Why does heat lightning differ from real lightning?
Alice Johnston, Winthrop
Other than distance from an observer, there is no difference between the two. “Heat lightning” is lightning generated by a thunderstorm so distant that its light has outrun the much slower moving and more easily disrupted sound waves from the storm. (Thunder rarely carries more than 15 miles.) Heat lightning is often observed on hot evenings or nights during which airborne dust or pollutants bend the light emitted from lightning flashes. This bending can give heat lightning an orangish cast.
It’s a myth that heat lightning is a unique form of “thunderless” lightning. The fact is, all lightning produces thunder. In the case of “heat lightning,” it is merely the distance at which the storm’s lightning is being viewed that prevents us from hearing the crash of its thunder.