The most common type of lightning is the cloud-to-cloud type; only about 20 percent of all lightning strikes are directed toward the ground. But the cloud-to-ground variety is the one that affects people more often, so it receives the most study.
When lightning strikes sand or certain rock, the heat immediately melts and fuses the material it encounters, creating long underground glass tubes called fulgurites. These channels are usually from 0.5 to 2 inches across and can burrow as far as 20 yards into the ground. Fulgurites are fragile, and because they are difficult to remove from the soil, large specimens are rare.
For lightning to occur, there have to be areas of opposite electrical charges within a cumulonimbus cloud. How those regions develop and why they separate is an undiscovered secret, but one theory says that the collisions between small hailstones and ice crystals within a storm cause a positive charge in the upper regions while the lower and middle parts of the cloud gain a negative charge due to the influence of downdrafts and gravity.
Because unlike charges attract each other, the ground below the thunderstorm becomes positively charged, and as the cloud moves it drags this area of positive charge behind it. The resulting electrical field continues to build, but because the air between the cloud and the ground acts as an insulator, no current flows between them. If you measured the positive charges under the storm at this point (not recommended!), you’d find that protruding structures like radio towers, church steeples, and stubborn golfers have a stronger charge than the ground, making them a much more likely target for lightning.
Eventually this charge becomes so powerful that it overcomes the air’s insulating effect, and a bolt of lightning zaps out of the cloud. It’s over in less than a second, but in that time as many as ten separate lightning strokes can be generated. The fact that it looks like one continuous streak is because the human eye can’t register the distinct pulses fast enough; instead of separate flashes, your retinas see a flickering effect, because each pulse lasts only a few millionths of a second.
Lightning is very difficult to study. Not only is it extremely short-lived, but nobody wants to get too close to a billion volts of electricity. Imagine for a moment that you could slow down the passage of time, so that microseconds lasted for minutes. You’d be able to clearly see each phase of a lightning bolt as it developed, and what you saw would probably surprise you.
The first thing you’d see in a cloud-to-ground flash is a barely visible streamer of lightning, called a stepped leader, emerging from the cloud. The stepped leader travels the distance of about a city block in a microsecond and then pauses for up to fifty microseconds to decide where to go next. If there’s a stronger electrical field in a different direction, the leader will change course and head toward it, creating a crooked appearance. This stop-and-start process continues until the leader gets close to the ground, when it will often branch into several forks.
As it gets closer to the ground, the leader induces a rapid increase in the strength of electrical fields on the ground (around ten million volts’ worth), especially in taller objects. Suddenly another bolt of lightning jumps from the ground up toward the stepped leader, completing a cloud/ground electrical circuit.
Although the stepped leader had to go through the trial-and-error process of finding the best path to the ground, the return stroke doesn’t have that limitation. Taking advantage of its prefab pathway, it leaps back toward the cloud with a flash as bright as a million light bulbs, making the trip all the way up to the cloud and back down as many as ten times in a fraction of a second.
The pencil-thin bolt instantly heats the air in this corridor to 54,000°F—hotter than the surface of the Sun—and since heated air expands, a shock wave explodes outward from the lightning channel at the speed of sound, creating a blast of thunder. The bright flash caused by the bolt is expanding outward too, but at the speed of light (186,000 miles per second), a million times faster than the thunder. The flash reaches us almost instantly, while the thunderclap moseys along toward our eardrums at a leisurely 1,125 feet per second or so. If the lightning strike is more than about 15 miles away, you probably won’t hear the thunder, since sound waves generally curve upward around thunderstorms.
Most often there will be at least four distinct bolts traveling through the channel before the strike is over. Stepped leaders that form after the first one are called dart leaders, and they’re usually less powerful than the initial bolt. Dart leaders that depart from the original channel on their way back to the ground will give the lightning stroke a forked appearance. If the wind blows the lightning channel sideways at high speed, ribbon lightning results. It is as if the moving corridor creates a luminous strip in the air.
Just because you’re quite a distance from a thunderstorm doesn’t mean you should let your guard down. Cumulonimbus clouds can spawn “positive giants”: lightning strikes that come from the storm’s anvil-shaped head and can blast outward for up to 20 miles.
Other forms of lightning include heat lightning, which is just ordinary lightning seen from a great distance, and sheet lightning, which is lightning striking within a cloud or from one cloud to another. When the actual bolt is obscured by the cloud, it lights up the entire structure, momentarily making it look like a huge white sheet.
Sometimes the positive charge that accumulates on flagpoles, ship masts, and other high points doesn’t provoke a lightning strike, but instead appears as a halo of sparks or a weird glow around the top of the object. That’s called St. Elmo’s fire, named after the patron saint of mariners. The effect is caused by charged plasma called a corona discharge and is similar to the glow given off by a fluorescent light.
St. Elmo’s fire isn’t really lightning, but at least the basic electrical phenomenon that causes it is understood. Not so with the phenomenon of ball lightning, which continues to confound scientists. About as far from split-a-tree-in-your-backyard lightning as you can get, ball lightning is a true original. Although it has never been photographed, so many people have reported seeing the phenomenon that there’s little doubt of its existence.
Most observers describe ball lightning as a luminous red, orange, or yellow sphere floating along in the air a few yards or less from the ground, often near a thunderstorm. Some people hear a hissing sound or smell an odor like ozone. The spheres range in size from a few inches to a few feet, although most are in the 4- to 8-inch category and last only a few seconds.
Theories on the nature of ball lightning are a dime a dozen, ranging from plasma suspended in a magnetic field to swamp gas ignited by a lightning strike. One recent theory supposes that when lightning strikes certain types of soils, the ground can vaporize, creating hot gases that “burp” out of the soil and then condense into tiny, electricity-conducting wires that form a glowing sphere.