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Tornadoes....
Nature's Most Violent Storms
Adapted from: A PREPAREDNESS GUIDE Including Safety
Information for Schools U.S. DEPARTMENT OF COMMERCE National Oceanic and
Atmospheric Administration National Weather Service September 1992 (NOAA,
FEMA, The American Red Cross);
PDF version
Tornado!
Greg Stumpf
Although tornadoes occur in many parts of the world, these destructive
forces of nature are found most frequently in the United States east of
the Rocky Mountains during the spring and summer months. In an average
year, 800 tornadoes are reported nationwide, resulting in 80 deaths and
over 1,500 injuries. A tornado is defined as a violently rotating column
of air extending from a thunderstorm to the ground. The most violent
tornadoes are capable of tremendous destruction with wind speeds of 250
mph or more. Damage paths can be in excess of one mile wide and 50 miles
long. Once a tornado in Broken Bow, Oklahoma, carried a motel sign 30
miles and dropped it in Arkansas!
What causes tornadoes?
Thunderstorms develop in warm, moist air in advance of
eastward-moving cold fronts. These thunderstorms often produce large
hail, strong winds, and tornadoes. Tornadoes in the winter and early
spring are often associated with strong, frontal systems that form in
the Central States and move east. Occasionally, large outbreaks of
tornadoes occur with this type of weather pattern. Several states may be
affected by numerous severe thunderstorms and tornadoes.
During the spring in the Central Plains, thunderstorms frequently
develop along a "dryline," which separates very warm, moist air to the
east from hot, dry air to the west. Tornado-producing thunderstorms may
form as the dryline moves east during the afternoon hours.
Along the front range of the Rocky Mountains, in the Texas panhandle,
and in the southern High Plains, thunderstorms frequently form as air
near the ground flows "upslope" toward higher terrain. If other
favorable conditions exist, these thunderstorms can produce tornadoes.
Tornadoes occasionally accompany tropical
storms and hurricanes that move over land. Tornadoes are most common to
the right and ahead of the path of the storm center as it comes onshore.
Colorado Tornado (David Blanchard)
Dr. Joseph Golden (NOAA)
Tornado Variations
- Some tornadoes may form during the early stages of
rapidly developing thunderstorms. This type of tornado is most common
along the front range of the Rocky Mountains, the Plains, and the
Western States.
- Tornadoes may appear nearly transparent until dust
and debris are picked up.
- Occasionally, two or more tornadoes may occur at
the same time.
Waterspout
- Waterspouts are weak tornadoes that form over warm
water.
- Waterspouts are most common along the Gulf Coast
and southeastern states. In the western United States, they occur with
cold late fall or late winter storms, during a time when you least
expect tornado development.
- Waterspouts occasionally move inland becoming
tornadoes causing damage and injuries.
How Do Tornadoes Form?
 Before
thunderstorms develop, 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 thunderstorm 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. |
|
Woodward OK (Ron Przybylinski)
A
lower cloud base in the center of the photograph identifies an area of
rotation known as a rotating wall cloud. This area is often nearly
rain-free. Note rain in the background. |
Woodward OK (Ron Przybylinski)
Moments
later a strong tornado develops in this area. Softball-size hail and
damaging "straight-line" winds also occurred with this storm. |
|
Tornadoes Take Many Shapes and Sizes
Weak Tornadoes
- 69% of all tornadoes
- Less than 5% of tornado deaths
- Lifetime 1-10+ minutes
- Winds less than 110 mph
 |
Strong Tornadoes
- 29% of all tornadoes
- Nearly 30% of all tornado deaths
- May last 20 minutes or longer
- Winds 110-205 mph
 |
Violent Tornadoes
- Only 2% of all tornadoes
- 70% of all tornado deaths
- Lifetime can exceed 1 hour
- Lifetime can exceed 1 hour
 |
Tornado Myths:
MYTH: Areas near rivers, lakes, and mountains are
safe from tornadoes.
FACT: No place is safe from tornadoes. In the late
1980's, a tornado swept through Yellowstone National Park leaving a path
of destruction up and down a 10,000 ft. mountain.
MYTH: The low pressure with a tornado causes
buildings to "explode" as the tornado passes overhead.
FACT: Violent winds and debris slamming into buildings
cause most structural damage.
MYTH: Windows should be
opened before a tornado approaches to equalize pressure and minimize
damage.
FACT: Opening windows allows damaging winds to enter
the structure. Leave the windows alone; instead, immediately go to a
safe place.
Tornadoes Occur Anywhere
Carolinas Outbreak:
- March 28, 1984, afternoon-evening
- 22 tornadoes
- 57 deaths
- 1,248 injuries
- damage $200 million
- 37% of fatalities in mobile homes
|
Pennsylvania-Ohio Outbreak:
- May 31, 1985, late afternoon-evening
- 41 tornadoes, including 27 in PA and OH
- 75 deaths in U.S.
- 1,025 injuries
- damage $450 million
|
Plains Outbreak:
- April 26-27, 1991, afternoon of 26th through
early morning 27th
- 54 tornadoes
- 21 deaths
- 308 injuries
- damage $277+ million
- 15 deaths in/near mobile homes, 2 deaths in
vehicles
|
Weather Radar Watches the Sky
Meteorologists rely on weather radar to provide information on
developing storms. The National Weather Service is strategically locating
Doppler radars across the country which can detect air movement toward or
away from the radar. Early detection of increasing rotation aloft within a
thunderstorm can allow life-saving warnings to be issued before the
tornado forms.
Doppler Radial Velocity
Jim Ladue
Frequency of Tornadoes
Tornadoes can occur at any time of the year.
- In the southern states, peak tornado occurrence is
in March through May, while peak months in the northern states are
during the summer.
- Note, in some states, a secondary tornado maximum
occurs in the fall.
- Tornadoes are most likely to occur between 3 and 9
p.m. but have been known to occur at all hours of the day or night.
- The average tornado moves from southwest to
northeast, but tornadoes have been known to move in any direction. The
average forward speed is 30 mph but may vary from nearly stationary to
70 mph.
- The total number of tornadoes is probably higher
than indicated in the western states. Sparce population reduces the
number reported.
STAY INFORMED ABOUT THE STORM
by listening to NOAA Weather Radio, commercial radio,
and television for the latest tornado WATCHES
and WARNINGS.
 When
conditions are favorable for severe weather to develop, a severe
thunderstorm or tornado WATCH is issued
Weather Service personnel use information from weather radar, spotters,
and other sources to issue severe thunderstorm and tornado
WARNINGS for areas where severe weather is
imminent.
Severe thunderstorm warnings are passed to local radio and television
stations and are broadcast over local NOAA Weather Radio stations serving
the warned areas. These warnings are also relayed to local emergency
management and public safety officials who can activate local warning
systems to alert communities.
NOAA WEATHER RADIO IS THE BEST MEANS TO
RECEIVE WARNINGS FROM THE NATIONAL WEATHER SERVICE
The National Weather Service continuously
broadcasts updated weather warnings and forecasts that can be received
by NOAA Weather Radios sold in many stores. The average range is 40
miles, depending on topography. Your National Weather Service recommends
purchasing a radio that has both a battery backup and a tone-alert
feature which automatically alerts you when a watch or warning is
issued.
What To Listen For...
- TORNADO WATCH: Tornadoes are
possible in your area. Remain alert for approaching storms.
- TORNADO WARNING: A tornado
has been sighted or indicated by weather radar. If a tornado warning is
issued for your area and the sky becomes threatening, move to your
pre-designated place of safety.
- SEVERE THUNDERSTORM WATCH:
Severe thunderstorms are possible in your area.
- SEVERE THUNDERSTORM WARNING:
Severe thunderstorms are occurring.
Remember, tornadoes occasionally develop
in areas in which a severe thunderstorm watch or warning is in effect.
Remain alert to signs of an approaching tornado and seek shelter if
threatening conditions exist.
Environmental Clues
Look out for:
- Dark, often greenish sky
- Wall cloud
- Large hail
- Loud roar; similar to a freight train
Caution:
- Some tornadoes appear as a visible funnel
extending only partially to the ground. Look for signs of debris
below the visible funnel.
Alma, NE (Gene Rhoden) |
- Some tornadoes are clearly visible while
others are obscured by rain or nearby low-hanging clouds.
Mike Emlaw |
Other Thunderstorm Hazards
Center photo Bill Bunting
These dangers often accompany thunderstorms:
- Flash Floods: Number ONE weather killer - 146
deaths annually
- Lightning: Kills 75-100 people each year
- Damaging Straight-line Winds: Can reach 140 mph
- Large Hail: Can reach the size of a grapefruit -
causes several hundred million dollars in damage annually to property
and crops
Contact your local National Weather Service office, American Red Cross
chapter, or Federal Emergency Management Agency office for a copy of the
"Thunderstorms and Lightning...The Underrated Killers" brochure (NOAA PA
92053) and the "Flash Floods and Floods...The Awesome Power" brochure (NOAA
PA 92050).
Tornado Safety
What YOU Can Do
Before the Storm:
- Develop a plan for you and your family for home,
work, school and when outdoors.
- Have frequent drills.
- Know the county/parish in which you live, and keep
a highway map nearby to follow storm movement from weather bulletins.
- Have a NOAA
Weather Radio with a warning alarm tone and battery back-up to
receive warnings.
- Listen to radio and television for information.
- If planning a trip outdoors, listen to the latest
forecasts and take necessary action if threatening weather is possible.
Top: James Campbell; Bottom: Gene Rhoden
If a Warning is issued or if threatening weather
approaches:
- In a home or building, move to a pre-designated
shelter, such as a basement.
- If an underground shelter is not available, move
to an interior room or hallway on the lowest floor and get under a
sturdy piece of furniture.
- Stay away from windows.
- Get out of automobiles.
- Do not try to outrun a tornado in your car;
instead, leave it immediately.
- Mobile homes, even if tied down, offer little
protection from tornadoes and should be abandoned.
Occasionally, tornadoes
develop so rapidly that advance warning is not possible. Remain alert
for signs of an approaching tornado. Flying debris from tornadoes causes
most deaths and injuries.
It's Up To YOU!
Each year, many people are killed or seriously injured by tornadoes
despite advance warning. Some did not hear the warning while others
received the warning but did not believe a tornado would actually affect
them. The preparedness information in this brochure, combined with timely
severe weather watches and warnings, could save your life in the event a
tornado threatens your area. After you have received the warning or
observed threatening skies, YOU must make the decision to seek shelter
before the storm arrives. It could be the most important decision you
will ever make.
Who's Most At Risk?
- People in automobiles
- The elderly, very young, and the physically or
mentally impaired
- People in mobile homes
- People who may not understand the warning due to
a language barrier
Tornado Safety in Schools
EVERY School Should Have A Plan!
-
  Develop
a severe weather action plan and have frequent drills,
- Each school should be inspected and tornado
shelter areas designated by a registered engineer or architect.
Basements offer the best protection. Schools without basements should
use interior rooms and hallways on the lowest floor and away from
windows.
- Those responsible for activating the plan should
monitor weather information from NOAA Weather Radio and local
radio/television.
- If the school's alarm system relies on
electricity, have a compressed air horn or megaphone to activate the
alarm in case of power failure.
- Make special provisions for disabled students
and those in portable classrooms.
- Make sure someone knows how to turn off
electricity and gas in the event the school is damaged.
- Keep children at school beyond regular hours if
threatening weather is expected. Children are safer at school than in
a bus or car. Students should not be sent home early if severe weather
is approaching.
- Lunches or assemblies in large rooms should be
delayed if severe weather is anticipated. Gymnasiums, cafeterias, and
auditoriums offer no protection from tornado-strength winds.
- Move students quickly into interior rooms or
hallways on the lowest floor. Have them assume the tornado protection
position (shown at right).
Hospitals, nursing homes, and other institutions should develop a
similar plan
Paul Bowen
Your National Weather Service, Federal Emergency Management Agency, and
American Red Cross educate community officials and the public concerning
the dangers posed by tornadoes. YOU can prepare for the possibility of a
tornado by learning the safest places to seek shelter when at home, work,
school, or outdoors. You should also understand basic weather terms and
danger signs related to tornadoes. Your chances of staying safe during a
tornado are greater if you have a plan for you and your family, and
practice the plan frequently.
FAMILY DISASTER PLAN
Families should be prepared for all hazards
that affect their area. NOAA's National Weather Service, the Federal
Emergency Management Agency, and the American Red Cross urge each family
to develop a family disaster plan.
Where will your family be when disaster
strikes? They could be anywhere - at work, at school, or in the car. How
will you find each other? Will you know if your children are safe?
Disasters may force you to evacuate your neighborhood or confine you to
your home. What would you do if basic services - water, gas, electricity
or telephones - were cut off?
Follow these basic steps to develop a family disaster
plan...
- I.Gather information
about hazards.
Contact your local National Weather Service office, emergency management
or civil defense office, and American Red Cross chapter. Find out what
type of disasters could occur and how you should respond. Learn your
community's warning signals and evacuation plans.
- II.Meet with your
family to create a plan.
Discuss the information you have gathered. Pick two places to meet: a
spot outside your home for an emergency, such as fire, and a place away
from your neighborhood in case you can't return home. Choose an
out-of-state friend as your "family check-in contact" for everyone to
call if the family gets separated. Discuss what you would do if advised
to evacuate.
- III.Implement your
plan
(1) Post emergency telephone numbers by phones; (2) Install safety
features in your house, such as smoke detectors and fire extinguishers;
(3) Inspect your home for potential hazards (such as items that can
move, fall, break, or catch fire) and correct them; (4) Have your family
learn basic safety measures, such as CPR and first aid; how to use a
fire extinguisher; and how and when to turn off water, gas, and
electricity in your home; (5) Teach children how and when to call 911 or
your local Emergency Medical Services number; (6) Keep enough supplies
in your home to meet your needs for at least three days. Assemble a
disaster supplies kit with items you may need in case of an evacuation.
Store these supplies in sturdy, easy-to-carry containers, such as
backpacks or duffle bags. Keep important family documents in a
waterproof container. Keep a smaller disaster supplies kit in the trunk
of your car.
A DISASTER SUPPLIES KIT SHOULD INCLUDE:
A 3-day supply of water (one gallon per person per day) and food that
won't spoil 
one change of clothing and footwear per person
one blanket or sleeping bag per person
a first-aid kit, including prescription medicines
emergency tools, including a battery-powered NOAA Weather Radio and a
portable radio, flashlight, and plenty of extra batteries
an extra set of car keys and a credit card or cash
special items for infant, elderly, or disabled family members.
- IV.Practice and
maintain your plan.
Ask questions to make sure your family remembers meeting places, phone
numbers, and safety rules. Conduct drills. Test your smoke detectors
monthly and change the batteries at least once a year. Test and recharge
your fire extinguisher(s) according to manufacturer's instructions.
Replace stored water and food every six months.
LOCAL SPONSORSHIP:
NOAA/PA 92052 ARC 5002
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