High powered lightning discharges are happening somewhere on Earth 100 times every second - 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.
Also see: Terrestrial gamma-ray flashes