What is heat lightning?

Dear Tom,

My father served on a WWII weather ship in the South Pacific and often talked about heat lightning, but my daughter who minored in meteorology said there’s no such thing. Can you clarify?

Dan Buba
Sugar Grove

Dear Dan,

Your daughter is correct. The term came into use because distant lightning was frequently observed on hot, humid summer nights when people remained outdoors well after dark.
What is actually being seen is regular thunderstorm lightning from a storm so far away that the thunder can’t be heard. Lightning sometimes occurs at the top of 50,000 foot plus thunderheads and if no clouds exist, is visible for well over 100 miles; but the sound of thunder rarely carries more than 15 miles. At large distances, the thunderhead may actually be below the horizon and the flash does not even appear to originate in a cloud.