The United States has about 100,000 thunderstorms annually. It's estimated that at any given moment nearly 2,000 thunderstorms are in progress over the earth's surface and that lightning strikes the earth 100 times each second. A thunderstorm may extend several miles across its base and often towers to altitudes of 40,000 feet or more. Some thunderstorm clouds stand alone while others can be seen in groups. These groups are known as a squall line.
Lightning always accompanies a thunderstorm.
Electricity produced in a thunderstorm can be as strong as 100 million volts. Lightning strikes before it thunders. Thunder is the sound of air suddenly expanding due to the heat of the lightning. When lightning is near, thunder sounds like a sharp crack. Thunder from distant lightning sounds like groaning and rumbling by the time it reaches you.