Ask a Nerd

AskANerd: June 16, 1999

Gail ???? of Cincinnati, OH asks:

What causes thunder and lightning?  Does one cause the other, and which one really comes first?  I've heard that thunder really comes first but that because lightning travels faster than sound, it always appears that lightning comes first.   Is this true? 

The Nerd responds:

Thunder and lightning is very interesting stuff.  Saying one causes the other might depend on the way you look at it.  It is probably safer to say that they are both caused by the same thing.

A good way to understand what  typically happens in a thunder/lightning scenario, is to make some tiny lightning of your own.  You've probably petted a dog or walked across a carpet and then reached out to touch a doorknob and zap! you get a shock.   What causes these carpet shocks  is pretty much the same thing that causes lightning--only on a much much smaller scale, of course.  Here's what happens:   When you walk across a carpet floor or pet a dog, or anything like that, the rubbing motion actually strips electrons away from one surface and transfers these electrons to you.  With these excess electrons you have now become negatively charged and immediately your body (as would any object that doesn't have a net charge of zero) starts looking for a way to shed these extra electrons so you can become neutrally charged again.  But the problem is that once this rubbing motion has stopped, there isn't enough energy to transfer these electrons back to from where they came.  Your body, clothes, and certainly the air around you are all pretty poor conductors of electricity.   So now these electrons are stuck with you and they can't be drained away unless they can find a very good conductor through which to escape.  Of course, metal IS a very good conductor, so when you touch that doorknob, the electrons suddenly have an easy route out of your body.  In fact, as your hand approaches the knob, the excess electrons are all bunched up in your hand in anticipation of an escape route.  They can't escape yet though because the air is such a poor conductor (or an insulator) that they cannot make the jump from your hand to the knob until your hand is just nearly touching.  Then, when your hand is very close, there isn't enough air between your fingers and the knob to keep the electrons from making their escape.  They leap from your fingertips towards the doorknob, ionizing the air between to make a more energy efficient path.  The ionizing of the air makes a spark and the sudden change in your net charge from negative to neutral as the electrons leave is the zapping sensation you feel.

Now your feet rubbing against a carpet floor pick up enough electrons to give you a bit of a jolt.  Now imagine how many electrons something with the surface area of a cloud could pick up if it was doing something similar. 

First, as a storm starts brewing lighter clouds start to rise up in the atmosphere and rub against the heavier, water-bearing clouds.  As the light clouds rub against heavy clouds, the heavy clouds strip them of many of their electrons just as your feet do to the carpet.  cloud image

Now, the heavy clouds have excess electrons and have become negatively charged.  Now the clouds immediately start to look for a way to drain off these extra electrons but they have an even worse problem than you--they are completely surrounded by air and have nothing to conduct their electricity.  This rubbing can continue building up HUGE amounts of electrons in the clouds. 

lightning strike

Eventually, even the air can no longer insolate the negatively charged cloud from its nearest conductor--the ground (or sometimes another cloud, causing cloud-to-cloud lightning strikes).  At this point, just as when you reached for the doorknob, the electrons jump from the cloud and drain themselves into the ground.  Of course, a 'spark' of this magnitude is what we call lightning.

Now that we have seen what produces lightning, we still have to figure out what causes the thunder.  Let's go back to the carpet shock you get when touch a doorknob.   I'm sure you've noticed that along with the flash of light from the spark you also get a quick snapping sound.  Could that be miniature thunder?  Yes!

Both the flash of light and the snapping sound is a result of the electrons ionizing the air as they prepare to pass through.  The air resists this effect and the resistance causes friction which in turns releases heat.  The energy of the heat is given off as both light and sound (flash and snap). Now on a much, much greater scale (with heat actually hotter than the surface of the Sun!) this flash and snap effect becomes thunder and lightning. 

The immense energy given off by the escaping electrons causes another effect that gives thunder its more booming sound.  Particles in the air are forced away from the electron path at speeds faster than the speed of sound.  This causes all of the little snapping sounds to become piled up on top of each other, augmenting each other until they release a very large booming sound.  This is known as a sonic boom.

Now at last, we can answer the second part of your question.  Thunder and lightning happen at approximately the same time, but as you have heard, lightning travels faster than thunder (even though it is moving a supersonic speeds) and thus we see the flash before we hear the thunder.