What is lightning?

A stepped leader descends from the cloud in steps, branching as it goes, carrying charge downward along a channel of partly ionised air. As it nears the ground, an upward connecting discharge of opposite polarity rises and meets it, typically about thirty metres up.

When they meet, the return stroke propagates back to the cloud at about a third of the speed of light. Peak currents in return strokes to negative leaders are typically about 30,000 amperes, and the channel reaches about 30,000 degrees Celsius. A typical flash to the ground contains three or four leader and return-stroke sequences in rapid succession.

All of this has been measured. The engineering follows: a lightning rod protects a structure by intercepting the current and diverting it into the ground.

Lightning emits bursts of X-rays. Measurements beside five natural strikes in Florida caught them about a millisecond before the first return stroke, during the stepped-leader phase, at energies up to a few hundred thousand electronvolts, in discrete bursts coinciding with the formation of the leader steps. Thunderstorms also produce flashes of gamma radiation energetic enough to be detected from orbit, and those were found by a satellite instrument built to watch for gamma-ray bursts in space.

That means electrons inside thunderclouds are being accelerated to relativistic energies, by runaway processes of the kind Wilson suggested in the 1920s.

The gamma-ray flashes were first reported in 1994, from a satellite instrument built to watch for cosmic gamma-ray bursts. The bursts it kept recording were coming from below it.

At least a dozen had been recorded in two years. They were brief, they looked like bremsstrahlung from electrons at millions of electronvolts, and the discovery team placed the source above thirty kilometres, above the storms.

Later spectra, fitted with simulations, brought it down to between fifteen and twenty-one kilometres, low enough to point at the thunderstorms themselves. Radio measurements since suggest the flashes come early in lightning that stays inside the cloud. The gamma-ray burst monitor on the Fermi space telescope has recorded photons from them at around thirty million electronvolts.

The discovery paper called them "an unexplained terrestrial phenomenon". They were found by an instrument watching the whole sky for something else, which noticed what arrived from the wrong direction.

The fields measured inside thunderclouds are lower than most of the proposed mechanisms require. Something must locally exceed the threshold or lower it.

The leading proposals are that water drops and ice particles concentrate the field at their surfaces, or that the steady background of cosmic rays seeds an avalanche of high-energy electrons that runs away and amplifies the field until conventional breakdown can happen.

They are not equally supported. Interferometry of the first microseconds of a flash finds a burst of fast positive breakdown that looks like streamers started by corona on ice crystals or drops, and the same group reports little or no evidence that electron avalanches are involved at all. A follow-up in Florida by several of the same researchers, with an improved interferometer, found fast breakdown that can also begin with negative polarity, which its authors say does not fit current understanding of how air breaks down. Neither account is settled, and the measurement that would settle it has to be made inside a thundercloud.

The charge comes from collisions. A developed thundercloud carries ice crystals in its upper levels and a mixture of ice and small hail in the middle, and collisions between them in the rising air leave the lighter ice crystals positively charged and carried upward while the heavier hail takes the negative charge and is held up by the rising air or falls. That is what separates the top of the cloud from the bottom.

What decides which way the charge goes is still being worked out. Laboratory experiments find that it depends on the temperature and on how much supercooled water the hail is collecting, and they disagree about where the sign reverses.