What causes thunder?
Listed below are some interesting facts about thunder. For starters, 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 around the lightning bolt 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 light in the house 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 first hear the thunder. Divide that number by 5 and that will tell you about 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 pretty much the same.
Why does thunder rumble?
Thunder rumbles for a few different reasons. The first being, 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 second thing is the compression waves (or the thunder) will 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.
The other reason thunder rumbles is because higher frequencies tend to dissipate more quickly, unlike the lower frequencies that can travel much further before they dissipate. Think of a car with a lot of bass. You can hear the base long after the car has passed, even when the windows are up and doors closed. But the higher frequencies are 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. There’s really not all the much rumbling. 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?
First off, it’s impossible to have lightning without thunder and it’s impossible to have thunder without lightning. Heat lightning is nothing more than one seeing lightning and not hearing the thunder, because the storm is too far away. Lightning is pretty darn bright and can be seen for tens of miles. Thunder is a compression wave and will dissipate with time and distance. Under excellent conditions, thunder can be heard about 12 miles away from the lightning strike. However most of the time conditions aren’t this good and we usually can’t hear thunder any more than about 5 miles away. Conversely, if you hear thunder and don’t see lightning, it’s probably because the lightning occurred inside the cloud and was obscured by the cloud itself, or rain. On average, about 85% of all lightning is intra-cloud, that is to say, it occurs inside the cloud and never touches the ground.