Steven Cummer is a professor of electrical and computer engineering at Duke University. Here he describes some of the unseen wonders associated with nature’s light show. Questions and answers have been edited.
Q. When there is a thunderstorm, we see the flashes of lightning, and we hear the thunder. But what else is going on?
A. Definitely a lot. Lightning moves and develops too fast for people to see all the details, but high-speed video cameras can slow it down and show what is really happening. Cloud-to-ground lightning starts as a thin, bright channel that exits the bottom of the cloud and then often splits into many separate branches. These channels move toward the ground at a speed around 50 miles per second. When one channel gets within 100 yards or so of the ground, a tall object on the ground – often a tree – will launch its own upward-moving lightning channel. When these two channels connect, a huge surge of electric current flows, making the bright flash and loud thunder we see and hear.
Q. So lightning can go up, not just down?
A. Yes, and there are a number of amazing things happening above thunderstorms that have only been discovered in the past 20 years. Very powerful lightning flashes create high altitude sparks called sprites that can be 20 miles in length. More rarely, storms can launch a lightning bolt that travels upward out of the cloud about 40 miles into the air – these are called gigantic jets. And then there is a poorly understood process in which a thunderstorm acts as a giant particle accelerator and shoots an intense beam of high-energy electrons, gamma rays and even anti-matter out into space. This is called a terrestrial gamma-ray flash.
Q. What do radio waves have to do with lightning?
A. In addition to producing bright light, lightning flashes emit very strong radio wave pulses that can give clues about what the lightning is doing. These pulses are often strong enough to be measured hundreds and even thousands of miles away from the lightning itself. Radio measurements are thus a great tool for remotely studying how lightning creates some of its unusual effects.
Also, the lightning channels we see are only the tip of the iceberg. There are many more lightning channels inside clouds that we can’t see clearly because the light can’t escape the cloud. But radio waves travel straight through clouds. By measuring lightning radio pulses with very precise timing from multiple locations, we can map in-cloud lightning channels as they propagate and expand. This is the only way to measure how lightning channels initiate, because they almost always start inside clouds.
Q. How is lightning different in different parts of the country and different parts of the world?
A. The basic lightning processes are the same everywhere. But the storms themselves can be much wider or taller in certain parts of the world, which makes them more likely to produce very powerful lightning and also some of the more exotic lightning-driven phenomena. For example, central Africa has huge storms that produce more sprites than anywhere else in the world. And, for reasons we don’t yet understand, most terrestrial gamma-ray flashes are produced by tropical storms close to the equator.