What Is a Thunderstorm?

Thunderstorms are more than just rain. Here is some information to help you understand what causes thunderstorms and what damage they can cause.

• A thunderstorm is formed from a combination of moisture, rapidly rising warm air and a force capable of lifting air such as a warm and cold front, a sea breeze, or a mountain.
• 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.
• While thunderstorms and lightning can be found throughout the United States, they are most likely to occur in the central and southern states. The state with the highest number of thunderstorm days is Florida.
• All thunderstorms are dangerous.
• Thunderstorms can bring heavy rains (which can cause flash flooding), strong winds, hail, lightning and tornadoes.
• About 10% of the thunderstorms that occur each year in the United States are classified as severe. (A thunderstorm is considered severe if it produces hail at least 3/4 inch in diameter, winds 58mph or greater, or tornadoes.)

Estimating the Distance From a Thunderstorm

Because light travels much faster than sound, lightning flashes can be seen long before the resulting thunder is heard.

1. Estimate the number of miles you are from a thunderstorm by counting the number of seconds between a flash of lightning and the next clap of thunder.
2. Divide this number by five.
3. If you can hear thunder, you are close enough to the storm to be struck by lightning.

Other Terms

Lightning

Lightning is an electrical discharge that results from the build-up of positive and negative charges within a thunderstorm. When the buildup becomes strong enough, lightning appears as a bolt. This flash of light usually occurs within the clouds or between the clouds and the ground. A bolt of lightning reaches a temperature approaching 50,000 degrees Fahrenheit in a split-second.

The rapid heating and cooling of air near the lightning is what causes thunder.

In the United States, 75 to 100 Americans are hit and killed each year by lightning. It is a myth that lightning never strikes twice in the same place. In fact, lightning will strike several times in the same place in the course of one discharge.

Heat lightning

Heat lightning is actually lightning from a thunderstorm too far away for thunder to be heard. However, the storm may be moving in your direction.

Hail

Hail forms when updrafts in thunderclouds carry raindrops upward into extremely cold areas of the atmosphere. The raindrops freeze and are bounced around in the powerful winds within thunderclouds while new layers of ice are added. Eventually, the hailstones grow too heavy to be supported by the updrafts and fall to the ground. Some hailstones are the size of peas while others can be as big as grapefruits.

Signs and Alerts

• A SEVERE THUNDERSTORM WATCH tells you when and where severe thunderstorms are more likely to occur.
• A SEVERE THUNDERSTORM WARNING is 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 a Tornado Watch or Warning and Flash Flood Watch or Warning.

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For More Information

Ready.gov/Thunderstorms and Lightning
This section has information about thunderstorms and lightning, with advice on how to be safe. Ready.gov is a national public service campaign designed to educate and empower the American people to prepare for, respond to and mitigate emergencies, including natural and man-made disasters. The goal of the campaign is to promote preparedness through public involvement.

National Weather Service–Lightning Safety Tips and Resources
Check out the National Weather Service’s Lightning Safety site to learn more safety tips, stories, activities and facts about lightning. The National Weather Service (NWS) provides weather, hydrologic, and climate forecasts and warnings for the United States, its territories, adjacent waters and ocean areas.

National Lightning Safety Institute
Read Personal Lightning Safety and Structural Lightning Safety. The National Lightning Safety Institute (NLSI) is a nonprofit, non-product advocacy of lightning safety for both people and structures.

Insurance Institute for Business & Home Safety
Click on the Resources tab for information on natural disasters and weather conditions, and what steps to take to protect yourself and your premises. The Institute for Business & Home Safety (IBHS) is a nonprofit association that engages in communication, education, engineering and research.

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