A tornado is
a violent rotating column of air extending from a thunderstorm to the ground.
The most violent tornadoes are capable of tremendous destruction with wind
speeds of up to 300 mph. They can destroy large buildings, uproot trees and
hurl vehicles hundreds of yards. They can also drive straw into trees. Damage
paths can be in excess of one mile wide to 50 miles long.
In an average year, 1000 tornadoes are reported
nationwide. Of the 1000 tornadoes that occur each year, about 2% of them are
rated EF4 or EF5. That means that there are as many as 20 devastating tornadoes
each year. It is possible that meteorologists have underestimated the number of
violent tornadoes that occur each year. Tornadoes are rated only by the damage
they do to man-made structures. Therefore, if a tornado doesn't hit a structure
of some kind, we cannot estimate its strength. Also, a tornado varies in
strength during its lifetime and could be its strongest while between areas of
houses or other buildings.
Below is
video of the strong 1 mile-wide EF5 tornado that moved through Tuscaloosa,
Alabama on April 27th, 2011.
Before a tornado reaches the ground, its called a
funnel cloud. A funnel cloud is a rotating cone-shaped column of air extending
downward from the base of a thunderstorm, but not touching the ground. When it
reaches the ground it is then called a tornado.
Most
tornadoes form from thunderstorms. You need warm, moist air from the Gulf of
Mexico and cool, dry air from Canada. When these two air masses meet, they
create instability in the atmosphere. A change in wind direction and an
increase in wind speed with increasing height creates an invisible, horizontal
spinning effect in the lower atmosphere. Rising air within the updraft tilts
the rotating air from horizontal to vertical. An area of rotation, 2-6 miles
wide, now extends through much of the storm. Most strong and violent tornadoes
form within this area of strong rotation.
It
is not fully understood about how exactly tornadoes form, grow and die. Tornado
researchers are still trying to solve the tornado puzzle, but for every piece
that seems to fit they often uncover new pieces that need to be
studied.
They can
happen at any time of the year and at any time of the day. In the southern
states, peak tornado season is from March through May. Peak times for tornadoes
in the northern states are during the summer. A few southern states have a
second peak time for tornado outbreaks in the fall. They are most likely to
occur between 3 p.m. and 9 p.m. Tornadoes have been known to occur in every
state in the United States, on any day of the year, and at any hour. They also
occur in many other parts of the world, including Australia, Europe, Africa,
Asia, and South America.
The
geography of the central part of the United States, known as the Great Plains,
is suited to bring all of the ingredients together to forms tornadoes. More
than 500 tornadoes typically occur in this area every year and is why it is
commonly known as "Tornado Alley".
Most tornadoes are associated with a wall
cloud. A wall cloud is an abrupt lowering of a rain-free cumulonimbus base into
a low-hanging accessory cloud. A wall cloud is usually situated in the
southwest portion of the storm. A rotating wall cloud usually develops before
tornadoes or funnel clouds.
A
supercell thunderstorm is a long-lived thunderstorm whose updrafts and
downdrafts are in near balance. These storms have the greatest tendency to
produce tornadoes that stay on the ground for long periods of time. Supercell
thunderstorms can produce violent tornadoes with winds exceeding 200
mph. A mesocyclone is a rotating vortex of air within
a supercell thunderstorm. Though, mesocyclones do not always produce
tornadoes.
Tornadoes often produce hail. Hail is created when
small water droplets are caught in the updraft of a thunderstorm. These water
droplets are lifted higher and higher into the sky until they freeze into ice.
Once they become heavy, they will start to fall. If the smaller hailstones get
caught in the updraft again, they will get more water on them and get lifted
higher in the sky and get bigger. Once they get lifted again, they freeze and
fall. This happens over and over again until the hailstone is too heavy and
then falls to the ground. According to the National
Weather Service, the largest hailstone is 8 inches in diameter and weights
approximately 2 pounds. It fell in Vivian, South Dakota on July 23,
2010.
Tornadoes can also form over water. These are called
waterspouts. They are most common along the Gulf Coast and are usually weak.
Waterspouts can sometimes move inland, becoming tornadoes causing damage and
injuries. A landspout is a very weak
tornado that is not associated with a wall cloud or a mesocyclone. It is the
land equivalent of a waterspout.
Gustnadoes can
also be mistaken for tornadoes. A gustnado is a short-lived, relatively weak
whirlwind that forms along a gust front. A gust front is the surge of very
gusty winds at the leading edge of a thunderstorm's outflow of air. Gustnadoes
are not tornadoes. They do not connect with any cloud-base rotation. But
because gustnadoes often have a spinning dust cloud at ground level, they are
sometimes wrongly reported as tornadoes. Gustnadoes can do minor
damage.
The development of Doppler radar has made it
possible, under certain circumstances, to detect a tornado's winds with a
radar. However, human beings remain an important part of the system to detect
tornadoes, because not all tornadoes occur in situations where the radar can
"see" them. Ordinary citizen volunteers make up what is called the SKYWARN
network of storm spotters, who work with their local communities to watch for
approaching tornadoes, so those communities can take appropriate action in the
event of a tornado. Spotter information is relayed to the National Weather
Service, which operates the national Doppler radar network and which issues
warnings to the public by radio, TV, and NOAA Weather Radio, using information
obtained from weather maps, weather radars, and local storm spotters.
Tornado Terms
Tornado Watch
Tornadoes are possible in
your area. Stay tuned to the radio or television news.
Tornado Warning
A tornado is either on the
ground or has been detected by Doppler radar. Seek shelter
immediately!
Fujita Scale of Tornado Intensity
SCALE
WIND
SPEED
POSSIBLE
DAMAGE
Enhanced,
Operational Fujita Scale
F0
40-72 mph
Light damage:
Branches broken off trees; minor roof damage
EF0 65-85 mph
F1
73-112
mph
Moderate damage:
Trees snapped; mobile home pushed off foundations; roofs damaged
EF1 86-110 mph
F2
113-157
mph
Considerable
damage: Mobile homes demolished; trees uprooted; strong built homes unroofed
EF2 111-135 mph
F3
158-206
mph
Severe damage:
Trains overturned; cars lifted off the ground; strong built homes have outside
walls blown away
EF3 136-165 mph
F4
207-260
mph
Devastating
damage: Houses leveled leaving piles of debris; cars thrown 300 yards or more
in the air
EF4 166-200 mph
F5
261-318
mph
Incredible
damage: Strongly built homes completely blown away; automobile-sized missiles
generated
EF5 over 200 mph
Tornado Safety
Tips
BEFORE A TORNADO: Have a
disaster plan. Make sure everyone knows where to go in case a tornado
threatens. Make sure you know which county or parish you live in. Prepare an
emergency kit for your home. Have enough food and water for at least 3 days.
DURING A TORNADO: Go to a
basement. If you do not have a basement, go to an interior room without windows
on the lowest floor such as a bathroom or closet. If you can, get under a
sturdy piece of furniture, like a table. If you live in a mobile home get out.
They offer little protection against tornadoes. Get out of automobiles. Do not
try to outrun a tornado in your car, leave it immediately. If you’re
outside, go to a ditch or low lying area and lie flat in it. Stay away from
fallen power lines and stay out of damaged areas.
IF YOU’RE AT SCHOOL DURING A TORNADO:
Every school should have a disaster plan and have frequent drills. Basements
offer the best protection. Schools without basements should use interior rooms
and hallways on the lowest floor away from windows. Crouch down on your knees
and protect your head with your arms.
AFTER A
TORNADO: Stay indoors until it is safe to come out. Check for injured or
trapped people, without putting yourself in danger. Watch out for downed power
lines. Use a flashlight to inspect your home.
