The Debunker: Can Warm Summer Nights Cause “Heat Lightning”?

by Ken Jennings

Lord Almighty, I feel my temperature rising. If you live in the Northern Hemisphere, July is the beginning of the “dog days” of summer, the hottest period of the year. But you know what’s cool on a hot day? Knowledge. Grab a tall glass of lemonade, settle down in a hammock under a shady tree, and let Jeopardy! wunderkind Ken Jennings set you straight on some shamefully persistent misinformation about hot stuff.

The Debunker: Can Warm Summer Nights Cause “Heat Lightning”?

You’re sitting on your porch on a warm, humid summer night. Without warning, off on the horizon, you see flashes of lightning. After a few minutes’ pause, the lightning continues. But the whole time, you haven’t felt a drop of rain—in fact, there’s not a cloud in the sky. Even weirder, none of the lightning was accompanied by thunder! This is clearly no ordinary lightning.

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This phenomenon is colloquially called “heat lightning.” The term originated in rural America in the 1820s, but confused naturalists had been writing about heat lightning since the previous century, when it was called vespertine. In 1858, meteorologists at the Smithsonian Institution wrote that “a number of physicists…regard this phenomenon as existing by itself independently of storms,” and wondered if heat lightning might be shooting upward from electrically charged soil, or might even be a result of meteors falling to Earth.

To this day, many people have the misconception that heat lightning, unaccompanied by rain or thunder, is its own mysterious phenomenon, distinct from regular old lightning. Nope! “There’s no such thing as ‘heat lightning,’” says the Washington Post’s weather column. It’s “a meteorological myth,” reports AccuWeather. As early as 1829, The American Journal of Science and Arts conjectured that so-called heat lightning might be nothing more than “thunder storms at so great distance that not only the sound of the thunder cannot possible reach the ear, but that the curvature of the earth conceals that clouds.” With Doppler radar, we can now confirm that this is true: heat lightning is just what regular lightning looks like from over the horizon. It has nothing to do with heat—summer’s just the time when people tend to be outdoors to notice nearby thunderstorms.

Quick Quiz: What movie franchise, with over $1 billion in worldwide grosses, follows the adventures of a character named Lightning McQueen?

Ken Jennings is the author of six books, most recently his Junior Genius Guides, Because I Said So!, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at ken-jennings.com or on Twitter as @KenJennings.