Thunderstorms are very common and affect great numbers of people each year. Despite their small size in comparison to hurricanes and winter storms, all thunderstorms are dangerous. Every thunderstorm produces lightning. Other associated dangers of thunderstorms include tornadoes, strong winds, hail, and flash flooding. Flash flooding is responsible for more fatalities— more than 140 annually— than any other thunderstorm-associated hazard.
Some thunderstorms do not produce rain that reaches the ground. These are generically referred to as dry thunderstorms and are most prevalent in the western United States. Known to spawn wildfires, these storms occur when there is a large layer of dry air between the base of the cloud and the ground. The falling raindrops evaporate, but lightning can still reach the ground.
What to do before thunderstorms approach
1. Know the terms used by weather forecasters:
• Severe Thunderstorm Watch— Tells you when and where severe thunderstorms are likely to occur. Watch the sky and stay tuned to radio or television to know when warnings are issued.
• 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.
2. Know thunderstorm facts:
• Thunderstorms may occur singly, in clusters, or in lines.
• Some of the most severe weather occurs when a single thunderstorm affects one location for an extended time.
• Thunderstorms typically produce heavy rain for a brief period, anywhere from 30 minutes to an hour.
• Warm, humid conditions are very favorable for thunderstorm development.
• A typical thunderstorm is 15 miles in diameter and lasts an average of 30 minutes.
• Of the estimated 100,000 thunderstorms each year in the United States, about 10 percent are classified as severe.
• A thunderstorm is classified as severe if it produces hail at least three-quarters of an inch in diameter, has winds of 58 miles per hour or higher, or produces a tornado.
3. Know the calculation to determine how close you are to a thunderstorm:
• Count the number of seconds between a flash of lightning and the next clap of thunder. Divide this number by 5 to determine the distance to the lightning in miles.
4. Remove dead or rotting trees and branches that could fall and cause injury or damage during a severe thunderstorm.
5. When a thunderstorm approaches, secure outdoor objects that could blow away or cause damage. Shutter windows, if possible, and secure outside doors. If shutters are not available, close window blinds, shades, or curtains.