and a negative spark is launched from the lower cloud. The
descending spark creates a channel; upward positive sparks come
from the earth and when they meet, the stroke we see spreads in both
directions, superheating air and creating shock waves that produce
thunder. Lightning occurs when charges become strong enough to
overcome resistance in insulating air and current flows between the
two charges.
The
facts:
a.
Travels as fast as 100,000 miles a second.
b.
~ 50,000 degrees F [4 times hotter than the sun].
c.
Channels longer than 10 miles have been observed. Case
reports of "a bolt from the blue" where one may be
struck on an apparently clear day.
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d.
Lasts ~ .01 - .0001 of a second
e.
Potentials often as high as 10 million to 100 million volts.
f.
Current up to 50,000 + amps.
As the particles within a cloud (called hydrometeors) grow and interact,
some become charged possibly through collisions. It is thought that the smaller
particles tend to acquire positive charge, while the larger particles acquire more
negative charge. These particles tend to separate under the influences of
updrafts and gravity until the upper portion of the cloud acquires a net positive
charge and the lower portion of the cloud becomes negatively charged. This
separation of charge produces enormous electrical potential both within the cloud
and between the cloud and ground. This can amount to millions of volts, and
eventually the electrical resistance in the air breaks down and a flash begins.
Lightning, then, is an electrical discharge between positive and negative regions
of a thunderstorm.
A lightning flash is composed of a series of strokes with an average of about four.
The length and duration of each lightning stroke vary, but typically average about
30 microseconds. (The average peak power per stroke is about 10
12
watts.)
DESCRIPTION OF LIGHTNING DISCHARGE PROCESSES
W
ith the initial breakdown of the air in a region of strong electric fields, a streamer may
begin to propagate downward toward the Earth. It moves in discrete steps of about 50
meters each and is called a stepped leader. As it grows, it creates an ionized path
depositing charge along the channel, and as the stepped leader nears the Earth, a large
potential difference is generated between the end of the leader and the Earth. Typically, a
streamer is launched from the Earth and intercepts the descending stepped leader just
before it reaches the ground. Once a connecting path is achieved, a return stroke flies up
the already ionized path at close to the speed of light. This return stroke releases
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see end of syllabus for case report
