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05/01/2001
A Preparedness Guide to Nature’s Most Violent Storms
Editor’s note: As summer approaches, the threat of violent storms is enhanced. This preparedness guide explains thunderstorms and related hazards and suggests life-saving actions you can take. With this information, you can recognize severe weather, develop a plan, and be ready to act when threatening weather approaches. Remember... your safety is up to you!
Why Talk About Thunderstorms?
They Produce...
• Tornadoes
- Cause an average of 70 fatalities and 1,500 injuries each year.
- Produce wind speeds in excess of 250 mph.
- Can be one mile wide and stay on the ground for over 50 miles.
• Lightning
- Causes an average of 80 fatalities and 300 injuries each year.
- Occurs with all thunderstorms.
• Strong Winds
- Can exceed 100 mph.
- Can cause damage equal to a tornado.
- Can be extremely dangerous to aviation.
• Flash Flooding
- Is the #1 cause of deaths associated with thunderstorms... more than 140 fatalities each year.
• Hail
- Causes more than $1 billion in crop and property damage each year.
Thunderstorms affect relatively small areas when compared with hurricanes and winter storms. Despite their small size, ALL thunderstorms are dangerous! The typical thunderstorm is 15 miles in diameter and lasts an average of 30 minutes. Of the estimated 100,000 thunderstorms that occur each year in the United States, about 10 percent are classified as severe.
What Are Thunderstorms? What Causes Them?
The National Weather Service considers a thunderstorm severe if it produces hail at least 3/4-inch in diameter, winds of 58 mph or stronger, or a tornado.
Every thunderstorm needs:
• Moisture - to form clouds and rain.
• Unstable air - warm air that can rise rapidly.
• Lift - cold or warm fronts, sea breezes, mountains, or the sun’s heat are capable of lifting air to help form thunderstorms.
Life Cycle of a Thunderstorm
Developing Stage
• Towering cumulus cloud indicates rising air.
• Usually little if any rain during this stage.
• Lasts about 10 minutes.
• Occasional lightning.
Mature Stage
• Most likely time for hail, heavy rain, frequent lightning, strong winds, and tornadoes.
• Storm occasionally has a black or dark green appearance.
• Lasts an average of 10 to 20 minutes but may last much longer in some storms.
Dissipating Stage
• Rainfall decreases in intensity.
• Can still produce a burst of strong winds.
• Lightning remains a danger.
Tornadoes
Although tornadoes occur in many parts of the world, they are found most frequently in the United States. In an average year, 1,200 tornadoes cause 70 fatalities and 1,500 injuries nationwide. You can find statistical information on tornadoes at www.spc.noaa.gov.
How 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 tornadoes form within this area of strong rotation.
Tornadoes Take Many Shapes and Sizes
Weak Tornadoes
• 88% of all tornadoes
• Less than 5% of tornado deaths
• Lifetime 1 - 10+ minutes
• Winds less than 110 mph
Strong Tornadoes
• 11% of all tornadoes
• Nearly 30% of all tornado deaths
• May last 20 minutes or longer
• Winds 110-205 mph
Violent Tornadoes
• Less than 1% of all tornadoes
• 70% of all tornado deaths
• Lifetime can exceed 1 hour
• Winds greater than 205 mph
Weather Radar Watches the Sky
The National Weather Service has strategically located Doppler radars across the country that can detect air movement toward or away from a radar. Early detection of increasing rotation aloft within a thunderstorm can allow life-saving warnings to be issued before the tornado forms.
How Lightning Forms
Lightning results from the buildup and discharge of electrical energy between positively and negatively charged areas. Rising and descending air within a thunderstorm separates these positive and negative charges. Water and ice particles also affect charge distribution.
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 clouds and produces the visible lightning strike.
Lightning Facts
• Lightning causes an average of 80 fatalities and 300 injuries each year.
• Lightning occurs in all thunderstorms; each year lightning strikes the Earth 20 million times.
• The energy from one lightning flash could light a light bulb for more than 3 months.
• Most lightning fatalities and injuries occur when people are caught outdoors in the summer months during the afternoon and evening.
• Lightning can occur from cloud-to-cloud, within a cloud, cloud-to-ground, or cloud-to-air.
• Many fires in the western United States and Alaska are started by lightning.
• The air near a lightning strike is heated to 50,000˚ F - hotter than the surface of the sun! The rapid heating and cooling of the air near the lightning channel causes a shock wave that results in thunder.
In recent years, people have been killed by lightning while:
• boating
• standing under a tree
• riding a horse
• swimming
• riding on a lawnmower
• playing soccer
• golfing
• talking on the telephone
• fishing in a boat
• bike riding
• loading a truck
• mountain climbing
30/30 Lightning Safety Rule
Go indoors if, after seeing lightning, you cannot count to 30 before hearing thunder. Stay indoors for 30 minutes after hearing the last clap of thunder.
Straight-line Winds
Straight-line winds are responsible for most thunderstorm wind damage. Winds can exceed 100 mph! One type of straight-line wind, the downburst, is a small area of rapidly descending air beneath a thunderstorm. A downburst can cause damage equivalent to a strong tornado and can be extremely dangerous to aviation. A “dry microburst” is a downburst that occurs with little or no rain. These destructive winds are most common in the western United States.
Flash Floods
Flash floods and floods are the #1 cause of deaths associated with thunderstorms...more than 140 fatalities each year.
Most flash flood fatalities occur at night and most victims are people who become trapped in automobiles.
Six inches of fast-moving water can knock you off your feet; a depth of two feet will cause most vehicles to float.
Large Hail
Large hail causes more than $1 billion in damage to property and crops each year. Large stones fall at speeds faster than 100 mph. Strong rising currents of air within a storm, called updrafts, carry water droplets to a height where freezing occurs. Ice particles grow in size, becoming too heavy to be supported by the updraft, and fall to the ground.
Who’s Most At Risk from Thunderstorms?
From Lightning - People who are outdoors, especially under or near tall trees; in or on water; or on or near hilltops.
From Flash Flooding - People who walk or drive through flood waters.
From Tornadoes - People who are in mobile homes and automobiles.
Be Prepared...It’s Up To You!
Each year, many people are killed or seriously injured by tornadoes and severe thunderstorms despite advance warning. Some did not hear the warning, while others heard the warning but did not believe it would happen to them. The following preparedness information, combined with timely severe weather watches and warnings, could save your life. Once you receive a warning or observe 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.
What You Can Do Before Severe Weather Strikes
• Develop a plan for you and your family at home, work, school, and when outdoors. The American Red Cross offers planning tips on their Internet site: www.redcross.org/disaster/safety/.
• Identify a safe place to take shelter. Information on how to build a Safe Room in your home or school is available from the Federal Emergency Management Agency at www.fema.gov/mit.
• Have frequent drills.
• Know the county/parish in which you live, visit, or work. The National Weather Service issues severe weather warnings on a county or parish basis.
• 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.
• National Weather Service watches and warnings are also available on the Internet. Select your local National Weather Service office at www.wrh.noaa.gov/wrhq/nwspage.html or go to the National Weather Service Home Page at www.nws.noaa.gov.
• Listen to radio and television for weather information.
• Check the weather forecast before leaving for extended periods outdoors. Watch for signs of approaching storms.
• If severe weather threatens, check on people who are elderly, very young, or physically or mentally disabled.
What You Can Do When Threatening Weather Approaches
Lightning Safety Rules
• Postpone outdoor activities if thunderstorms are imminent. This is your best way to avoid being caught in a dangerous situation.
• Move to a sturdy building or car. Do not take shelter in small sheds, under isolated trees, or in convertible automobiles. Stay away from tall objects such as towers, fences, telephone poles, and power lines.
• If lightning is occurring and a sturdy shelter is not available, get inside a hard top automobile and keep the windows up. Avoid touching any metal.
• Utility 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 emergency.
• Do not take a bath or shower during a thunderstorm.
• Turn off air conditioners. Power surges from lightning can cause serious damage.
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 shorter trees.
• 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 over your ears and your head between your knees. Make yourself the smallest target possible and minimize your contact with the ground. DO NOT lie down.
• If you are boating or swimming, get to land and find shelter immediately!
Tornado Safety Rules
• In a home or building, move to a pre-designated shelter, such as a basement.
• If an underground shelter is not available, move to a small interior room or hallway on the lowest floor and get under a sturdy piece of furniture. Put as many walls as possible between you and the outside.
• Stay away from windows.
• Get out of automobiles.
• Do not try to outrun a tornado in your car; instead, leave it immediately for safe shelter.
• If caught outside or in a vehicle, lie flat in a nearby ditch or depression and cover your head with your hands.
• Be aware of flying debris. Flying debris from tornadoes causes most fatalities and injuries.
• Mobile homes, even if tied down, offer little protection from tornadoes. You should leave a mobile home and go to the lowest floor of a sturdy nearby building or a storm shelter.
Flash Flood Safety Rules
• Avoid walking, swimming, or driving in flood waters.
• Stay away from high water, storm drains, ditches, ravines, or culverts. If it is moving swiftly, even water six inches deep can knock you off your feet.
• If you come upon flood waters, stop, turn around, and go another way. Climb to higher ground.
• Do not let children play near storm drains.
For More Information
Contact your local National Weather Service office, American Red Cross chapter, or the Federal Emergency Management Agency for a variety of weather-related brochures. Specific information on flash flooding can be found in the “Flash Floods and Floods...The Awesome Power” brochure (NOAA PA 92050). Brochures can be viewed and downloaded from the Internet at www.nws.noaa.gov/om/nwspub.htm. Preparedness information can be obtained from the Federal Emergency Management Agency at www.fema.gov/pte/prep.htm, and the American Red Cross address at www.redcross.org/disaster/ safety/. Frequently asked questions about tornadoes can be found on the Internet at www.spc.noaa.gov/faq/tornado/index.html.
Check with your local National Weather Service office or visit the Internet site www.nws.noaa.gov/nwr/ to determine if your county is covered by NOAA Weather Radio. National Weather Service watches and warnings are also available on the Internet by selecting your local National Weather Service office at www.wrh.noaa.gov/wrhq/nwspage.html or by going to the National Weather Service Home Page at www.nws.noaa.gov.
Sources: U.S. Department of Commerce, National Oceanic and Atmospheric Administration, and the National Weather Service
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