Lightning Origins Weather
What determines why lightning originates on
the ground or in the sky?
"The lightning discharge", which is the more precise term for
lightning is very complicated. Ulum defines the term as: "... the
transient, high-current electric discharge whose path length is
measured in kilometers." So it is an electric plasma (to use the
physicist's jargon). There are several categories of cloud-and-earth
discharges depending upon the direction of propagation (cloud-to-earth
or earth-to-cloud), the sign of the charge (positive or negative).
Cloud-to-ground is the most common. It is initiated by a downward
moving negatively charged leader, which is a partial breakdown of the
air that sets up a channel for the return stroke. This return stroke
travels from ground-to-cloud at 1/3 or greater times the speed of
light, but the speed is not constant. It decreases with altitude.
The current may be of the order of 100 amps, and a single return
stroke lasts for about 0.1 sec. Discharges may occur in clumps taking
different paths, and these successive return strokes can be
There are at least 2 theories of the generation of cloud dipoles,
that is, charge separation that sets up the necessary conditions for a
lightning discharge. (1) The precipitation theory posits that
falling raindrops in the thunder cloud and warm rising air charges the
drops negatively and charges the rising warm air positively. Thus
charge, produced by the friction is separated and the cloud becomes a
dipole positively charged at the top and negatively at the bottom. (2)
The convective mechanism claims that layers of charged air are move
"in bulk" to their observed location in the cloud or on the ground.
The pros and cons of these mechanism are beyond the scope of this
Being transient, and unpredictable lightning discharges are
difficult to study. Furthermore, until recent satellites were put in
orbit, it has not been possible to look at the "top" of a
discharge. And even now measuring what is going on inside the
cloud is problematic. The inside of a thunder cloud is not an
environment you want to stay in very long. Ultra high speed
photography and spectroscopy have provided valuable tools, but in the
case of both these methods you have to be looking in the right place
at the right time.
Probably the definitive modern book (2001) on the subject is: "The
Lightning Discharge" by Martin A. Uman. It is fairly readable and
thoroughly documented. However, I came away with more questions than
answers which speaks to the complexity of the subject, not anything
negative about the author.
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Update: June 2012