Tornadoes and Severe Thunderstorms

Lightning

Each year in the United States, there are about 25 million lightning flashes, and lightning remains one of the top three thunderstorm-related killers, along with floods and tornadoes. On average, some 54 people are killed by lightning each year, and hundreds are permanently injured with debilitating disorders. Approximately 70 percent of all injuries and deaths from lightning occur in the afternoon. Florida is the thunderstorm capital of the United States and leads the nation in lightning fatalities, with 463 reported deaths between 1959 and 2011, followed by Texas with 212 fatalities during the same period. Lightning can strike as far as 10 miles away from a thunderstorm, so to avoid being struck, one must remember that if thunder is audible, lightning is also close enough to strike. Lightning is also responsible for igniting wildfires that burned more than 8 million acres across the United States in 2011 and continue to threaten property and lives as the frequency of extreme drought events related to climate change increases.

Hail

Severe thunderstorms are capable of producing hail the size of a stone, dependent on the time spent in a cloud and, thus, the severity of the storm. Approximately 40 percent of severe hailstorms in the United States take place between the hours of 3:00 p.m. and 6:00 p.m. To date, the largest recorded hailstone fell in June 2010 at Vivian, South Dakota, with a diameter of 8 inches and weighing nearly 2 pounds. Cheyenne, Wyoming, is the hail capital of the United States, with an [Page 933]average of nine hailstorms a year. Although hail does not pose the same risk of injury or death as lightning, hail can cause significant property damage to automobiles, airplanes, and windows. The main threat from hail is to crops such as wheat, corn, and soybeans, with annual losses representing about 2 percent of all U.S. agricultural output.

Wind Gusts

Severe thunderstorms are capable of producing extreme wind gusts in the form of straight-line winds and downbursts. Straight-line winds can reach speeds of 100 mph and extend for hundreds of miles. Unlike tornadoes that scatter debris in a rotational fashion, straight-line winds leave a forward path of destruction. A rare super derecho (Spanish for straight-ahead) raged for 12 hours over an area from Indiana to the mid-Atlantic coast in July 2012, leaving behind 700 miles of devastation as the SPC received some 600 reports of damaging winds, and wind gusts as high as 91 mph were recorded. Downbursts are localized downdrafts that exit the base of a severe thunderstorm, striking the ground and spreading out horizontally. If the diameter is less than 2.5 miles, it is referred to as a microburst. Microburst wind speeds can reach 160 mph and pose the greatest threat to aircraft, as was the case with Pan Am Flight 759, which encountered a microburst on take off and crashed, killing 153 people in 1982.

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