Thunderstorm Fact Sheet
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.
The National Weather Service considers a thunderstorm severe if it produces hail at least ¾-inch in diameter, wind 58 m.p.h. or higher or tornadoes.
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 Warning.
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 130 m.p.h.!
- One type of straight-line wind, the downburst, can cause damage equivalent to a strong tornado and can be extremely dangerous to aviation.
Large Hail
- Causes nearly $1 billion in damage to property and crops annually in the United States.
Tornadoes
- Nature's most violent storms.
- Winds can exceed 300 m.p.h.
- 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.
Information adapted from: A Preparedness Guide. A joint publication of NOAA, FEMA and The American Red Cross. January 1994.
