What is heat lightning?

Heat lightning is ordinary lightning from a thunderstorm too far away for its thunder to reach you. Hot summer nights do not create a special kind of lightning; the silent flashes mean a real storm is out there, possibly moving your way.

What's actually true

"Heat lightning" is just lightning from a storm too far to hear. There is no special kind of lightning caused by hot summer nights. Those silent flashes on the horizon come from an ordinary thunderstorm that is simply far enough away that its thunder fades before reaching you. Often you are seeing the light bounce off high clouds, not the bolt itself.

Takeaway: Silent flashes mean a real storm is out there, and it may be heading your way.

Why people believe it

The name comes from timing, not physics. Silent flashes are most often noticed on warm summer evenings, when people are outside and storms are common, so heat looked like the cause. The missing thunder adds to the illusion: if you can see a flash but hear nothing, it is natural to assume it is a different, harmless thing. In reality thunder simply fades with distance faster than light does, and the flash may even be light reflecting off high clouds.

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