It has long been thought that the run-of-the mill thunderhead lacked sufficient energy to create the stupefying amount of power found in the lightning they produce. After all, a common bolt of cloud to ground lightning is as hot as the sun's corona, carries a current of 30 kiloamperes, transfers a charge of 5 coulombs, has a potential difference of about 100 megavolts, and dissipates 500 megajoules. That's a lot of juice for an uncompressed mist.
In 1992 Alex Gurevich proposed that lightning is the spawn of interstellar radiation. The theory was widely dismissed by the scientific community, but in the time since it has come more and more into favor. Partly because there has been no suitable scientific theory, and partly because new observations fit correctly into the theory.
Various agencies studying lighting discovered in 2002 that lightning produces a large pulse of x and gamma rays. This was a surprise since conventional wisdom said that the atmosphere is too dense to allow for electrons to accelerate to speeds high enough to create those high-energy emissions. It was perplexing, but the much older proposition of Runaway Breakdown explained it eloquently.
In a nutshell, Runaway Breakdown takes into account that the Earth is under perpetual assault by Cosmic Rays, a very high energy that roves the cosmos. Most of the time this energy has no effect on terrestrial life, including clouds, most being too diffuse for the radiation to care about. The difference comes in the thick, heavy, wet clouds that make up a thunderhead. The dark foreboding weather front a person sees bearing down on him on a warm summer's eve is only the tip of the proverbial iceberg. The thunderhead reaches up to 14 miles above sea level. The giant mass of unstable air and moisture acts somewhat like a solar cell to the cosmic rays, but lacks the power to trap the energy.
The high energy Cosmic Rays smack into air or water molecules in the conundrum, and cause the molecule to release a high energy electron. That electron has a reach and power usually unavailable in the atmosphere, and that electron is prone to hit another molecule which will release one or more electrons in turn. This cascading breakdown is somewhat analogous to a nuclear reaction in an open environment. but where a nuclear core must be cooled, inside a thunderhead the energy is dispersed when there are enough long-trajectory electrons created, and they conglomerate into a bolt of conductive plasma--lightning.
There are still some difficulties with the Runaway Breakdown theory, namely that the events described therein are exceedingly difficult to observe since storms rarely make appointments to appear at a designated time and place for observers. This isn't to say it's not provable, but like may things, it will take time. As Benjamin Franklin, the father of meteorology and electrodynamics said, "He that can have patience can have what he will."