High powered lightning discharges are happening somewhere on Earth 100 times every second.

A possible mechanism for the very substantial electrical charges within the clouds was put forward in 1978. Following lab-based experiments, it was suggested that the charges arise from the static-electric interactions of graupel (slush) and ice crystals moving within the cloud (due to convection currents caused by widely differing air temperatures) .

See : Journal of the Atmospheric Sciences Volume 35: Issue 8.

Since then however, tests performed on atmospheric thunder clouds have shown that the charges are far higher than would be expected - as yet there are no agreed explanations as to how or why.

One mystery is how thunderclouds become so highly charged. The best explanation is that collisions between small ice particles and heavier gobs of slush called graupel tend to transfer electrical charge, but the role of this process in real clouds is not proven.“

An even bigger puzzle is how the huge current of a lightning bolt ever begins to flow when air is an electrical insulator. It is possible to make air break down to form a conducting plasma, but this requires a fearsomely intense electric field of more than a million volts per metre. Although meteorologists have sent hundreds of instrument-laden balloons and rockets into thunderclouds to test local conditions, the strongest fields they have seen are only about a tenth of that critical value.”

Source : New Scientist, Feb. 2012 Strange skies: Lightning should be impossible

Although we've been sending balloons and aircraft into lightning-charged thunderstorms since the 1950s, we haven't observed that 3 million volts per metre electric field needed to cause breakdown. Instead, the field is typically 10 times weaker than the ones we generate [by walking] on deep pile carpets“

Source : New Scientist, 15 April 2017 'Bolt from the blue: Lightning doesn’t form like we thought'

Figures for the apparent (lack of) voltage anomaly are provided in a 2016 paper for the Journal of Geophysical Research - see:Positive streamer initiation from raindrops in thundercloud fields

The threshold field for the electric gas discharge in air is ≈26 kVcm−1atm⁻¹, yet the maximum field measured (from balloons) is ≈3 kVcm−1atm⁻¹. The question of how lightning is stimulated is therefore one of the outstanding problems in atmospheric electricity.”

The authors conclude that other as-yet-undocumented mechanisms may be operating :

Such mechanisms could be electric field space variations via collective effects of many hydrometeors or runaway breakdown.“

The possibility that high energy electrons (linked to cosmic rays originating in deep space) may trigger (or facilitate) the path of lightning strikes is still being debated.

Notes:

[1] The World Wide Lightning Location Network (WWLLN) publishes daily updated maps of major lightning hits across the world.

[2] Lightning discharges also commonly occur near volcanic eruptions. It's thought that ice crystals may also play a part in the generation of the electricity. Other theories include colliding dust particles, radioactive effects, rock fragmentation, etc etc. See Wikipedia

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