Ask Joe

Ask Joe | Speed of sound, air heats into thunderous rumbles

State College - Centre Daily TimesMay 24, 2014 

Q: Why do we hear such a loud sound when it thunders, and why does it rumble?

A: The sound of thunder comes from the quick expansion of air from the heat of lightning. The lightning is very hot, several times hotter than the surface of the sun.

When air heats, it expands, so when there is a bolt of hot lightning passing through a volume of air, it causes a quick expansion of that air. It’s much like noise created from popping a balloon. When the balloon breaks, the air rushes out and creates the sound you hear.

As you mentioned, the sound of thunder is different than a balloon popping, as it is often more of a rumble. This rumble is a result of several things, including the speed of sound and the echoing of the sound off objects on the ground. Sound travels at a speed of about a mile every five seconds. Light travels at a much faster speed, pretty much instantaneously for what you are seeing with lightning close enough to hear.

Therefore, if you see lightning and then count 10 seconds, you know that the lightning was about 2 miles away. A lot of the rumbling comes from the fact that the lighting bolt is long enough that you will hear the close part of the lightning first and then the farther part of the bolt. If you add in objects such as mountains or hills, an echoing of sound adds to the rumbling.

Not only does sound travel slower than light, it also doesn’t quite travel as far. In fact, even under the quietest conditions, the sound of thunder rarely makes it as farther than 7 to 10 miles from lightning. Because lightning can strike 10 miles from the rain of a thunderstorm, if you hear thunder, it’s time to take shelter from lightning by heading inside a house or a car. Remember, if you hear the roar, head indoors.

Q: Is the sun closer to Earth in the summer and farther away during winter?

A: No, actually the Earth’s closest point to the sun is during early January. The perihelion occurs around Jan. 4, when the Earth is just more than 91 million miles away from the sun. At the aphelion, which occurs July 5, the Earth is actually 94.5 million miles away from the sun.

While 3 million miles seems like a lot, it only accounts for about a 3 percent of the total distance from the sun and really is not enough to give us the seasons. Of course, we have noticed that because it is much colder in January.

What causes the seasons is more the angle of the Earth’s axis of 23.5 degrees. During the winter, the northern hemisphere is pointed away from the sun, making it lower in the sky than during the summer.

 

If you have a question about the weather, write to Joe Murgo at 5000 Sixth Ave., Altoona, PA 16602, or email him at murgo@wtajtv.com.

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