WeatherSavvy
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Did You Know?

Lightning kills more Americans in an average year than tornadoes.

You'll impress everyone you know after reading this Archived WeatherSavvy issue on lightning

What About Lightning?

     Lightning is the visible part of an electrical discharge.  Thunder is the resulting sound from the rapid expansion of the air after this electrical discharge.  Thus, thunder results from lightning.  So, if you see lightning there is always thunder (although you may be too far away to hear it, typically thunder isn't heard 15-20 miles from the lightning strike).

How Far Is Lightning From You?

Here's how to find out how far a lightning strike has occurred from your position:

Count the seconds in between seeing the Lightning and hearing the Thunder.   Multiply that number of seconds by .2,(that's not two, it's "point two") and you have roughly figured out the distance from your position to where the lightning has struck in miles.   So, If it takes 7 seconds to hear the thunder, then the lightning is roughly 1.4 miles from your position.

Is that confusing?  A good rule is that for every 5 seconds from seeing the lightning until hearing the thunder means the lightning is roughly 1 mile from your position.  This is easier to remember.

Here's why it works:

When lightning strikes, light and sound both travel in all directions away from the point of the lightning strike.  However, light travels much faster. In fact, it appears to travel instantaneously to humans.  Sound travels much slower, at 344 meters/second (m/s) or two tenths of a mile/second (.2).  So we see the lightning instantly, but we hear the thunder seconds later. 

I think that the situation is similar to sitting in the bleachers at a baseball game.  We see that the batter has hit the ball, but it isn't until a second later that we hear the crack of the bat.  This is because sound travels slowly compared to light.

So, if sound travels at .2 miles per second, then we count every second from seeing the lightning and multiply that number by .2, and then we have the miles between you and the lightning!  The reason this is a rough estimate is because the speed of sound is not constant.  It actually changes depending on the temperature and pressure of the air.  In fact, sound also has a different speed when it travels in a different medium.  For example, the speed of sound is 4 times faster when it travels in water than when it travels in air!  

Remember, next time you are in a thunderstorm, for every 5 seconds in between seeing the lightning and hearing the thunder, means that there is 1 mile between you and where the lightning has struck.

Warning:  Just because the lightning has struck 5 or more miles from you, or even if you can't hear the thunder, does not mean it is safe to be outside during a lightning storm.  In fact, an electrical charge could be building right above you, and you wouldn't know it until it was too late. Also, lightning can strike 10 miles from the storm. If you can hear the thunder, be safe and listen from inside your house.  

What is Heat Lightning

     Heat lightning is not a special kind of lightning, but rather it is a special way in which we see the lightning.  Heat lightning is lightning from a distant storm, usually so far away that we don't hear the thunder.  What happens is that the light we see is distant lightning reflected off of clouds and dust in the air to our eyes.  It just so happens that conditions are most favorable for heat lightning during hot summer months, thus the term "heat" lightning.  I guess it could have been called "reflected lightning" or "lightning sans thunder" or even "crazy flow of electrons causing rapid expansion of atmospheric gas and a highly visible glow" ( or CFECREAGHVG for short)  Anyway, heat lightning will do.

Does Lightning Go From the Ground Up or the Cloud to the Ground?

Actually it goes both ways!

     Charges usually separate in a cloud so that the negatively charged electrons are at the bottom of the cloud and the positively charged electrons are at the top of the cloud (the cause for charge separation in the cloud is still debatable despite several good theories).   What happens is that a negatively charged stepped leader slowly makes its way toward the ground in small steps (go figure? stepped leader).  At this point there is no lightning, just the electrical charges preparing for the strike.  The stepped leader induces a positive charge on the ground or objects on the ground, like a tree, a house, or even you and me.  


     Then, when the stepped leader is pretty close to the object or ground, a traveling spark is emitted from the object up to the stepped leader.  There is still no lightning yet, but watch out!  When the stepped leader and the traveling spark meet, huge amounts of electrons travel towards the ground.  Then, the actual lightning travels upward toward the cloud, following the path used by the stepped leader to reach near the object.  This is called the lightning stroke.   So lightning is a complex process combining movement of charges both up and down.  Usually the light portion is the result of charges moving upward towards the sky.  However, it is known to happen the other way around.  The stepped leader can begin from a tall building and then the light portion would result from  charges moving down towards the ground.

Lightning can strike almost anywhere.  It can strike another cloud, its own cloud, an airplane,  and even strike upwards toward space.  

To Interest your Intellect, Some Tantalizing Tidbits...

For more on lightning try this on for size...

http://www.lightningsafety.com/