Saturday, January 28, 2012

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.

But, why then is the rumble of thunder prolonged?  This is because the path taken by the lightning from the clouds to the surface of the Earth is a fairly long one.  Whatever the lightning is doing to produce the rumbling sound (we'll get to that later), it does it at all points along its length.  Assume that the path of the bolt is vertical (not diagonal).  Then, without question, the sound from the ground end of the bolt has a shorter distance to travel to you than that which emanates from the cloud end.

If lightning strikes the surface 1 km from you, but had a source in the clouds that are 2 km directly above the surface, then the distance from the lightning on the cloud end to you is 2.24 km (using the Pythagorean theorem).  Sound covers that distance in about 6.5 seconds.  In such a case, you will observe the lightning, then begin to hear the sound of thunder after a three second delay (when the sound from the ground side reaches you), and it will continue to rumble for three and a half more seconds (when the last of the sound produced by the lightning arrives to your ears from the cloud end).

Still, this does not capture the essence of what lightning is.

Lightning is a rapid discharge of electrical energy.  First, clouds can develop a net charge by way of friction with air molecules.  Once charge builds up in a cloud, it polarizes the surface of the Earth (the Earth itself is a huge conductor) below it.  As a result, a large potential difference builds up between the two ends.  This potential difference is 'relieved' when a stampede of electrons race from the clouds above to the ground below at over 200,000 km/hr.  The event can only occur if the electric field between the cloud and the surface below is sufficient to ionize a path of air between them (only an ionized path can conduct the charge).

The path of a lightning strike is indeed the path taken by the electrons.  So, can I then conclude that electrons are blue?  Not quite.

The reason that lightning is visible is due to both incandescence and luminescence.  Incandescence occurs due to the locally high temperature of air, which emits a blue-white glow.  The luminescence is a bit more complex.

The air in the atmosphere consists largely of nitrogen gas molecules.  The rapid flow of energy of the electrical discharge causes the electrons of the nitrogen molecules to move to higher energy states.  However, this state is very temporary - the molecules return to their original state soon after.  And it is this change of state that emits the distinctive blue-white colour that we associate with lightning, and electricity in general.

As such, the visual evidence of lightning is not the charge itself, but rather its effect on the air that it travels through.  And the same is true with the static electricity we see under our sheets on cold winter nights.  Here, we do not have the extreme high temperature that we had with the lightning (no incandescence), so the colour that manifests is due exclusively to the luminescence described above.  It turns out that those "sparks" travelling along your pajamas that freaked you out as a kid were just the flashing of nitrogen molecules.

Now, those static discharges on your pajamas do not produce a sound that resembles thunder.  The reason that lightning makes that thunderous sound is just as non-trivial as the reason it appears blue.  As mentioned above, part of the bluish appearance of the lightning is due to the high temperature air, which causes incandescence.  How hot does the air get?  Immediately after a lightning strike, the air in the vicinity of the lightning path reaches around 30,000 degrees Celsius - that is five times the temperature one would experience if one were hanging out on the surface of the Sun.

While the molecules along the lightning strike become scorching hot, the neighbouring volume of air remains unaffected in terms of temperature.  This massive difference in temperature translates to a giant pressure gradient.  The body of air, wishing for the equilibrium it had moments before the strike, seeks to restore it.  The pressure gradient causes the air to move... fast!  The result is a shock wave: the air moves at a speed faster than that at which it carries sound.  When we hear thunder, we are hearing these shock waves, which occur along the entire length of the lightning bolt.  If you listen to a supersonic jet airplane that is flying nearby, you would notice that it sounds like thunder.  That is because such jets also cause shock waves as they move faster than the speed of sound in air (supersonic).

Lightning is a very dramatic expression of charge redistribution.  The entire event, itself a complex dance of electrons within the atmosphere, is complete within a fraction of a second.  Due to the intense nature of this phenomenon, I would never wish to eat lightning or crap thunder, as Mickey encourages Rocky to do.  Just the same, I would hate to enter the ring with someone who does.

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