What Causes Thunder?
Perhaps one of the most exciting aspects of the warmer seasons is being able to experience the fierceness of a thunderstorm, watching lightning turn the night into day and being humbled by the roar of nature as thunder echoes in the air. What is it about a storm that produces thunder?
Sound waves
First, it is important to understand how sound works. In order for sound to travel, there has to be a medium for it to pass through. The medium that we're all used to is the atmosphere. While it is basically transparent as far as we are concerned, it's actually made up of billions of free-floating atoms (mostly nitrogen and oxygen).
What we perceive as sound is actually waves traveling through the air just as a pebble dropped in a lake sends waves through the water. A small vibration will excite surrounding air molecules, which then excite the ones surrounding them, and so on as the energy dissipates into the distance. When those air molecules transfer the energy to your ear drum, it is translated into a corresponding sound by the brain.
Where there's thunder, there's lightning
You may notice that during a thunderstorm you can always see lightning. This is no coincidence; in fact, if lightning were not present then you wouldn't be hearing thunder at all. This is because thunder is the direct result of a lightning strike. As a bolt shoots through the atmosphere, it superheats the surrounding air molecules, sending out massive shockwaves of energy in all directions.
Like other sound waves, these vibrations travel through the air, exciting molecules all the way down the line until they reach your ear. Again, the air molecules transfer the energy to your ear drum and get translated by your brain into the sound that we call thunder.
Distance matters
Anyone who has sat through a thunderstorm has surely heard thunder of different pitches in sound, with the loudest being when the storm is practically overhead, and quieter as the storm travels off into the distance. As is described in The science of the neighbor's music, low-pitched sounds travel further than high-frequency ones which dissipate their energy much quicker. When you are close to a lightning strike, the energy your ear receives contains a whole mix of the high- and low-frequency vibrations, causing a louder sound. If lightning strikes miles away, though, most of the high-energy waves have dissipated and are no longer traveling through the air. But the low-frequency sounds carry on, and when they reach your ear you don't hear a loud "crack!", but a low rumble.
What about heat lightning?
Heat lightning is not a special type of lightning, but rather it is simply lightning that's striking far away on the horizon in a distant storm. It's lightning that you can see, but not hear. It got its name to differentiate it from lightning that produces audible thunder and also because it is usually witnessed in the hot seasons, the light being reflected off clouds.
The only reason you do not hear thunder from these strikes is that because of their distance, even the low-frequency waves from the initial jolt have dissipated before reaching your ears.
