MadSci Network: Earth Sciences

Re: How is lightning formed and why does it come down so fast and crooked?

Date: Wed Dec 9 20:00:15 1998
Posted By: Dave Dixon, Assistant Research Physicist,University of California
Area of science: Earth Sciences
ID: 911418291.Es

Lightning is basically just the result of static electricity.
You can build up a static charge on your body, say, by shuffling
your feet across the floor.  This causes electrons an excess of
electrons to build up on your body (or maybe they leave and go
to the floor?  it's all the same in the end), which than then
create a spark if you get close to something.  Lightning is similar.
Thunderstorms build up because of something called "convection".
You probably know that hot air rises and cool air falls.  When
the sun heats the air near the ground, it tends to rise, but the
higher it goes the more it cools off, which eventually causes it
to fall back down again.  This circulation is called convection.
If the air carries with it moisture, you form thunderclouds.

The exact mechanism of how and why lightning occurs isn't really
well understood.  Here's one theory: as the convection process goes
on, warmer water droplets going up bump into cold ice crystals going
down.  Just like rubbing your feet on the floor builds up a static
charge on your body, so do these collisions between the water drops
and ice crystals build up a static charge in the cloud, which eventually
makes a spark - just a really big one.  But this is only one theory.

The mechanism behind the spark itself is better understood, and is
something called "dielectric breakdown".  Basically a dielectric
material is one that is also a good electric insulator, i.e., electrical
current flows very poorly through it.  The example you may be familiar
with is rubber.  Air is also a good insulator.  With an insulator,
you can build up a very large static charge on one side of the insulator,
and it will not flow through to the otherside UNLESS something
catastrophic happens.  This is dielectric breakdown, where basically
the stresses on the atoms from the electric field of the static charge
become large enough to pull electrons off the atoms - this then allows
electrical current to flow, and so the static charge blasts through.
So when enough charge builds up in a thundercloud, the air undergoes
dielectric breakdown, allowing the electricity to flow to the ground,
or perhaps another cloud.

The speed of lightning is simply the speed at which this breakdown
takes place - about 1/3 of the speed of light.  Lightning bolts are
crooked because air itself is irregular, especially in a storm where
winds cause turbulence, etc.  Even at the molecular level, the air
molecules aren't all lined up, but in random positions relative to
one another.  So there's no reason to think the dielectric breakdown
will occur is a straight line.  The color occurs because the
air through which the lightning passes is heated to a very high
temperature.  Just like an electric stove glows red when you push
electricity through the burner, the air glows when you push electricity
through it.  However, the lightning has a tremedous amount of energy,
and heats the air to much higher temperatures than a stove (tens of
thousands of degrees).  Higher temperature means it glows with a
"hotter" color of the rainbow, in this case purple-white instead of red.

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