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Exploding Trees

A single lightening bolt carries a peak current that's ten thousand times what an average light bulb would use.

Tall, green forest trees.

Photo: hsivonen (flickr)

What happens when lightning strikes a tree depends in part on how long it's been raining.

In a scene from Mary Shelley’s famous novel Frankenstein, one of the characters watches as a lightning bolt strikes a large oak tree.  The tree is utterly destroyed in a gigantic explosion. This might be good story‑telling, but could such a thing really happen?  Let’s find out.

Lightning is nothing more than electricity: a LOT of electricity.  A single bolt carries a peak current that’s ten thousand times what an average light bulb would use.  All this electricity has only one aim:  to travel between the clouds and the ground along the easiest possible route.

Trees… Natural Lightning Rods

Trees, which stick out of the ground like natural lightning rods, make this route a little bit easier.  What happens when lightning strikes a tree depends in part on how long it’s been raining.

Water is a much better electrical conductor than wood, and if the tree is thoroughly wet, the bolt will travel along the water that soaks the tree’s bark.  This might singe the tree, but it probably won’t kill it, and it certainly won’t make it explode.

Exploding Oaks

If the bark is dry however, the next easiest path for the bolt is inside the tree, through the damp, sugary sap.  A lightning bolt contains enough energy to bring a ton of water to the boil, so the sap is instantly vaporized.  Like a pressure cooker with no safety valve, the tree will indeed explode, just like Mary Shelley reported.

What’s more, Mary Shelley’s exploding oak tree is scientifically accurate in another way.  Oak trees have particularly rough bark, and it takes rain longer to soak them than smooth‑barked trees.  This makes oaks especially vulnerable to lightning explosions.

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