The initial sound of thunder echoes and is deflected by clouds and obstructions on the ground. This means that most often, a single thunder clap arrives at your ear having been scattered by these objects, often over many miles. The result is what's commonly referred to as rolling thunder, or the sound of the thunder clap arriving gradually, in waves. An exception is when a lightning bolt strikes close by and has next to nothing to deflect its sound. In this case, you hear the loud crash, almost without warning, followed by the rolling sounds of the thunder as the sound waves travel away from you, but bounce back off of trees, hills and clouds.
---David Murphy
