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

What is a heat lightning?

Heat lighting is the flashes of light produced by distant electrical storms.  The flashes of lightning occur too far away for thunder to be heard.

Sometimes, on warm summer evenings, we see so-called “heat lightning”—silent flashes of light in the distant sky.  Heat lightning is really light from a flash of chain lightning that takes place beyond the horizon.  Chain, or forked, lightning is the lightning you see zig-zagging across the sky in any thunderstorm.

We just don’t hear heat lightning’s thunder because we are too far away.  We don’t see the lightning either.  We see its light reflected from high clouds.  Lightning is a big electric spark that spears its way across the sky.  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 flashes across the sky, it quickly heats the air in its path.  The heated air expands (spreads out) rapidly and sets a great sound wave in motion which we hear a few moments later as thunder.