Heat lightning...it's something we have always heard about growing up. But, it's actually a myth! Many people think oppressive heat causes lightning to develop, but heat lightning is actually a thunderstorm happening a significant distance from an observer, who is only able to see the flash and not hear the thunder.
Thunder travels much more slowly than light, and it's unlikely to hear thunder from a distance greater than 10 miles.
However, because light travels much faster than sounds of thunder, it is possible to see lightning from thunderstorms over 100 miles away.