Why Does Lightning Appear Blue and Cause a Rumble?

Fans of the film Rocky are very familiar with the character played by Burgess Meredith.  While Mickey, Rocky's lovable, old, grumbling boxing trainer has a lot of memorable lines, I particularly enjoy the one he uses to motivate Rocky during his training: "Kid...You're gonna eat lightning and you're gonna crap thunder!"  This hilarious line, delivered ferociously by Meredith, is not without educational merit: the sound of thunder always follows a lightning event.    

While everyone has observed the awesome phenomenon known as lightning, most do not know what they are seeing, nor the cause for what they hear afterwards.  Still, most are aware that one precedes the other (Rocky would never, for example, eat thunder and then crap lighting).

The reason for the order is simple: a lightning bolt causes a flash of light, which travels at 300,000 km/s (the speed of light), and the sound it produces, thunder, travels through air at about 0.34 km/s (the speed of sound in air).  So, the light from a flash of lightning that strikes one kilometer from you reaches your eyes almost instantly (1/300,000 of one second), while the sound it produces reaches your ears three seconds later.  We cannot easily fathom a ratio of one million, but in our atmosphere, that is just how many times faster light travels than sound.