As we crawl through this, our second day of True Summer (not that fake astronomical stuff), many people who haven’t had thunderstorms yet this year are in for a flashing, crashing, startling treat. What exactly is it about lightning that makes that thunderous noise, and why does it seem to crackle, boom, and roll?

Lightning is one of the first bits of science they teach us in elementary school—these large-scale discharges of static electricity form in much the same way that you can torture someone in your living room on a dry day.

The ice crystals high in a thunderstorm and surface of the Earth both tend to take on a positive charge during a storm, while the base of the storm takes on a negative charge. The buildup between the regions of positive and negative charges can result in an explosive discharge we see and hear as lightning, occurring more than a billion times around the world every year.

This enormous electrostatic discharge is gorgeous, but it’s nothing to play around with. Not only is lightning dangerous because it’s a powerful jolt of electricity (duh), it’s also five times hotter than the surface of the sun—the temperature of lightning frequently clocks in at more than 50,000°F, while the surface of the sun is relatively chilly, sitting at around 10,000°F. Air expands when it’s heated, so when lightning flashes through the sky, the layer of air immediately around the bolt rapidly expands as it’s heated by tens of thousands of degrees in less than a second.

If you’ve ever watched one of the dozens of episodes where the Mythbusters blow stuff up, you know that the force and deafening sound of the explosion comes from the shockwave that emanates from the center of the blast. This is why we hear the loud crack of thunder after a flash of lightning—a small shockwave forms from the air expanding so rapidly in that split second the lightning existed. The closer you are to the bolt, the louder the thunder will be, and bigger shockwaves can rattle both you and your house, just like an explosion. (The above video shows a Russian Proton-M rocket exploding—the shockwave was so powerful that it shattered windows in nearby buildings.)

It makes sense that you hear the initial crack or boom of the thunder (and even the echoes of that boom off of nearby buildings and hills), but why does thunder seem to continuously crackle and roll? When you hear thunder that booms and rolls or crackles for another few seconds, what you’re hearing is actually the shockwave from the entire length of the bolt from the ground to the bolt’s source up in the clouds. You hear the blast of air closest to the surface, and the roll and crackle of the thunder fades as shockwaves from higher and higher up the bolt reach your ear holes as that soothing, summertime treat.

Oh, and if you were wondering, the old thing about judging the distance of lightning by counting the seconds between the thunder and the flash really does work. Depending on conditions like terrain, air temperature, and precipitation intensity, it takes thunder about five seconds to travel one mile. If you see a flash of lightning and count ten seconds before you hear the clap of thunder, the bolt of lightning was roughly two miles away. However, you should always do this indoors—if you can hear thunder outdoors, no matter how faint it is, you’re close enough to get struck.

[Top image by the author, taken during the June 30, 2012 derecho | Video via YouTube]

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