What is heat lightning?

Dear Tom,
What is heat lightning? I see it occasionally in the summer.
— Mala Yarko
Dear Mala,
Your question comes up frequently, so let’s revisit the subject. There is no such thing as “heat lightning.” What you are seeing is normal lightning, but it is so far from you that you cannot hear the thunder it generates.
Lightning flashes sometimes occur at the tops of thunderheads, at elevations of 50,000 feet or higher. If no clouds exist between you and the flash, the lightning can be seen at distances of 100 miles or greater. The sound of thunder, however, rarely carries more than 15 miles. That means we can often see the lightning flash but not hear its thunder.
At a distance of 100 miles, the thunderhead may be at or below your horizon and it will not be apparent that the flash you are seeing actually originates in a cloud, but it surely does.
There is no such thing as “heat lightning.”