the.com/heat lightning

means Flashes of light seen on the horizon on a warm night, from a distant thunderstorm whose thunder is too far off to reach your ears.

from The name is a well-meaning misnomer from folk weather-watching — people once thought summer's heat itself could trigger silent flashes, so they named the phenomenon for what caused the sultry nights, not for its real source. There's no separate kind of lightning behind it: it's ordinary lightning from a storm beyond the horizon, its light bending and scattering through miles of atmosphere until only the flash survives the trip and the thunder dies out along the way.