A Channel 3 viewer asks What comes first thunder or lightning? 

Lightning always happens first because it causes thunder. The electric spark that constitutes a lightning bolt heats the air through which it passes to

at least 50,000 degrees in thousandths of a second. Explosive expansion of the air immediately occurs, followed by cooling and sudden contraction. These events trigger shock waves in the air which we hear as thunder.

The initial thunder that you hear comes from the point on the lightning channel closest to you. The last sound comes from the most distant point.

It takes about five seconds for thunder to travel one mile, so you can tell how far away a storm is by counting between seeing the lightning and hearing the thunder!