Lightning is a common manifestation of the raw power
: something as humble as
can shatter rocks, set fire
to buildings and kill people
. We all know what
it looks like: A big line of angry bluish white light
, combined with a thunder
that comes a
few seconds later. But, how does it work? Why does it happen? It turns out the answer to that
question is not entirely clear.
The physics of lightning
Lightning starts with a large cloud. In this cloud, the rubbing of ice crystals against each
other liberates electrons. For some reason that's not currently understood, the negative
crystals go to the bottom and the positive ones go to the top, although the opposite-positive
bottom, negative top- also happens. This produces an electric field, between the top and bottom
of the cloud and the bottom of the cloud and the earth.
The next step is the formation of streamers. The electric field causes free electrons to
accelerate, and these ionize air molecules. So, now we have free positive and
negative charge, creating a plasma. The electric field causes these charges to reorganize.
The head of this thing, called a streamer, becomes negative, and the part behind the head
becomes positive. This field compression causes enhanced ionization. This means the streamer,
known as a stepper leader, can propagate. Due to inherent
instabilities in this process, the streamer will have a tendency to branch. In less than a
second, typically, a whole tree-like structure of streamers becomes attached to the cloud. In
this short time, the plasma behind the streamer heads hasn't recombined fully, so the whole
tree is a moderately good conductor of electricity, as it contains free charges.
When the tree, with the negatively-charged tips, comes close to the ground, the electric field
caused by the tips causes the ground, and in particular high, pointy things on the ground, to
send out their own streamers. At some point, a streamer from the ground and from the cloud will
connect. At this point, we have a conductive channel, over which there is an enormous voltage
difference, in the order of millions of volts.
Now, things go very, very fast. There is nothing stopping an electric current from flowing
through the channel between earth and cloud. As the current increases, the plasma will become
hotter, increasing ionization and, consequently, conduction. The size of the plasma will also
increase, reaching typical sizes of a centimeter or so. These mechanisms cause even more current
to flow. In principle, this process is self-increasing, until the charge separation has been
rectified. This plasma is what we commonly call lightning. The plasma reaches a temperature in
the order of 25,000 K. There will be many free electrons and ions. Hot electrons can excite atoms
and molecules, causing them to emit light. This is the visible lightning.
The air has been suddenly heated up from 300 to 25,000 K, and extra particles have been produced,
in the form of free nitrogen atoms, free oxygen atoms, all sort of atomic and molecular ions
and electrons. Via the ideal gas law we see that the pressure is proportional to both
temperature and the amount of particles. So, suddenly the local pressure has increased
hundredfold. The subsequent explosion of this high-pressure region into the cool surrounding
gas produces awesome shockwaves and is audible as thunder. Because sound travels much
slower than light, you will hear the thunder generally after you have seen the lightning. As a
rough guide, the lightning is one kilometer away for each 3 seconds of time difference.
The residual plasma recombines after the lightning strike. However, the free nitrogen and
oxygen atoms do not necessarily recombine to nitrogen and oxygen molecules. Some will
recombine to nitrous oxides, NOx, and others to ozone, O3. These
substances are the cause of the acrid smell after a thunderstorm.
How to prevent being struck by lightning
Being hit with a hot plasma bolt that conducts several kiloampères of current can be extremely
detrimental to one's health. Generally, extensive burns, both where the bolt enters the body and
where it leaves the body occur. The biggest danger however is that the massive current may cause
a cardiac arrest which is fatal if not treated. In short, we don't want to be hit by
lightning. So, how can we avoid it?
- First, establish how far the lighting is away from you. If it's 5 kilometers away, there is
little need to get all excited yet, as it may not come near.
- If you are in a city, don't sweat it. There is likely a high building nearby, which has a
much greater chance of getting the lightning strike than you have. However, unplugging
nonessential electronic equipment can be a good idea, as the current may pass through the
electric wiring in the ground, and completely toast every electric device that is plugged in.
And most insurances are bitches when it comes to paying for them. Fuses offer little
protection, as their melting time is too long. Do stay away from large windows and metal piping.
- If you are in a car, just stay in. The metal of your car is an excellent conductor, and
the current will flow trough the outer metallic shell rather than through you. If your car is hit
by lightning, it may be a good idea to gently bump an obstacle to let the residual charges drain.
- In a forest - and only in a forest! - just take shelter under a tree, preferably one that isn't very tall. The
chance that your tree will be hit is pretty minimal.
- If caught in the open, you may have a problem. Don't take shelter under high objects, as
these attract lightning. Don't be the highest object yourself. If you are the highest object,
squat, while touching the ground only with your feet. If you lie down, you have a very large
contact area with the ground, and nearby lightning strikes may harm you, by draining their charge
not only through the ground but also through you. Professor Pi points out that putting your feet close together is a good idea, and he's right, as it will minimize the potential difference between them, thus making it less likely for current to flow through your body.
Lightning is a beautiful manifestation of the power of nature. Reaching temperatures several
times more than the temperature of the surface of the sun, producing shock waves that can be
heard from several kilometers distance, and reorganizing the very molecules of the air, it
continues to be a symbol
. It is, like most things powerful, dangerous,
and proper precautions should be taken not to be struck by it.
- Lecture notes from a lecture by Professor Raizer
- My own understanding of plasma physics