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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 milliseconds apart.

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 website.

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.

Vince Calder

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