A couple of weeks ago, after a Saturday thunderstorm, our running buddy Kate told me on a run that it had “sparked” a family debate about whether lightning strikes upward, or downward.  (I didn’t know it at the time, but that lightning storm had started some forest fires in Big Sur that would change our lives.)  Some were claiming that a bolt of lightning actually moves from the ground to the cloud overhead.  But what could that mean?  That when we are struck by a lightning bolt, it actually originates underfoot, and leaps up to the clouds through us?  Clearly some research was called for.  I leapt into action.

Alas, like so many things in nature, it is not as simple as either of those explanations.  But it’s pretty interesting.  Here’s how it all comes down—pardon me, here’s how it all transpires.  (There are a zillion kinds of lightning.  I’ll just describe the most common type of ground strike.)

Your basic thunderhead has a negative electrical charge at its bottom.  This induces an opposite electrical charge in the earth below it, which actually follows the cloud around like its shadow as it moves across the landscape.  So you have a humongous negative charge in the cloud, and a humongous positive charge in the ground, and they’re following each other across hill and dale.  They’re just itching to get together.  But there’s a problem.  Air does not conduct electricity.

What ends up happening is that a “leader” of ionized air molecules starts reaching down from the cloud toward the ground.  Ionized molecules have missing electrons, so they do conduct.  So this little tendril is a is a conductive channel, reaching tentatively downward, and often branching out in several directions, irresistably attracted to the positive charges below.  Its voltage is pretty small, as lightning goes, and it’s almost invisible.  It takes a long time to do this reaching down thing—a large fraction of a second.  But when one of its branches finally touches the ground, POW!  It’s like touching a hot terminal with a piece of wire.  As soon as it touches, all hell breaks loose.  That’s called the return stroke.

It’s the return stroke that does all the damage.  That’s the one you see, as roughly 100 trillion watts run explosively back up the channel opened by the leader, with all the ferocity that’s been stored up in the positively charged earth.  The return stroke is faster, only a few milliseconds.  The average bolt is about half an inch across—the diameter of a pencil—and it instantly superheats the air around it to twice the temperature of the surface of the sun (I’m not making this up), which creates a shock wave which we hear as thunder.  Usually it’s followed by repeat strokes, along the same ionized channel, until the charges are equalized or the channel dissipates.  Your average lightning strike is composed of three or four separate strikes back up the same path, which is why lightning flickers.

So here are the multiple answers to the debate:

  • In the sense that the whole thing starts with a leader reaching down from the cloud, lightning strikes downward.
  • In the sense that the major bolt is the return stroke following that leader back up, lightning strikes upward.
  • In the sense that in any electrical current the electrons move toward the positive, lightning strikes downward.

Glad I could clarify.  Now you know.

 

 

 
Copyright © 2011 Randy Fry