From: Paul Doherty (pauld@exploratorium.edu)
Date: Wed Nov 01 2000 - 10:34:09 PST
Message-Id: <l03110785b6260f766a03@[192.174.2.173]> Date: Wed, 1 Nov 2000 10:34:09 -0800 From: Paul Doherty <pauld@exploratorium.edu> Subject: Re: pinhole Questions about lightning and aurora borealis
Hi Regan
The answers to your questions can be found in "the Lightning Book" by
Martin Uman.
Scientists are still unsure about what makes the path of a lightning bolt
bent and forked. (I went to a talk at the APS meeting just three years ago
where researchers were argueing this very question.)
My temporary hypothesis is that the odd shaped mass of charge at the bottom
of the cloud produces a distribution of electric field around its perimeter
and that the first place that exceeds the local breakdown field for air
(between 300,000 and 1 million volts per meter depending on the pressure
and humidity.) causes a spark. Photography shows that the spark called a
step leader goes an average of 50 meters before stopping and pausing for an
average of 50 microseconds. During this pause charge from the cloud
redistributes within the cloud and along the step leader channel. When the
electric field once again increases to the required value another 50 meter
step leader spark is generated, this always happens from the end of the
first step leader. However the charge distribution in the cloud has charged
and so the new spark goes off in a new direction. That's why lightning is
jagged and turns.
I don't like the snow pushing analogy since the air resistance remains the
same, what happens is you get further from the ball of charge in the cloud
so the electric field (the push) decreases.
The usually negative charges in the lightning step leader as it approaches
the ground induces positive charges in the ground. The better conductor the
ground it the larger the induced charge. The closer the ground is to the
lightning channel the higher the electric field. So lightning will strike
highthings and/or conductive things.
Benjamin Franklins kite was not struck by a lightning bolt. The second
person to repeat this experiment Georg Richmann (The spelling is correct)
he had his kite struck by lightning and was killed.
Paul Doherty
Paul "But it is more complicated than that!" Doherty,
Senior Staff Scientist, The Exploratorium.
pauld@exploratorium.edu, www.exo.net/~pauld
This archive was generated by hypermail 2b29 : Tue Oct 16 2001 - 12:21:59 PDT