Thunder is the acoustic shock wave from the extreme heat of a lightning flash. The latest edition of Science Behind the Forecast explains how thunder is created.
This interview has been edited for brevity and clarity.
Bill Burton: It's time for us to take a look at the Science Behind the Forecast as I am joined by WAVE 3 meteorologist Tawana Andrew. Good morning, Tawana.
Tawana Andrew: Good morning. Last week and the week before, we were talking about lightning. Now we're talking about the thing that comes after lightning.
BB: We know all about negative lightning and now positive lightning. And of course, you can't have lightning without thunder. You've got to talk about them both. So that's our topic today. What do we need to know about thunder?
TA: So thunder is created by the acoustic shock wave from the extreme heat of a lightning flash. So lightning can reach temperatures of 54,000 degrees Fahrenheit. That is five times hotter than the surface of the sun.
BB: Wait. Five times hotter than the surface of the sun?
TA: Yes, that is how hot lightning can be. That is incredible to think of, and that is happening all the time in our atmosphere. So that just kind of blows your mind a little bit there. So the air surrounding a lightning strike will obviously heat up a lot in the fraction of a second, causing what's what we call explosive expansion. So the air expands so quickly due to the rapid heating, then it will compress the air ahead of it, and this will form basically a sonic boom like shockwave. And that shockwave is referred to as the thunder clap, or if you're really cool, the peal of thunder.
BB: The peal of thunder. It sounds so refined.
TA: It does. You have to poke your pinky up when you say it. So that loud, sharp, initial crack will be heard closer to the actual lightning strike due to that air rapidly contracting after the explosive expansion. Now the rumbling and rolling sound that usually follows is actually caused by the air column continuing to vibrate, or the sound of the initial thunder clap propagating and reverberating throughout the atmosphere. So the atmosphere's temperature can actually influence the sound of thunder and how far it travels. So typically, sound will move faster in warm air, and that could lead to thunder being heard up to 10 miles away in stereotypical atmospheric conditions.
Now let's say we have an inversion, and that is when air temperatures actually increase instead of decreasing as you increase with altitude, and with that inversion in place, it basically keeps the sound waves deflecting towards the ground, amplifying the sound of thunder. And this would be more common in winter, where you have these thunderstorms forming in that layer of warm air above the cooler air closer to the ground.
So an interesting thing with that kind of scenario is that if lightning remains within a cloud, which we know that most lightning actually happens within a cloud. So if lightning remains within the cloud above the inversion, then so will most of the thunder's sound. So you would see the lightning, but not really hear much of the thunder in that scenario. Now, if we have a cloud to ground lightning strike, then the inversion will basically be like a lid on the atmosphere, and that inversion will cause the thunder to rumble a little bit more, and that sound will be much louder because it's trapped closer to the ground. So that's why sometimes that thunder can seem louder in the winter months than the summer months.
And while lightning and thunder form simultaneously, lightning will always be seen before thunder is heard, because the speed of light is 671 million miles per hour. While the speed of sound is just 760 miles per hour.
BB: It's just poking along.
TA: It's moving a little bit slower.
