Maybe it’s because, in the last couple of years, I’ve taken up golfing, but I’ve become increasingly fascinated by lightning. (There’s something about standing in the open holding a metal rod that’ll do that to you.)
I’m certainly not alone. Lightning has fascinated people throughout history. It took Benjamin Franklin, though, to demonstrate its electrical nature, through his famous (if somewhat foolhardy) experiment of flying a kite in a thunderstorm.
Since Franklin’s time, we’ve learned a lot more about lightning, though certainly not everything. We have a pretty good grasp of what it is, but not what causes it.
What it is is a massive but short-lived electrical discharge in the atmosphere, usually several kilometres long–in fact, sometimes as much as 150 kilometres long. Although most commonly associated with thunderstorms (obviously), lightning has also occurred in snowstorms, sandstorms, the clouds of ash over erupting volcanoes, and even from the clear blue sky.
Although we normally think of lightning as striking from the sky to the ground, in fact it can also occur within a cloud, between clouds, or between a cloud and the air around it.
Actually, that whole business of lightning “striking the ground” is misleading, because in many ways it’s the ground striking the sky, although the sky initiates the process.
Lightning arises because of a charge separation in a cloud. A “charge separation” just means that there are more electrons in one place than another. Cloud-to-ground lightning occurs when there are lots of free electrons in the base of the cloud. These electrons are discharged in what is called a stepped leader: “stepped” because it descends from the cloud in discrete steps, each about 50 metres long (which is what gives lightning its jagged appearance), and “leader” because that’s what it is–the precursor for the main bolt.
Since electrons are negatively charged, this stepped leader has a very strong negative charge. When it gets within 100 metres or less of the positively charged ground, a leader moves up to meet it, often through handy protruding objects like buildings, trees and golfers.
What we think is the main stroke of lightning is actually what is called the return stroke, which propagates upward from the ground along the path formed by the leader and the stepped leader. Several subsequent strokes usually follow along the same path–the same path, because the stepped leader knocked electrons loose from molecules of atmospheric gas along the way, creating a channel of positively charged air, a path of least resistance for the return stroke. All these strokes come and go with the space of a second, traversing the distance between cloud and sky at up to half the speed of light, until the surplus of electrons in the lower part of the cloud is eliminated. And this awe-inspiring phenomenon is anything but rare: it happens as many as 100 times per second in a large thunderstorm.
The big question in all of this is exactly how the original charge separation arises. One school of thought holds that large bits of precipitation such as hailstones and raindrops (collectively called “hydrometeors”–next time it rains, tells someone we’re undergoing a “hydrometeor shower”), which naturally descend to the lowest part of clouds because of their weight, are charged differently than smaller particles such as cloud droplets or ice crystals. Another theory holds that the small particles are the principle charge carriers and charge separation occurs because of the varying air motions in the cloud. There’s even a relatively new theory that holds that charge separation occurs because the top of the cloud is being bombarded with high-energy particles produced by cosmic rays.
Whatever, the charge separation happens, the lightning follows, and down on the ground, as often as not, we’re left holding the bill. And quite a hefty bill it is, too, including between 100 and 200 people killed every year across North America and many more hundreds injured (in the U.S., the death rate from lightning is higher than that from tornadoes), hundreds of millions of dollars in property damage and thousands of forest fires.
Lightning is dangerous because of the enormous power it packs into each bolt: anywhere from 10 million to two billion volts, at anywhere from a few thousand to 300,000 amperes (household wiring, by contrast, typically carries only a few tens of amperes). This understandably produces a lot of heat–so much heat that the air along the lightning’s path is superheated to 8,000 degrees Celsius. This air expands explosively, creating a shock wave that we hear as thunder–a short, sharp clap if the path of the lightning is short and direct, or a long, rolling rumble if the path of the lightning is long and varies in distance from our ears.
This same effect is why people who are struck by lightning often have bits of their clothing blown off: the sweat on their body superheats and explodes into steam, literally “blowing their socks off.” Of course, it has other effects, too. Those who die from lightning (about one out of three victims) suffer heart failure. Those who survive often experience after-effects including memory loss, muscle aches and pains and central nervous system damage, because the bolt literally cooks nerves and blood vessels. It can cause brain damage or cataracts, rupture eardrums, break bones and sizzle skin. One man compared it to being in a microwave oven. (Saskatchewan, by the waym, has the highest per capita rate of lightning deaths in Canada, probably because we have so many wide-open spaces and also some of the world’s most severe thunderstorms.)
It used to be that lots of Saskatchewan buildings, especially in rural areas, had lightning rods, a device first suggested by Benjamin Franklin in 1749 as protection against lightning, the theory being that they provide a safe path for the discharge. They work, too, although not perfectly. Lightning sometimes seems to have a mind of its own and strikes where it wants to.
In the last few decades, several attempts have been made to divert lightning using lasers, on the theory that you could use a laser to ionize air, forming a channel for the lightning to follow that would take it away from things you don’t want hit: airports, transformers, nuclear power plants. Until recently, those attempts have been very unsuccessful. Now, however, a group of scientists headed by Jean-Claude Diels, a professor of physics and astronomy at the University of New Mexico, and Xin Miao Zhao, a researcher at Los Alamos National Laboratory, have come up with a ultraviolet laser that holds promise of doing the trick–at least, in the laboratory, it has been able to trigger an electrical discharge between two highly charged electrodes 25 centimetres apart. They’ve now built a much more powerful laser and soon hope to begin attempts to trigger actual lightning.
Meanwhile, other scientists are studying a whole new form of lightning–or rather, a form we’ve just now begun to realize exists. For years, observors, especially pilots, have occasionally reported seeing very strange flashes of light above thunderstorms, in the upper atmosphere. These reports were generally discounted until, in 1990, John R. Winckler and colleagues at the University of Minnesota captured one with a video camera.
It turns out that lightning not only strikes between a cloud and the ground, but between a cloud and the highly charged ionosphere, which gets its charge from the ultraviolet rays striking gas molecules and knocking loose electrons. This lightning above the clouds–as much as 90 kilometres above them–is quite different from the lightning we’re familiar with. Because the air is thinner, fewer gas molecules get heated up by the electrical discharge, and different colors appear: red, for instance. This high-level lightning comes in a variety of shapes, two of which scientists, in a rare burst of whimsy, have dubbed elves and sprites (the other two are called blue jets and gamma-ray events, in rather jarring contrast).
Scientists are continuing to gather data on this high-level lightning, just one more example that there are still more things in heaven and earth than are dreamt of in our philosophy.
Next time you’re caught on a golf course in a thunderstorm, give lighting the awe and respect it deserves–and get the heck out of there!
