Thunderstorms pose a safety hazard in several ways. Lightning is the most dangerous part of a thunderstorm, and it kills more people than hurricanes and tornadoes combined. Thunderstorms produce heavy rain, which can cause flash flooding. The strong winds from a thunderstorm can also cause widespread damage. Each of these components requires specific precautions to minimize their danger.
A thunderstorm typically has a diameter of 15 miles and lasts for 30 minutes, which is much smaller than a hurricane or winter storm. The National Weather Service places specific criteria on a severe thunderstorm. These include thunderstorms that produce hail with a diameter of at least ¾ inch, wind gusts of at least 58 mph or a tornado. Severe thunderstorms comprise about 10 percent of all thunderstorms.
Lightning from a thunderstorm can strike as far as 10 miles away in from any rain. This generally means that you are at risk from lightning. Lightning travels much faster than sound, which allows you to estimate your distance from the lightning. You can do this by measuring the number of seconds between the lightning and thunder. Divide this result by five to obtain the distance to the lightning in miles.
The NWS recommends that you stay indoors for 30 minutes after the last clap of thunder when you hear thunder less than 30 seconds after the lightning. This rule is known as the 30/30 lightning safety rule.
You may need to take emergency shelter in a thunderstorm if you don’t have time to get back to your main residence before the thunderstorm hits. A shelter should generally be a sturdy structure that is closed on the top. It should also have some type of wiring or plumbing through the structure to ground it from the lightning. A vehicle with a hard top is better than no shelter at all.
Avoid taking emergency shelter in structures that are vulnerable to high winds, such as a mobile home. Outdoors shelters such as sheds, pavilions and golf’s typically will not protect you from flying debris and lightning. Avoid and report downed power lines. Bring pets and livestock under shelter to protect them during hailstorms.
You must also take additional precautions once you are safely indoors. Unplug any electrical appliances that you don’t need immediately since lightning can severely damage electrical appliances when they are plugged in. Refrain from activities that involve running water, such as taking a bath or shower. Running water can provide a conduit for lightning. Draw the blinds to protect yourself from broken windows.