heat lightning

Meanings

1. Lightning and/or thunder which the observer attributes to heat rather than a storm (e.g. because the storm/rain is too distant to be seen, or because there is no rain, as with dry lighting).
2. Visible lightning that occurs too far away for the resulting thunder to be audible by the observer.
3. Thunder that is heard without lightning being seen by the observer; heat thunder.

Etymology

So called because it is common on hot summer nights and is sometimes popularly thought to be caused by heat rather storms.