What causes the “rolling effect” from thunder?
The phenomenal heating experienced by air exposed to lightning produces explosive expansion. Rather than originating from a single point, these sound waves are generated along the length of a lightning stroke. Radiating outward in all directions and traveling at roughly 1,100 feet per second, the precise speed at which the sound waves move often varies. The fading rumbling occurs as sound waves from increasingly distant stretches of the lightning column reach our ears. But other factors can be involved. Sound waves travel faster in warm air than cold. In addition, thunder may be reflected by hills or even scattered by small swirls of air. Each factor can affect the speed of the sound waves.