My Backyard Struck By Lightning
by Ian Kluft
San Jose, California

On the afternoon of Wednesday, September 8, 1999, tropical moisture that had been part of Hurricane Greg off the coast of Mexico travelled northward off the coast of California and was pulled into a low-pressure trough. Usually a trough with no associated moisture might go unnoticed by most people (except pilots) as the wind just changes directions. But with the infusion of unstable tropical air, the trough provided uplift that turned it into a line of thunderstorms unusually intense for California.

Well, this can happen in California and September is the time of year for it. But even compared to previous years, this was unusually strong. Thousands of lightning strikes happenned all over Silicon Valley and the San Francisco Bay Area. Storm clouds kept travelling northward up the line as the line barely moved across the valley. It literally lasted all night before reaching my neighborhood in the northeast San Jose area after dawn.

Just after 6:00AM on Thursday, September 9, lightning struck my neighbor's driveway. Everyone in the neighborhood was awake after that. It woke me up too. All the car alarms were set off by the loud thunder for several hundred feet.

A couple minutes later, after the car alarms had all stopped, I saw out my back window a bright flash and heard a loud boom with no delay in between. All the car alarms were set off yet again so I knew it had to be close too. But didn't realize for months that it had actually struck my backyard.

Then in April 2000, after the rainy season was over, I went to pull down a broken branch that had fallen but was still snagged up in the peach tree in my backyard. When I got a good look at the break point on the branch, it was obvious from the burn marks around the edge of it that I had found the location of last September's lightning strike.

Before taking the branch down, I took some pictures. The irony was immediately obvious to me that one of the owners of the "thunder.com" and "thunder.net" domains should have a lightning strike in his yard. I'll never hear the end of that one. :-)

[picture]
(992x1488 791K) (500x750 94K) 		The tree branch that had been broken by the lightning: This is the part still attached to the tree. Notice that there's a darker coloration around the edge of the branch. Even after 6 months of rains, the burn marks are clearly visible.
[picture]
(1488x992 716K) (500x334 47K) 		The branch break point from a different angle.
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(1488x992 811K) (500x334 54K) 		The broken branch is shown here before it was pulled down out of the tree. It has similar discolorations and burn marks, though they're not clearly visibile in this picture.
[picture]
(1488x992 873K) (500x334 58K) 		A wider shot of the broken branch. It was pretty big. The tree probably lost at least 5 feet of height when this fell from the top.
[picture]
(1488x992 815K) (500x334 40K) 		Close up on the burn marks on the broken branch.
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(1488x992 793K) (500x334 33K) 		After pulling the broken branch down out of the tree, this spot was noticed where it had split open with more burn-looking marks. I'm assuming this is also from the lightning, maybe where it entered the branch.
[picture]
(1488x992 758K) (500x334 32K) 		A different angle on the split in the branch and possible lightning entry point.
[picture]
(1488x992 715K) (500x334 29K) 		A different angle on the split in the branch and possible lightning entry point.
[picture]
(1488x992 961K) (500x334 52K) 		Now the branch has been taken down, cut up and put out on the street (next to the neighbor's pickup truck) for the city's garbage/recycling crews to pick up.

Lightning Safety Tips
(Or "What I learned...")

I learned a lot about lightning from this. Fortunately, I found out that I had done some things right. But now, after going back to my college Meteorology class textbook, I think know why this happenned the way it did.

First of all, we all know the rule of thumb that lightning will strike the tallest object in the area. Well, it's only a rule of thumb and it has exceptions. One first thing puzzled me when my neighbor showed me where the lightning struck his driveway. I looked at the 40-foot mast and Ham Radio antenna on his house. I looked at the 30-foot mast and Ham Radio antenna on my house. I looked at my palm trees, more than 50 feet tall on the other side of my house. Then I looked back at the driveway again. I knew it didn't strike the tallest thing in the area. Why?

I got out the textbook from a Meteorology class I took in college as an elective. (It wasn't required for my degree in Computer Science. :-) I re-learned that the ground naturally has a negative charge. But the thunderstorm comes along with a much stronger negative charge so it attracts positively charged particles to follow along on the ground underneath it. The positive-charge can usually accumulate in the highest objects, which is why they're prone to get struck. But anywhere they can accumulate enough charge, lightning will strike.

So why didn't it strike my palm trees? One of them has a hidden Ham Radio "G5RV" wire antenna in it, which is connected to a commercial lightning arrestor and an 8-foot copper grounding rod. Honestly, when I bought the lightning arrestor five years ago, I was sure it was just a good precaution but that I'd never use it.

In my opinion, the positive charge must have travelled up the palm trees but were bled off to the natural negative charge of the ground by the 8-foot grounding rod. That much current caused the lightning arrestor to short to ground, as it's designed to do. My other antennas were grounded too, but not quite this well. (They are now!)

Why did it strike my neighbor's driveway?! That puzzled us for a while. It seems so strange. But he had also grounded all his antennas. He used a cold water pipe for the grounding. The results indicate that was good enough. The positive charge was also bled off as it undoubtedly (in my opinion) probed his house and antennas for places to build up a charge.

The crack on his driveway from the lightning was nearly along a straight line. It looked like it might be lined up with steel reinforcement bars ("rebar") inside the concrete of the driveway, which must have expanded during the lightning strike. So we think the rebar in the driveway was not well grounded and gave enough place for the charge to build up for the lightning strike.

What can you do to protect your house? In places which get lots of lightning, they use lightning rods for this purpose. I wasn't aware until after this that proper grounding can prevent a direct lightning strike. (It certainly isn't guaranteed but I'm happy for any chance I can turn in my favor.) So after reading this, you can probably see there's more of a point to properly grounding all your antennas than just to get a better signal. A cold water pipe was good enough for my neighbor. An 8-foot grounding rod was definitely good enough in my case.

For all I know, these may have been little lightning bolts compared to what other areas get. I was inside my house, about 100 feet from the strike in the neighbor's yard about 50 feet from the strike in my yard. Maybe only because I was indoors, the thunder was uncomfortably loud but not enough to make my ears ring after either of them.

Related Links

Since I'm one of the owners of the thunder.com and thunder.net domains, I've occasionally received requests from school children for help with research about thunder and lightning. And in light of these recent experiences on the subject, I may as well provide some links. :-)

(This list led to the creation of lightning.thunder.net.)

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Copyright (c) 2000 Ian Kluft