The reason why lightning is always accompanied by thunder is because the thunder is caused by the bolt of electricity produced in a lightning strike. The deep rumbling and sharp cracks of thunder are produced as the air around the lightning bolt is super heated—up to 60,000 degrees Fahrenheit (33,315 C)—and rapidly expands. As the air cools and contracts, it creates a shock wave which manifests itself as thunder. The closer the lightning is, the louder the clap of thunder will be.

Before science could provide an explanation based on scientific data in the 20th century, the cause of thunder was undetermined, and a matter of much dispute. Early Greeks believed it was caused by clouds colliding. Other theories include vacuums, exploding gases and steam. Thor, a god of Norse mythology is named for the Old Norse word for thunder, and is nearly always portrayed wielding a thunder clapping hammer.

Over 100,000 thunderstorms are recorded every year but only a small percentage is considered “severe” by the National Weather Service. Thunderstorms are capable of producing dangerous lightning, tornados, strong winds, hail and heavy rain, which can lead to flash flooding. Although thunder can be frightening for some, and a few people actually suffer from astraphobia, the fear of thunder, it is helpful to regard it as a warning that lightning is near.

The following tips are recommended if you hear a thunderstorm approaching:

• If outside, take cover in a building or inside a car. If this is not possible, find a clear area and squat as close to the ground as possible. Never seek shelter under trees or near power lines, telephone lines, or fences. Avoid contact with metal items such as golf clubs and bicycles as these can attract the lightning.
• If inside, stay off the telephone and away from electrical appliances. Avoid bathing or doing dishes as metal pipes can serve as conduits for the lightning’s electrical charge.
• If in a car, pull to the side of the road with your emergency lights on, until the storm subsides. Be sure not to park beneath trees or other tall structures.

A weather radio should be a permanent fixture in your home, especially if you live in an area with severe weather. These inexpensive radios are available with a battery backup, and will instantly turn on and notify you of severe weather when activated by the local weather service. It is recommended you have one both in your home and in your vehicles.

You can determine how far away the lightning is from your location by counting the seconds from the lightning strike to the first rumble of thunder, then dividing by five. The resulting number is the distance in miles the storm is from you. For kilometers, divide the number of seconds by three.