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By Jeff Danner Jeff has worked in both the chemical and biotech industries and is the veteran of thousands of science debates at cocktail parties and holiday dinners across the nation. In his Common Science blog, Jeff aims to make technological and scientific concepts accessible to all.

Shelter from the Storm

By Jeff Danner Posted May 20, 2011 at 2:42 pm

This question to Common Science was submitted by Boykin Bell of Chapel Hill.

 

“It’s the bottom of the first inning.  Rain is falling steady, a little thunder.  Kids are holding metal bats and standing in the wide open baseball diamond.  Lightning flashes once, twice, and then the rain is heavy enough that the bats might slide out of the kids’ hands. The umpire sees more lighting and calls a minimum 30 minute hiatus.  Kids start tossing balls and dancing around the infield.  One coach calls his team into the dugout.  The other coach calls his team into the woods beside the field.  The woods are at lower elevation and thick enough so that, even in the downpour, the kids are out of the rain.    Which kids are less likely to get hit by lightning?”

 

Before answering Boykin’s question, let’s talk about lightning for a while (this is a science blog after all). Probably, the most interesting thing about lightning is how little we know about it.  Even today when we can send rockets to Mars and identify sub-atomic quarks that make up matter, we are not quite sure how the charge accumulates in a cloud to start a lightening bolt.  While there are various theories the general view is that the movement of water droplets and ice particles within the cloud result in an accumulation of negative charge at the bottom and positive charge at the top. The segregation of positive and negative charges results in a strong electric field.  As the negative charge on the bottom of the cloud grows an induced positive charge on the ground is formed with moves with the cloud.  Remember the positive charge accumulating on the ground under the cloud, it will be important later.

 

Air molecules in their normal state are very poor conductors of electricity.  However, under the influence of the electric field of the cloud this changes. Once the cloud has a strong enough electric field it starts to ionize nearby air by knocking loose some of electrons, giving you a mixture of negatively charged electrons and positively charged molecules (they now have positive charge because they have lost one or more of their negatively charged electrons).  The ionized air is now an excellent conductor of electricity.

 

Now we get to another part of lightning which is not completely understood, the formation of a “stepped leader”.  Extending out of the bottom of the cloud straight lines of ionized air are formed.  These lines of ionized air changes direction and also often branch as they approach the ground giving lightening its classic shape.  As the stepped leader descends, the positive charge on the ground begins to ionize air from the ground up rising as a “positive streamer”.  Although they can been seen with the proper photographic equipment both the stepped leader and the positive streamer are effectively invisible.

 

Once the leader from the cloud meets the streamer from the ground there is complete electrical connection and now we are in business.  The moment the connection is made, there is a huge and rapid flow of electricity from the ground to the cloud (yes, from the ground to the cloud, though there are often return strokes from cloud to ground afterward as well) which is the lightening that we see.  The statistics on lightening are hard to fathom.  On average lightning carries 30,000 amps of current at 200 million volts and can heat the air to 40,000 oF (3 times as hot as the surface of the Sun). 

 

OK, so back to our intrepid baseball players.  (Men reading this blog please play special attention as our life habits and feelings of invincibility make us 4 times as likely as women to be struck by lightning).  What our ball players want to avoid is becoming or being next to the source of a positive streamer.  The positive streamer tends to, but does not always, come from the tallest most conductive point in the vicinity below the stepped leader.  (Though this is not a guarantee, a positive streamer can also arise from the ground next to a tall tree or building.  There is more randomness to the process than the phrase “lightning always strikes the highest point” implies).

 

The team in the dugout has made a poor choice for shelter.  The dugout is made of metal fencing which is directly connected to the backstop which is both tall and conductive.  A lighting strike in the area of the field is quite likely to hit the metal fencing putting the boys in great danger.

 

The team in the woods has made a better choice, but as I will explain below, not the best choice available.  Standing next to an isolated tall tree is a particularly poor choice in a thunderstorm.   A tree in the woods described in the question is also a likely target for a lightning strike. I’ll give the boys in the woods partial credit here since there are a lot of trees in the woods so they would have to have back luck to be standing next to the wrong tree.  If this post was not getting too long already (clearly a flaw of mine as a blogger) I could go on to discuss which types of trees are more or less likely to be struck by lightning as a function of the conductivity of the sap.

 

Let me finish on a serious note.  Lightning can cause grave injury or death and knowing the proper safety procedures is important and possibly life saving.  The best place to be in a thunderstorm is in a large, well-grounded permanent building.  This game was played at Cedar Falls Park on Weaver Dairy Road where no such structure is readily available.  In this situation the players, coaches, siblings, and fans should have all sheltered in their cars.  You car is not very tall, less likely to be the origin of a positive streamer due to your rubber tires, and if involved in a lightning strike will provide some protection to you from the electricity by conducting it along the outside surface of the car through the tires and into the ground.

 

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