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 thunderstorms. 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!

Flash Floods/Floods

• The number ONE thunderstorm killer...nearly 140 fatalities each year.
• Most flash flood deaths occur at night and when people become trapped in automobiles.

Lightning

• Occurs with ALL thunderstorms.
• Averages 93 deaths and 300 injuries each year.
• Causes several hundred million dollars in damage to property and forests annually.

Straight-line Winds

• 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 to aviation.
• During the summer in the western states, thunderstorms often produce little rain but very strong wind gusts and dust storms.

Large Hail

• 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.

Tornadoes

• 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?