Lightning.... it's not caused by some dude with a hammer!

Lightning.  It can often be one of the scariest parts of thunderstorms, especially to a little kid (if you've even been woken at 2 a.m. during a thunderstorm by your scared son or daughter... or have done it to your parents, you know what I'm talking about).  Lightning is the severe weather element most associated with thunderstorms and is easily identified world-wide.  It can cause awe and wonder while at the same time fear and worry.  So how and why does lightning happen?

To understand how lightning forms, you need a little background on the electric field of the Earth-atmosphere system.  The Earth has what is known as a fair weather electric field that exists in the absence of clouds.  In the clear air, the atmosphere will carry a net positive charge while the Earth below carries a net negative charge.  This results from the action of past thunderstorms, which act to deposit electrons on the Earth's surface and remove them from the atmosphere.  The fair weather electric field is about 100 volts per meter which, compared to the field just before a lightning strike (1,000,000 volts per meter), is quite small.  Luckily, air is an excellent insulator and so its conductivity is close to zero.

So now let's examine what happens when a rain cloud is in the area.  It all begins in the cloud where ice crystals and hail collide with each other, transferring electrical charges with each other as they collide.  The smaller ice crystals gain a positive charge while the larger particles, like hail, gain a negative charge.  The smaller, positively charged ice crystals are then swept higher into the cloud by the storm's updraft, while the larger, negatively charged particles fall toward the cloud base.  In so doing, the top of the cloud becomes positively charges, while the cloud base near the surface becomes negatively charged.    The negatively charged cloud base then induces a positive charge at the ground level (it's like a magnet... opposites attract!).

The negative charges in the cloud base then begin searching for a path of least resistance to the ground, creating the stepped leader which ionizes the air around it, forming a narrow conductive path.  As one of the branches of the stepped leader approaches the surface, the electric field between the surface and the stepped leader becomes so great that positive charges jump upward off the object to meet the descending stepped leader.  When this traveling spark connects with the stepped leader, the channel for electrons flow opens up and the bright and powerful return stroke occurs.  This process continues until the negative charge in the cloud base has been drained and deposited on the ground. 

What about thunder?
When lightning occurs, the air surrounding the stroke is heated to 54,000°F, causing the air to expand explosively.  This creates a shock wave that evolves rapidly into crashing sound waves and results in the noise we know as thunder.  The sound waves travel at approximately 330 meters per second, which means that it takes the sound about 5 seconds to travel a mile while the light from the flash travels so fast that it essentially arrives at our eyes in an instant.  Therefore, you can estimate the approximate distance between you and a lightning strike by counting the seconds between the flash of light and the sound of the thunder.

So now when you see lightning and hear thunder occur during a thunderstorm, you will know the cause behind it.  It's just a discharge of negatively charged particles from the cloud base to the ground followed by sound waves caused by exploding, superheated air.  That's right... it's not Thor with his mighty hammer.  Sorry big guy.

I think I just made Thor mad.