heat lightning

lightning seen near the horizon, esp. on hot evenings, and thought to be reflections of lightning on clouds below the horizon: its thunder is too distant to hear

See heat lightning in American Heritage Dictionary 4

noun
Intermittent flashes of light near the horizon, usually seen on a hot summer evening, unaccompanied by thunder and thought to be cloud reflections of distant lightning.

Learn more about heat lightning

link/cite print suggestion box