain that slopes downward from west
to east. Where the dry air and the moist air meet the ground a boundary
known as a “dry line” forms to the west of Oklahoma. A storm
system moving out of the Southern Plains or Rocky Mountains may push
the “dry line” eastward, with severe thunderstorms or tornadoes
forming along the “dry line” or in the moist air or just
ahead of it. Peak tornado months are from April to June, but tornadoes
can occur at any time and in any location depending on the convergence
of the required conditions.
Wind Speeds in Tornadoes and the F-Scale
In general, tornado wind speeds must be determined from a study of
damage left in its wake. From damage studies a scale was developed
called the Fujita, or F-Scale. The F-Scale rates tornadoes from F0
(F Zero) to F5 based on wind speed estimated from damage to homes.
The weakest tornadoes are rated F0 while the strongest and most destructive
tornadoes are given an F5 rating.
The most destructive force of a tornado is the wind, which may reach
speeds close to 300 miles per hour in extreme cases. On April 26, 1991,
University of Oklahoma researchers measured 287 mile per hour winds
at about 600 feet above the ground in a tornado in Northern Oklahoma
using a portable Doppler radar device. This was the first time direct
measurement of a tornado of that intensity has ever been made. Until
then, no wind measuring equipment had ever survived a direct hit from
a tornado.
The Tulsa tornado of April 24, 1993 and the Red Rock tornado of April
26, 1991 were both rated F4, while the tornado that struck Wichita,
Kansas on April 26, 1991 was rated F5. Tornadoes of F5 intensity are
rare, with several years going by without a single F5 tornado across
the United States.
Tornadoes do their destructive work through the combined action of
winds and the action of windblown debris. In one of the most common
cases the roof is peeled off a structure or wind enters the structure
through a window or door, such as a garage door blown in by the winds.
The appearance to someone outside the structure is that the building
has exploded. It is now known that buildings do not explode. Rather,
wind coming inside the structure forces the walls out. The damage pattern
can vary widely from one home to the next based on factors such as
shape, building quality, orientation to the storm, the amount and size
of windblown debris, and the intensity of the tornado itself.
An important point to remember is that while most tornado damage
is caused by the violent winds, flying debris causes most tornado injuries
and deaths. Wood splinters, glass, lawn furniture, or even cars can
become deadly missiles when tossed about by tornado force winds.
Forecasting Tornadoes – Watches and Warnings
Tornado forecasting began in 1948 when Tinker Air Force Base was
hit by a tornado. The first tornado forecast was issued for the base
just one week later, and turned out to be correct as another tornado
struck the base that day.
National forecasting of tornadoes and severe thunderstorms in the
United States is done at the Storm Prediction Center in Norman, Oklahoma.
At the Storm Prediction Center, the meteorologists monitor weather
conditions around the clock over the continental United States. They
look for conditions that are favorable for severe weather. When such
conditions arise, they issue a tornado or severe thunderstorm “Watch.”
A “Watch” normally is issued for a period of up to 6 hours
and covers an area between 10,000 and 25,000 square miles; possibly
including several states. The “Watch” covers the area where
severe weather is most likely to occur. When a tornado “Watch”
is issued for your area, it means you may continue normal activities,
but you should remain alert for threatening weather conditions and
be ready to take action if threatening weather approaches or a “Warning
“is issued.
A tornado “Warning” is an urgent statement that a tornado
has actually been sighted or indicated as developing by radar. Warnings
are issued by local offices of the National Weather Service for areas
much smaller than for watches, usually only one or two counties. If
a tornado “Warning” is issued for your area, take action
immediately since the tornado may be headed in your direction.
Tornado Safety Rules
When a tornado is sighted, or if a “Warning” is issued,
follow these basic safety rules:
If you are in your home or a small building, go to the basement or
interior part of the lowest floor. Stay away from exterior walls and
windows. Get under something sturdy and protect yourself from flying
debris. First floor closets and bathrooms near the center of a building
without a basement often provide good protection.
If you are in a school, hospital or office building, go to designated
shelters if available. Otherwise, get away from the exterior walls
and windows and take cover in the basement or the interior part of
the building on the lowest floor possible. Hallways, corridors and
closets often provide good protection, but stay away from doors, windows,
skylights, and outside entrances.
If you are in a mobile home or a motor vehicle, leave them and get
to a more substantial structure if available, where shelter can be
sought in a basement or interior part of the lowest floor. If no shelter
is available go to a ditch or culvert and cover your head. A ditch
or low area is often your only chance if caught in the open with no
available shelter.
If you are in stores, churches, movie theatres, or like buildings
with no basement or interior walls, try to get under or behind something
sturdy to put some protection between yourself and the approaching
tornado. Do not try to escape in a car.
Remember, when a tornado strikes, the biggest threat of death or
injury comes from head injuries caused by flying and falling debris.
If you can protect your head and neck you have a much better chance
of survival.
A tornado “Watch” means the weather conditions are favorable
for the development of severe thunderstorms and tornadoes. When a “Watch”
is issued, stay in touch with the updated news and weather reports.
Watch for signs of threatening weather and listen for “Warnings.”
A tornado “Warning” means that a tornado has been visually
sighted or Doppler Radar indicates one may be developing. If you are
in the path of the storm, take cover.
Remember that some severe thunderstorms can produce winds as strong
as those winds in some tornadoes. These winds can occur in or slightly
ahead of an approaching severe thunderstorm. These winds can cause
a roar that may sound like a tornado, as well as produce tornado-like
damage.
Deadliest Tornadoes In The U.S.
The deadliest tornadoes in the United States showed no preference
for any single state. Even California is visited by these deadly events
today.
Date: Deaths: Location:
April 8, 1998 40 Alabama, North Carolina, Mississippi, Georgia
February 23, 1998 42 Florida
May 31, 1985 90 Ohio, Pennsylvania
March 28, 1984 67 North and South Carolina
April 3-4, 1974 350 Alabama, Georgia, Tennessee, Kentucky, Ohio
February 21, 1971 110 Mississippi Delta Region
April 11, 1965 271 Indiana, Illinois, Ohio, Michigan, Wisconsin
March 31, 1962 17 Florida
May 25, 1955 115 Kansas, Missouri, Oklahoma, Texas
June 8, 1953 142 Michigan, Ohio
May 11, 1953 114 Texas
March 21, 1952 208 Arkansas, Missouri, Texas
April 9, 1947 169 Texas, Oklahoma, Kansas
June 23, 1944 150 Ohio, Pennsylvania, West Virginia, Maryland
April 5-6, 1936 658 Mississippi, Georgia
March 21, 1932 268 Alabama
March 18, 1925 695 Missouri, Illinois, Indiana
May 18, 1902 114 Texas
May 27, 1896 300 Missouri, Illinois
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