A PREPAREDNESS GUIDE
U.S. DEPARTMENT OF COMMERCE
National Oceanic & Atmospheric Administration National Weather Service
NOAA, FEMA, The American Red Cross
Thunderstorms...& Their Offspring
Thunderstorms affect relatively small areas when
compared with hurricanes and winter storms. The typical thunderstorm is 15 miles
in diameter and lasts an average of 30 minutes. Nearly 1,800 thunderstorms are
occurring at any moment around the world. That's 16 million a year!
Despite their small size, all thunderstorms are
dangerous. Every thunderstorm produces lightning, which kills more people each
year than tornadoes. Heavy rain from thunderstorms can lead to flash flooding.
Strong winds, hail, and tornadoes are also dangers associated with some
Of the estimated 100,000 thunderstorms that occur
each year in the United States, only about 10 percent are classified as severe.
Your National Weather Service considers a
thunderstorm severe if it produces hail at least 3/4-inch in diameter, wind 58
mph or higher, or tornadoes.
Take the time NOW
to understand these dangers and learn basic safety rules!
Occurs with ALL thunderstorms.
Averages 93 deaths and 300 injuries each year.
Causes several hundred million dollars in damage
to property and forests annually.
Responsible for most thunderstorm wind damage.
Winds can exceed 100 mph!
One type of straight-line wind, the downburst,
can cause damage equivalent to a strong tornado and can be extremely dangerous
During the summer in the western states,
thunderstorms often produce little rain but very strong wind gusts and dust
Causes nearly $1 billion in damage to property
and crops annually.
Costliest United States hailstorm: Denver,
Colorado, July 11, 1990. Total damage was $625 million.
Nature's most violent storms.
Winds can exceed 200 mph.
Result in an average of 80 deaths and 1,500
injuries each year.
Most fatalities occur when people do not leave
mobile homes and automobiles.
Contact your local National Weather
Service office, American Red Cross chapter, or local emergency management office
for a copy of "Flash Floods and Floods...The Awesome Power" (NOAA PA 92050/ARC
4493) and "Tornadoes...Nature's Most Violent Storms" (NOAA PA 92052/ARC 5002).
What Makes A Thunderstorm?
Every Thunderstorm Needs:
Moisture - to form clouds and
Unstable Air - relatively warm
air that can rise rapidly.
Lift - fronts, sea breezes, and
mountains are capable of lifting air to help form thunderstorms.
Life Cycle of a Thunderstorm
Towering cumulus cloud indicates rising air.
Usually little if any rain during this stage.
Lasts about 10 minutes.
Occasional lightning during this stage.
Most likely time for hail, heavy rain, frequent
lightning, strong winds, and tornadoes.
Storm occasionally has a black or dark green
Lasts an average of 10 to 20 minutes but may last
much longer in some storms.
Rainfall decreases in intensity.
Some thunderstorms produce a burst of strong
winds during this stage.
Lightning remains a danger during this stage.
When Are Thunderstorms Most Likely?
Thunderstorms are most likely to happen in the
spring and summer months and during the afternoon and evening hours but can
occur year-round and at all hours.
Along the Gulf Coast and across the southeastern
and western states, most thunderstorms occur during the afternoon.
Thunderstorms frequently occur in the late afternoon and at night in the Plains
Thunder and lightning occasionally accompany snow or freezing rain. During the
blizzard of March 1993, lightning resulted in power outages near Washington,
Who's Most At Risk From Thunderstorms?
People who are: outdoors, especially under or
near tall trees; in or on water; or on or near hilltops.
People who are in automobiles when flash flooding
occurs near them.
People who are in mobile homes and automobiles.
Thunderstorm Winds & Hail
A small area of rapidly descending air beneath a
Can cause damaging winds in excess of 100 mph.
The strong winds usually approach from one
direction and may be known as "straight-line" winds.
In extreme cases, straight-line winds can reach
speeds equal to a strong tornado, causing significant damage to some buildings.
Strong winds may or may not be accompanied by
The strong rising currents of air within a storm,
called updrafts, carry water droplets to a height where freezing occurs.
Ice particles grow in size, finally becoming too
heavy to be supported by the updraft and fall to the ground.
Large hailstones fall at speeds faster than 100
What is Lightning?
The action of rising and descending air within a
thunderstorm separates positive and negative charges. Water and ice particles
also affect the distribution of electrical charge.
Lightning results from the buildup and discharge
of electrical energy between positively and negatively charged areas.
The average flash could light a 100-watt light
bulb for more than 3 months.
Most lightning occurs within the cloud or between
the cloud and ground.
Your chances of being struck by lightning are
estimated to be 1 in 600,000 but could be reduced by following safety rules.
Most lightning deaths and injuries occur when
people are caught outdoors.
Most lightning casualties occur in the summer
months and during the afternoon and early evening.
The air near a lightning strike is heated to
50,000øF hotter than the surface of the sun! The rapid heating and cooling of
air near the lightning channel causes a shock wave that results in thunder.
Many fires in the western United States and
Alaska are started by lightning. In the past decade, over 15,000
lightning-induced fires nationwide have resulted in several hundred million
dollars a year in damage and the loss of 2 million acres of forest.
In recent years, people have been
killed by lightning while:
standing under a tree
riding on a lawnmower
fishing in a boat
talking on the telephone
loading a truck
Lightning Can Strike Anywhere!
In recent years, sophisticated lightning
detection equipment has monitored cloud-to-ground lightning strikes. The map at
right shows which areas were MOST
prone to lightning during one year.
Which way does lightning travel?
A cloud-to-ground lightning strike begins as an
invisible channel of electrically charged air moving from the cloud toward the
ground. When one channel nears an object on the ground, a powerful surge of
electricity from the ground moves upward to the cloud and produces the visible
Lightning Myths and Facts
MYTH: If it is not raining, then there is no
danger from lightning.
FACT: Lightning often strikes outside of heavy rain and may occur as far as 10
miles away from any rainfall.
MYTH: The rubber soles of shoes or rubber tires on a car will protect you from
being struck by lightning.
FACT: Rubber-soled shoes and rubber tires provide NO protection from lightning.
However, the steel frame of a hard-topped vehicle provides increased protection
if you are not touching metal. Although you may be injured if lightning strikes
your car, you are much safer inside a vehicle than outside.
MYTH: People struck by lightning carry an electrical charge and should not be
FACT: Lightning-strike victims carry no electrical charge and should be attended
to immediately. Contact your local American Red Cross chapter for information on
CPR and first aid classes.
MYTH: "Heat lightning" occurs after very hot summer days and poses no threat.
FACT: What is referred to as "heat lightning" is actually lightning from a
thunderstorm too far away for thunder to be heard. However, the storm may be
moving in your direction!
When skies darken or thunderstorms are
forecast, look & listen for
Flashes of lightning.
Sound of thunder.
Static on your AM radio.
To estimate the distance in miles
between you and the lightning flash, count the seconds between the lightning and
the thunder and divide by five.
Advanced Weather Radar Sees "Inside"
Doppler Radars, which are being strategically
deployed around the country, are capable of seeing "inside" a thunderstorm to
detect hazardous weather conditions. New storms often form along leading edge of
the storm's cool-air outflow; this feature is able to be detected on Doppler
What YOU Can
Before the Storm...
Know the county or parish in which you live and
the names of nearby major cities. Severe weather warnings are issued on a county
or parish basis.
Check the weather forecast before leaving for
extended periods outdoors.
Watch for signs of approaching storms.
If a storm is approaching, keep a NOAA Weather
Radio or AM/FM radio with you.
Postpone outdoor activities if thunderstorms are
imminent. This is your best way to avoid being caught in a dangerous situation.
Check on those who have trouble taking shelter if
severe weather threatens.
When Thunderstorms Approach...
Remember: if you can hear thunder, you are close
enough to the storm to be struck by lightning. Go to safe shelter immediately!
Move to a sturdy building or car. Do not take
shelter in small sheds, under isolated trees, or in convertible automobiles.
If lightning is occurring and a sturdy shelter is
not available, get inside a hard top automobile and keep windows up.
Get out of boats and away from water.
Telephone lines and metal pipes can conduct
electricity. Unplug appliances not necessary for obtaining weather information.
Avoid using the telephone or any electrical appliances. Use phones ONLY in an
Do not take a bath or shower.
Turn off air conditioners. Power surges from
lightning can overload the compressors.
Get to higher ground if flash flooding or
flooding is possible. Once flooding begins, abandon cars and climb to higher
ground. Do not attempt to drive to safety. Note: Most flash flood deaths occur
If Caught Outdoors and No Shelter Is Nearby...
Find a low spot away from trees, fences, and
poles. Make sure the place you pick is not subject to flooding.
If you are in the woods, take shelter under the
If you feel your skin tingle or your hair stand
on end, squat low to the ground on the balls of your feet. Place your hands on
your knees with your head between them. Make yourself the smallest target
possible, and minimize your contact with the ground.
If you are boating or swimming, get to land and
find shelter immediately!
Stay Informed About The Storm
by listening to NOAA Weather Radio,
commercial radio, and television for the latest severe thunderstorm WATCHES and
When conditions are favorable for severe weather
to develop, a severe thunderstorm WATCH is issued.
Weather Service personnel use information from
weather radar, satellite, lightning detection, spotters, and other sources to
issue severe thunderstorm 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, which are 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 that automatically
alerts you when a watch or warning is issued.
What to Listen For...
SEVERE THUNDERSTORM WATCH: tells
you when and where severe thunderstorms are more likely to occur. Watch the sky
and stay tuned to know when warnings are issued. Watches are intended to
heighten public awareness and should not be confused with warnings.
SEVERE THUNDERSTORM WARNING:
issued when severe weather has been reported by spotters or indicated by radar.
Warnings indicate imminent danger to life and property to those in the path of
the storm. Also listen for Tornado Watch or Warning and Flash Flood Watch or
Also...listen for Tornado Watch or Warning and
Flash Flood Watch or Warning.