One of the best ways to spend a summer evening is to stand outside and watch a distant storm, the soft rumbles of thunder distracting you from the mosquitoes eating you alive. The most well-known part of these summertime thunderstorms is a phenomenon known as “heat lightning,” which doesn’t really exist.
Heat lightning is a term mostly used in the south, where nighttime thunderstorms are commonplace in the warm months. The term refers to distant, sky-filling flashes of lightning on the dark horizon that often seem to form from nothing—residents confidently explain to the uninitiated that the lightning forms from the warm, muggy air itself instead of a thunderstorm, hence the name.
The concept of heat lightning is a relic of a time when there wasn’t much widespread knowledge of how different weather events occurred. This era is also the one that brought us wildly unhelpful ideas such as “open all of your windows during a tornado” and one even worse, “lightning strike victims are still electrified after being struck, so you should just let ‘em die instead of reviving them.”
Lightning is produced by the discharge of static electricity that builds-up in and around a thunderstorm due to precipitation and ice crystals moving around inside. The process is similar to rubbing your feet across the carpet on a dry January day and vindictively shocking someone (who totally deserved it, I’m sure).
These charges can build up in such a way that we have three main types of lightning: cloud-to-ground, cloud-to-cloud, and intra-cloud, which is lightning that happens within the cloud itself. Some variations include anvil crawlers, bolts from the blue, ball lightning, and sprites (which are something else entirely), but the phenomenon commonly known as “heat lightning” encompasses almost all of the above.
Your average bolt of lightning is five times hotter than the surface of the sun, so when there’s a flash of lightning, the column of air immediately around the bolt gets incredibly hot very quickly. The shockwave that results from the rapid expansion of air is the thunder we hear, the sound of which moves just a little bit slower (by about 882,000 times) than the speed of the light emitted by the flash itself.
Under ideal conditions, it really does take about five seconds for the sound of thunder to reach you if lightning strikes a mile away, and the farther away the lightning strikes, the longer it’ll take for the thunder to reach you. If you take into account limiting factors like heavy rain, hail, wind direction, terrain, and your own hearing ability, lightning that’s even just a couple of miles away could produce thunder that’s inaudible to you.
The event known as “heat lightning” is just regular lightning produced by a thunderstorm off in the distance, too far away for you to hear the thunder and often too dark to see the clouds that make up the storm itself. On hazy nights, lightning dozens of miles away can illuminate the entire sky, giving rise to the idea that lightning can just spontaneously form from the muggy ick that hugs the ground on a summer night.
The gif at the top of this post shows what “heat lightning” looks like with just a little bit of twilight left to illuminate the clouds. If it were completely dark (and the storm were a few miles farther down on the horizon), it would be a classic example of what people are referring to when they talk about this phenomenon.
Heat lightning doesn’t exist. People can use the term all they want—I’m not here to tell you to stop calling it that—but it always helps to understand that there’s not some mystical phenomenon occurring on a warm night when the sky lights up and there’s not a drop of rain to be had or a clap of thunder to be heard.