What causes thunder?

Thunder occurs when a flash of lightning heats the air in its path.  The heated air expands, for wing a sound wave which we hear as thunder.  What makes the loud clap of thunder that often follows a flash of lightning?  

Lightning is a big electrical spark.  During a thunderstorm, electrical charges are built up in the clouds.  If the charges become great enough, a flash of lightning occurs. As the lightning bolt jumps across the sky, it quickly heats the air in its path.  

The flash comes from a hot glowing gas in the lightning channel.  The heated air quickly expands outward as violently as if there had been an explosion.  This causes a great wave of air (or sound wave) which we hear as thunder.

Each part of the zigzag bolt causes an air wave, so you often hear, thunder as a short, sharp clap followed by a series of rumbles set up by the part of the lightning bold that is farthest away.  Thunder crackles when lightning forks out into many branches.  It takes about five seconds for the sound of thunder to travel one mile. – Dick Rogers