Read more: 鈥�Strange skies: Seven wonders of the atmosphere鈥�
Lightning is almost as puzzling to modern science as it was to our awestruck ancestors. Although we might now be confident that lightning is not a weapon wielded by the gods, the true mechanisms behind it are not well understood.
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.
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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.
Perhaps lightning needs some kind of catalyst to let fly? One theory is that cosmic rays are involved. These charged particles are probably generated by supernova explosions far away in the galaxy. A cosmic-ray proton can have enough energy to generate a cascade of relativistic particles when it collides with a molecule in the atmosphere. This cascade ionises the air, producing a conical shower of free electrons where a current might begin to flow.
鈥淧erhaps lightning needs some kind of catalyst to let fly? Cosmic rays, charged particles from supernovae, could be involved鈥�
Lightning often produces flashes of X-rays and gamma rays, and even beams of antimatter. These phenomena imply that some relativistic process is involved, but they don鈥檛 prove that cosmic rays are the trigger.
To find out whether supernovae really are implicated, meteorologist at the University of Oklahoma is collaborating with a team of physicists to develop a ground-based monitoring system. 鈥淲e are putting out cosmic-ray detectors along with our lightning mapping array to see if they coincide,鈥� he says.
Even if they do, the puzzle will not be solved. The electrons liberated by a cosmic-ray shower are free for just a few microseconds 鈥� not long enough to maintain a large current. That is long enough, though, to boost the electric field in a cloud to a few hundred kilovolts per metre, says Beasley鈥檚 colleague Danyal Petersen at the University of Nevada, Reno. A strong field might allow another process to kick in, as it can stretch raindrops inside a cloud into pointed, needle-like shapes. Like the point of a lightning conductor, these rain needles could enhance the local electric field, forming coronas of charged air. These could spread and merge, eventually forming an ionised path called a leader that can carry the full fury of a lightning bolt. Measuring the sequence of radio frequencies emitted by the whole process could test this theory.