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Treebeard's Stumper Answer
1 October 1999

Unlucky Strikes

The week before school began, lightning started the Camuesa Fire near Red Rock. Julie and I were hiking up Mono Creek that day, into the remote Dick Smith Wilderness behind the fire. That trail starts after a two hour drive on a dead-end dirt road. We smelled smoke and got out as fast as we could! On our drive out, we saw dozens of lightning strikes in the backcountry mountains, but only one started a fire. Lightning is the major natural cause of wildfires. But why do direct lightning strikes to ground usually not start fires?

Fire on the hills
Lightning-caused Camuesa Fire burning into the backcountry behind Red Rock on August 26, 1999.


Lightning is certainly hot enough to start fires, as much as 50,000° F in the core. But trees (and people) often survive direct hits. Lightning strikes are explosive events that only last a few dozen microseconds. It's like trying to start a fire with a firecracker. It's hard enough to start a fire with just one match, and even matches sometimes go out. Trees also have their sap near the outside which might carry electricity to ground, protecting the tree like a car protects the people inside. Lightning is still dangerous, and I'm glad it's not more common here!

Notes:

I like this stumper because the answer seems so obvious when you finally think of it. DMS student Nick makes it short and simple: "I think the lightning bolt goes down so fast that sometimes it doesn't catch."

Graybear adds some details:

I think that most fires caused by lightning involve pine trees. Their sap contains lots of volatile chemicals. When the lightning strikes the top of the tree, the electrical charge must pass through the tree from top to bottom (or is it bottom to top?). Trees are not great conductors, so the resistance to flow generates enough heat to cause the sap to boil, releasing these volatile chemicals above their flashpoint and igniting the tree. The rapid conversion/expansion of sap to steam creates enough pressure in the tree to literally explode into flame. Even though the temperature of lightning is thought to be around 50,000 degrees, enough to incinerate most organisms, its duration is too short for ignition. When lightning strikes the ground, the path to GROUND is too short to generate much heat in the material it hits. One noticeable exception to this is fulgurite, a glassy material produced sometimes when lightning strikes sand and fuses it together.
I have a piece of fulgurite that I found in the Coso Mountains south of Owens Lake while looking for fossils. It's a hard white glassy tube about 6 inches long with longitudinal ribs. It's not hollow, but the center is less dense. It seemed very mysterious when I found it, vaguely anatomical (and unmentionable). I thought it was possibly a worm tube without much internal structure, though this is not a marine deposit. (But the movie Tremors was filmed nearby!) Then I saw another fulgurite in the local museum and instantly recognized it.

Lightning storms are not common here in Central California. But one of the most interesting results of recent satelite studies of the earth is that lightning is a fairly constant global phenomenon. At any given moment, there are about 2,000 storms in progress around the world, with about 30 to 120 cloud-to-ground flashes per second. A source at NOAA estimates about 20,000,000 flashes per year in the U.S. Another source estimates about 1,500 lightning-induced fires per year. That's about 1 fire for every 10,000+ strikes. It could be worse!

People don't do as well after lightning strikes. The fatality rate is about 20%. We're more complicated than trees, and our brains and hearts and nerves use electricity and are more sensitive to overload. Lightning is more dangerous than rattlesnakes, but not as likely as winning the lottery. Be careful when you go to pick up your winning check!

Cars protect people inside because they are hollow metal shells that function as a natural Faraday cage (with Skin Effect) that carries charge mostly on the outside and on to ground. The rubber tires have nothing to do with it, so don't feel safe in tennis shoes or on a bike. After all, that lightning bolt just burned a path through miles of atmosphere, a pretty good insulator!

Trees also carry their fluids on the outside, and this protects them. Lightning usually finds the lowest resistance path to ground and burns a trail along one side of the tree, leaving a visible scar. Twisted Juniper trees at timber line in the High Sierra sometimes show spiral scars where lightning followed the twisted wood down to ground.

Coulter pine on Zaca Ridge
Coulter pines on Zaca Ridge. Note the middle tree is missing it's top, probably from a lightning strike in this exposed place.

There's lots of information about lightning on the Web. Here are a few starting links for more research:

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Copyright © 1999 by Marc Kummel / mkummel@rain.org