Explosive thunderstorms: Why are they so loud?


The intense booms and menacing rumbles all come down to lightning

Lightning strikes over Rose Valley, WA on Aug. 29, 2019. (Courtesy: Brad Pearson)

PORTLAND, Ore. (KOIN) — Powerful thunderstorms shook the Pacific Northwest early Thursday morning. Chances are, your heart leaped, you held your breath for a second, then raced to look outside.

We know that lightning causes thunder, but why do some storms come with a KABOOM, and others just a rumble? What causes that sound in the first place?

During a thunderstorm, opposing electrical charges build up. This occurs between the ground and the storm cloud, within the storm cloud itself, or sometimes between two separate clouds. 

Lightning is the electrical discharge that occurs when a pathway between these two opposing charges is formed. The intense amount of energy flowing through a lightning bolt can cause it to reach a temperature of 50,000 degrees Fahrenheit (5 times hotter than the surface of the sun!).

This extreme temperature increase occurs in a fraction of a second. The air around the lightning expands rapidly, causing compressions in the surrounding air. 

These compressions are the sound waves sent out by the lighting bolt which we know as thunder. Start counting as soon as you see lighting. You are approximately one mile away if you hear the thunder in 5 seconds.

So why do we sometimes hear a faint rumble and other times a loud clap? First, sound travels from its source like a ripple in a pond when you throw a rock. As you get farther from the source, the ripple will eventually disappear. Ten miles is typically the maximum distance thunder will travel, though greater distances are possible. 

There are many things that can interfere with the sound wave as it travels including trees, buildings, sound waves from other sources, wind and even the clouds themselves. All of these things can dampen the sound of the thunder as it approaches you. 

Secondly, and this is important in regards to the rumbling we hear, the soundwave doesn’t propagate from a single point. The sound emanates from the entirety of the lighting bolt from start to end which is measured in units of miles. A rumble will start when the sound from the point closest arrives and will continue until the sound from the farthest point passes.

Our proximity to the lighting and its shape will determine how loud the thunder is and how long it lasts.

So the next time your ears perk up at the sound of thunder, you will understand some of the science behind the rumbles and kabooms of thunder. 

The most important thing to remember about thunder, however, is that if you can hear it, you are close enough to be struck by the lightning. So be aware and observe this awesome natural phenomenon from a safe location.

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