[TowerTalk] lightning & trees

[Tom Rauch]

> I guess the only remaining question in my mind is the
> process by which the streamers are generated, that is,
> what variables cause streamers in one object versus
> another?

Doc,

Think of spark plugs.

Think of HV leads to the CRT in TV sets.

Think of HV RF capacitors with wide spacing.

Sharp corners, even at a micropolish level, are where
streamers and arcs like to start. The plugs in your car, for
example, arc at progressively higher voltages as the
electrodes become rounded. Same for air variable capacitors.
The plates have to be polished by tumbling them in some soft
abrasive like walnut shells. Get the slightest burr or edge,
even one you can't see by naked eye, and the voltage
breakdown decreases dramatically.

Connect a resistor in series with two equal spaced gaps, one
rounded and smooth and the other with sharp edges, and the
sharp edged gap (or a smooth one with one single small nick)
will arc first every time...even when it has high series
resistance.

Get a sharp strand on a TV CRT lead and it will hiss and
sizzle. It may even flash an extraordinary distance to
ground.

This happens because the electric field around the sharp
point concentrates, and the electric field (force) extending
from that point into space becomes much higher than other
smooth areas where charges can't pile up in a point and leak
off.

Lightning is no different.

Now I suppose there are some cases where it is possible to
spray enough charges off to make something look "blunt" when
seen form thousands of feet away, but it would be never my
choice to stick anything out in the air sizzling and begging
for a streamer to form. The most likely points to arc are
always the sharpest points and of course closer points that
are sharp. It all comes down the voltage gradient or
electric field concentration near the object.

A sharp pointed object is more likely to be hit (arc) than a
blunt smooth object, and a taller object more likely than a
shorter object if the shapes (even at a small close view)
are the same. Series resistance in the path has almost
nothing to do with it, that's why they don't worry about
using resistor plugs and wires in car engines but they care
a great deal about the sparkplug's gap edges being sharp.

73 Tom