Dictionary Definition

The booming sound produced by rapidly expanding air along the path of the electrical discharge of lightning.

Formation

Explanation

Thunder is the product of lightning. When lightning strikes, it rears a path through the air. The intense heat makes the nearby particles of air spread out. The heated particles collide with others, which collide with other particles, and so on. Through these collision, a wave of sound energy speeds out from the lightning. You hear the thunder after you have seen a flash of lightning because sound travels slower. If lightning strikes near you from a cloud one mile high, the first sound comes from the end of the stroke, perhaps only one hundred yards away. Then, the sound waves gradually reach you from the higher parts of the flash. Several seconds after the first sound, those from the top of the lightning stroke reach you. Also, the thunder may have rolling and rumbling effects because of the irregular path of the lightning. Echoes from hills or building can also lengthen the sound of the thunder.

Experiment #1: Sound

What You'll Need:

• paper lunch bag

Directions:

1. Fill the bag by blowing into it.
2. Twist the open end and hold it closed with your hand.
3. Hit the bag with your free hand, hard and quick.
4. Listen to the sound you produce.

What's Happening?

When you hit the bag, the air inside compresses so quickly it breaks the bag. The air rushing out of the broken bag pushes the air outside the bag away from the bag, continuing to move in a wave motion. When this moving air reaches your ear, you hear the sound. Thunder is also a result of moving air. When lightning strikes, energy is given off that heats the air through which it passes. This heated air quickly expands to produce energetic waves of air.

Types

The thunder you hear may change sound because you are hearing it far away or close by. Here are two versions of thunder.

• Thunderclap: This thunder is from nearby lightning sounds, and is loud and sharp. If it is far away, you may hear only a distant booming sound.
• Loud Thunder: This is a rolling, rumbling kind of thunder that is sometimes caused by a thunderclap that is echoing across the countryside.

Sequence

Explanation

Lightning arrives before thunder because light travels faster than sound. Light travels at 186,000 miles (300,000 km) per second, while sound travels at about 1100 feet (330 meters) per second. So, the first sounds of the thunder reach your ears several seconds later. If the thunderstorm is one mile away, it takes the thunder five more seconds to get to you. The same thing happens for a thunderstorm that is three miles away; it takes the thunder fifteen seconds longer to reach you. Look below for an experiment on this.

Experiment #2: Where is the Storm?

What You'll Need:

• thunderstorm
• ears

Directions:

1. Count the seconds between the lightning and the thunder of a storm.
2. Divide that number by three to find the distance in kilometers, or
3. Divide by five to find the distance in miles.

What's Happening?

You see lightning before you hear the thunder because light travels faster than sound. So, use the experiment above to find out how far away the storm is. The sooner the thunder arrives, the closer the storm is. You can also tell if the storm is coming towards you or heading away from you by comparing the distance it is each time you count between the lightning and thunder. If the distance is decreasing, the storm is heading towards you. Also, if the storm is extremely close you may want to find a safe place in case it gets closer.