National Lightning Safety Council

The terminology "heat lightning" is commonly used to describe lightning from a distant thunderstorm that is just too far away to see the actual cloud-to-ground flash or to hear the accompanying thunder.

While many people incorrectly think that "heat lightning" is a specific type of lightning, it is simply the light produced by a distant thunderstorm.

Often, mountains, hills, trees, or just the curvature of the earth prevent the observer from seeing the actual lightning flash. Instead, the faint flash seen by the observer is light being reflected off higher-level clouds.

Also, for a distant thunderstorm, the sound of thunder can't be heard since thunder can only be heard for about 10 miles from a flash.

However, heat lightning may be a sign of an approaching thunderstorm and if you are some distance from safety, it might be a good time to head toward a safe place.