Heat Lightning - Curvature

Curvature

The Earth's curvature also contributes to distant observers not hearing the thunderclap. Thunder is more likely to be bounced off the Earth's surface before it reaches an observer far from the strike, and only the right refraction and reflection of the sound off of the atmosphere will give it range it needs to be heard far away. The reflection and refraction in the troposphere determines who hears the strike and who doesn't. More often than not, the troposphere will reflect the light, and leave out the sound - in these cases some fraction of the light emanating from distant thunderstorms (whose distant clouds may be so low to the horizon as to be essentially invisible) is scattered by the upper atmosphere and thus visible to remote observers.