tornadoes...
Nature's Most Violent Storms
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)
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Tornado!
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.
|
© Greg Stumpf
|
| 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.
|
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.
|
(Colorado Tornado) © David Blanchard
|
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.
|
© Dr. Joseph Golden, NOAA
|
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.
|
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|
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(Woodward, OK) © Ron Przybylinski
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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
- Winds greater than 205 mph
|
|
Left: © Charles Doswell III Center:
© Mary Hurley Right:
© Peter Willing
|
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 US
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
Doppler
Radial Velocity
© Jim LaDue
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 lifesaving
warnings to be issued before the tornado forms.
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. Sparse population reduces the number
reported.
Months of Peak Tornado Occurrence
United States Totals 1961-1993
US Reported Tornadoes and Average Number
of Deaths Per Year 1961-1993
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.
|
Some tornadoes are clearly visible while others are obscured
by rain or nearby low-hanging clouds.
|
|
(Alma, NE) © Gene Rhoden
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© Mike Emlaw
|
Other Thunderstorm Hazards
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).
|
Center Photo © Bill Bunting |
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.
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.
|
Top: © James Campbell
Bottom: © Gene Rhoden |
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 above right).
Hospitals, nursing homes, and other institutions
should develop a similar plan
| 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. |
© Paul Bowen
|
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.
NOAA/PA 92052 ARC 5002
National
Weather Service
Page prepared by AKM, National Severe Storms Laboratory May
12, 1998
Updated March 13, 2002
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