Thunder is caused by the rapid expansion and contraction of the air surrounding a lighting bolt. On average, a lightning bolt is about 5 times hotter than the surface of the sun, or about 50,000°F! This causes the air to become super-heated and as a result, the air rapidly expands in fractions of a second. However, the air doesn’t stay super-heated for long and quickly dissipates the heat in fractions of second.
It’s this rapid expansion and contraction of the air that creates a compression wave we hear as thunder. Contrary to what some think, thunder is not caused by the lightning bolt “breaking the sound barrier”. If this were true, then every time we turned on a house light we would hear a sonic boom!
Why is there a delay between lightning and thunder?
The speed of light is about 186,282 miles per second. The speed of sound is about 0.211 miles per second at sea level. So the light from a lightning bolt reaches us many orders of magnitude faster than the compression wave it creates.
To get a rough estimate of how far away the closest part of the lightning is to you, count the seconds from when you first saw the flash and when you heard the thunder. Divide that number by 5 and that will tell you roughly how many miles away it was. Another way to think of it is, every 5 seconds equals about 1 mile. This varies slightly with altitude, but it’s good enough for an estimate.
Why does thunder rumble?
Thunder rumbles for a few different reasons. A lightning bolt is very rarely a straight line and is never equally distant from you at all points. A lightning bolt is on average 4 miles long, zig-zags all over the place, and can have many limbs that branch out in many different directions separated by many miles. As a result, the compression waves created by each part of the lightning bolt reach you at different times. The sound wave that has traveled a greater distance will be softer and arrive later than a compression wave created by a part of the lightning bolt that was closer to you, which will be louder.
The compression waves (or the thunder) also bounce around and off the clouds, the terrain, and other objects. Much like your voice echoes in a canyon or large auditorium, so do the compression waves generated by lightning.
Lastly, lower frequencies will travel much further before they dissipate. Think of a car with a lot of bass. You can hear the low frequency bass long after the car has passed. But the higher frequencies of the engine are relatively much quieter and have a harder time going longer distances.
If you’ve ever had a lightning bolt crash down really close to you, the thunder doesn’t rumble that much and sounds more like a large explosion or a “clap of thunder”. That’s because the compression waves didn’t have a chance to bounce off many objects before you heard it, the higher frequencies didn’t have time to dissipate, and most of the compression waves pass you at the same time. However, to somebody that is further away, that same lightning bolt will sound much different.
What is heat lightning and why is there no thunder?
It’s impossible to have lightning without thunder, and it’s impossible to have thunder without lightning. Heat lightning is nothing more than seeing lightning and not hearing the thunder, simply because the storm is too far away. Lightning is pretty darn bright and can be seen from many miles away.
Thunder is a compression wave and will dissipate quickly with time and distance. Under excellent conditions, thunder can probably be heard at most about 10 miles away from the lightning strike (50 second count). However most of the time conditions aren’t this good and we usually can’t hear thunder any more than about 5 miles away (25 second count). Conversely, if you hear thunder and don’t see lightning, it’s probably because the lightning occurred inside the cloud obscuring the visibility. Especially in day time. Interestingly, about 85% of all lightning is intra-cloud, meaning, it occurs inside the cloud and never touches the ground.