YO3CTK wrote:
> We were surrounded by dozen of apartment buildings,
> 10-story high, with plenty of antennas on the roofs. Why the lightning
> has stricken this particular tree, when all surrounding buildings were
> made of reinforced concrete and (presumably) well grounded, is a mystery
> for me. Maybe someone will come up with a theory.
In South Africa, thatched roofs are relatively common. They look
good, and they offer excellent insulation against the weather. In
some areas, they are also cheap, as you can harvest the building
materials where you build the house!
We also have a very high lightning ground strike density. The
highest strike density in the world occurs in Swaziland, less than
four hours from here by road.
It goes without saying that virtually every thatch roof has a
lightning arrestor (or more) associated with it. Most of these are
simply poles, made from galvanised steel, with sharp tips. They are
typically over 20 m tall.
The recommendation is that all flammable components must be contained
within a 45 degree angle of the tip of the rod. The term used in the
lightning protection industry is a "cone of protection", with an apex
angle of 90 degrees. The angle is probably determined primarily from
anecdotal information, but insurance companies do accept it as
adequate protection when they have to underwrite your mansion.
Us humans tend to exaggerate vertical dimensions, probably as a self-
preservation measure. 30 m is a lot when you're looking down from the
top of your tower, but not a lot if it represents a horizontal
distance. I would not want to be 30 m from a charging bull elephant,
for example! If you consider that a 10 storey building is typically
only 30 m tall, you can understand that a 10 m tall tree at a 20 m
from the base is at risk.
You didn't mention how tall the tree was, or how far it was from the
buildings, but this effect will probably help to explain what
happened.
When I was at university, lightning destroyed a 50 year old tree
around 5 m from my res window. I was studying at the time, and my
books were covered in sawdust. The tree had simply disintegrated for
something like 300 mm above ground level, and the upper part of the
tree toppled. There was no visible damage to the upper part of the
tree.
As an aside, I grew up with the admonition that I had to stay away
from trees during thunderstorms. In fact, if you're out on the
grasslands, the safest thing you can do is to lie down until the
storm has subsided. You just don't want to be the highest point in
the area!
Chris R. Burger
ZS6EZ
PS The tree didn't collapse within 70 mm of the base, but then it
wasn't guyed!
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