There is something about the winding ray that involves random chaos. However, blue screws not only reach the same places regularly, but successive discharges often reuse the same channel.
It was never clear how the path laid by a screw extends to repeat performances, but new research uncovered persistent charge pockets following a single lightning strike, which could provide a map for more to follow.
An international team of physicists has collected an unprecedented level of detail about radio waves emitted by lightning to determine why the pockets of charged air defining the lightning path behave the way they do.
Using a radio-telescope network called the Low-Frequency Array, or LOFAR, the researchers were able to gather data that offered nanosecond resolution of attacks over an area of several thousand square kilometers.
"These data enable us to detect the propagation of lightning on a scale where, for the first time, we can distinguish primary processes," says physicist Brian Hare of the University of Groningen in the Netherlands.
"In addition, the use of radio waves allows us to look into the thunderstorm cloud, where most of the radius resides."
Despite all its impressive flashing and booming, the thunderbolt is really just a grandaddy electric spark caused by a difference of positive and negative charges.
These opposing charges are separated by streams of air rotating around pieces of hail called graupel, causing them to physically strike and grind their electrons in a passing game of the pack.
A stable buildup of hundreds of millions of volts can arise in and between separate clouds, or between a cloud and the ground. Wherever it is formed, however, lightning has the opportunity to jump, but only if the conditions are right.
What we see as the zigzag screw is merely the end of a complex process that we are still gathering.
The first step involves the formation of a small plasma bag – a bubble of heated gas composed of charged particles. This tiny lightning seed branches in many directions, with one or more forming a mile-long channel that acts like a giant wire hanging from the sky.
The ends of this channel, called leaders, can be positive or negative, each of which moves in unique ways depending on its charge.
Negative leaders tend to move in a discontinuous fashion called step by step, producing a high frequency radio signal as they jump. Positive leaders do not take the same path, so do not produce the same signal as you grow. Yet its channels still buzz with a distinct pattern in radio waves.
These contrasting signals provide researchers with insight into the rapid generation of lightning, from the growth of the plasma channel to the light show at the end.
One curious observation made in the past was that positive leaders can separate from their plasma channel. No one knows why this division occurs, mainly because most of the studies so far have not had the necessary resolution.
The vast array of antennas that make up the LOFAR gives researchers exactly what they need to focus on the details of a branched plasma channel.
"Near the central LOFAR area, where the antenna density is highest, the spatial accuracy was about one meter," says Groningen University physicist Olaf Scholten.
With such a good level of detail, the researchers were able to map the dynamic changes that occurred on the plasma channels as they approached, and this allowed the team to make a rather strange discovery.
As leaders arrive in areas with a sufficient voltage difference, electrons pass through the plasma, burning the air at warmer temperatures than the surface of the sun.
It turns out that not all of this current reaches the same end points. Some residual load leaks through interruptions in the main discharge channel, hanging from small, thin structures called needles.
"This finding is in stark contrast to the current image, where the charge flows along the plasma channels directly from one part of the cloud to another, or to the ground," says Scholten.
"These needles can have a length of 100 meters and a diameter of less than five meters, and are very small and very short for other lightning detection systems," adds Hare.
If the voltage difference increases again in the cloud over a relatively short period of time, these charged needles can provide a map for further attacks, explaining why we often see cat-like lightning striking the same mark over and over again.
The clip below shows a blow forming in slow motion with each yellow dot depicting a radio pulse describing the path of the pending discharge. At the top of the clip, you can see positive leaders glistening when they find needles from a previous screw.
"From these observations we see that a part of the cloud is recharged and we can understand why an electric discharge in the ground can repeat itself a few times," says Hare.
For such a primitive force that we marvel at forever, it's amazing to think that we're still learning a lot about how lightning works.
This research was published in Nature.