Dramatic lightning before the launch of STS-8
NASA photo - Marshall Space Flight Center
The few lightning storms that we had in San Francisco when I was a kid were frightening spectacles that caused me to dive under my covers in bed. As I got older, fear gave way (partially) to curiosity. How did the lightning work? I had heard that the flash of lightning happened when two clouds bumped together. But even at a tender and impressionable age, that explanation seemed pretty silly. During most lightning storms the sky was completely covered with clouds, and I never saw lightning when there were only individual clouds in the sky. Something else was at work here.
To understand lightning I had to learn about the nature of electricity: what it is, and how it moves. I found out that everything in the world is made of electricity and that electrical forces are responsible for holding things together--and sometimes for pushing them apart.
Everything is made of atoms that are, in turn, made out of charged particles. All charged particles come in one of two types: positive and negative (or plus and minus). The minus particles are the electrons, and the plus particles are the much heavier protons which are buried deep in the nucleus. (There are also heavy neutral particles in the nucleus called neutrons, but they really don't enter into our story.)
When you bring two charged objects near each other, something happens. If two charges have the same sign (plus and plus, or minus and minus), they will repel each other. Two charges with opposite signs will attract. The size of the electrical force depends on two things; how highly charged the objects are, and the distance between them. The more charge there is, the more force each charge feels. As the distance between the charges is increased, the force quickly becomes smaller. The force of gravity, a different kind of attractive force, is billions of billions of times weaker than the electrical force.
One of nature's displays of electrical forces is the lightning storm. A lightning stroke is a brief but large current of negative charge that travels from cloud to ground along a "wire" of air molecules that have been ionized or ripped apart.
Inside a thunderhead, electrical charges become separated. Warm updrafts sweep positive charges aloft, leaving the bottom of the cloud negatively charged. The attraction between the ground and the negative charges in the bottom of the cloud creates the lightning stroke, a brief current of negative charge that travels from cloud to ground.
The awesome power of the lightning stroke originates in the thunderstorm cloud where charges somehow become separated. There are several complicated theories that try to explain the actual mechanism of this charge separation, but no one really knows what pulls the charges apart in a thunderstorm cloud. It is believed that somehow water drops in the cloud become negatively charged and, being heavier than the surrounding air, fall to the bottom of the cloud. Meanwhile, the positive ions left behind are swept upward to the top of the cloud by the warm updrafts within the thunderhead. As more and more charges separate, parts of the cloud become so highly charged that the electrical forces tear nearby air molecules apart, making more charged fragments.
Since the ground beneath the cloud has far fewer negative charges on it than the bottom of the cloud, there is an attraction between the ground and the bottom of the cloud. Therefore, any electrons liberated near the cloud are pulled down toward the ground. As these electrons move, they bash into air molecules that are in their way, breaking the molecules up and creating more charged fragments. All the new negative fragments are dragged downward along with the original electrons and we have the makings of an electrical avalanche.
The avalanche would continue unabated were it not for the heavier and more sluggish positive charges that are left behind. They tend to attract the accelerating army of electrons back toward the cloud. But more electrons are continually being liberated up in the cloud, and they stream to the rescue of the slowing electrons below, reinforcing their race downward. This process of electrons slowing and then being rescued by reinforcements repeats itself over and over again. The initial party of electrons makes its way in jerky 150-foot steps along a sinuous path toward the ground.
This initial exploratory mission forms what is called a "stepped leader," named for its start-stop motion. The stepped leader takes about 5/1000 of a second, moving at about 240 miles per second, to reach from cloud to ground. When the leader gets near the ground, it may draw a stream of positive charges (called a streamer) up from the ground to meet it. When either the stepped leader reaches the ground or a streamer runs up to join the stepped leader, an electrical connection is completed between the cloud and the ground. The ionized air molecules of the leader conduct electricity quite well, and the path of charged particles acts as a wire, connecting the highly negative cloud and the positive ground. This ionized air becomes the path of the main bolt of lightning.
The first charges to feel the connection are those near the ground. The light and mobile negative charges quickly accelerate along the wire of ionized air. In their mad rush to the ground the negative charges collide with the air, causing it to glow like a neon sign--only thousands of times brighter and with a bluish-white color. The air near the ground is the first to start glowing, but as the electrons further and further up feel the connection and begin to accelerate, the air further and further up also starts to glow. Even though the negative charges all move from cloud to ground, the bright flash of lightning moves from ground to cloud in a speedy 1/10,000 of a second, moving 61,000 miles per second! The super-heated air expands outward explosively, producing the shock wave we hear as thunder. The bright flash of glowing air is called the return stroke since it moves from ground to cloud, opposite to the moving charges.
The return stroke discharges a region of the cloud, but the cloud can reorganize quickly and as many as 40 strokes have been observed to use the same charged channel. If you've been told that lightning never strikes twice in the same place, don't believe it! Lightning usually strikes more than once!
There is quite a lot of energy in a lightning stroke, about 250 kilowatt-hours. At the current cost of energy, this would be about $16.75 worth. Doesn't sound like much, but with that amount of energy, you could lift a 2000-pound car 62 miles high!
Lightning doesn't always travel from cloud to ground. If two parts of the cloud are charged highly (and oppositely), a lightning bolt can actually occur inside the cloud. Lightning can also arc from one cloud to another.
The typical type of lightning is called streak lightning, or forked lightning. (Photo to the right from NASA - Marshall Space Flight Center.) If the lightning channel is blown by the wind during a multiple discharge, each succeeding stroke is displaced by a short distance, making it appear as ribbon lightning. On rare occasions the lightning seems to break up into beads that persist for as long as one second, an unexplained form called bead or chain lightning. Sometimes the lightning flash is obscured by clouds, which are then brightly illuminated. During this sheet lightning, the flash seems to come from everywhere. The most controversial form of lightning is ball lightning. Ball lightning has never been observed scientifically and many doubt its existence altogether. It is reported to occur with or right after a nearby lightning stroke and is described as a luminous ball of light that floats along fences, rooftops, or through the open air. The jury is still out on ball lightning.
Lightning seems to strike some objects more than others. In New York, the Empire State Building is a favorite target. Tall trees are also preferred. In general, tall objects have a higher probability of being hit. Why is this? In simple terms, the tall object brings the ground closer to the cloud. The leader, in seeking the easiest path, will naturally head for any piece of elevated ground. Having observed this tendency with his famous kite-flying experiments, Benjamin Franklin decided that he would mount a metal rod on the highest part of his roof. From the rod on the roof he ran a heavy wire to another metal rod that he had driven deep into the ground. He figured that if lightning were to strike his house it would most likely strike the highest point, the metal rod, and the wire would safely conduct the electricity into the ground through the metal stake. He was right. His invention of the lightning rod has saved millions of dollars and thousands of lives ever since.(Photo to the left from NASA - Marshall Space Flight Center.)
In a thunderstorm, the safest place to be is inside a large building equipped with lightning rods. A vehicle such as a car provides complete protection by surrounding you with metal, which will conduct the lightning's charge safely to the ground. But it you are caught outside, don't stand under a tree. The tree acts just like a lightning rod and if you become part of the conducting path to the ground, it's good-bye you. Even if the current from the lightning stroke doesn't hurt or kill you, the tree might. When the current of a lightning bolt passes through a tree, the sappy interior can be heated to the boiling point, and the tree can explode! When lightning strikes the ground, the charges flow outward along the ground. If you are standing nearby with your feet apart, the current will flow up one leg and down the other, possibly killing you. Many cattle are lost to lightning because they can't keep their feet together. If they did, they'd lose their balance.
So, now I know a few things about lightning. But the thought of all those charges rushing to and fro, although interesting, does not make the spectacle less impressive or less frightening. Now I sit up and watch the light show, but still feel that urge to dive for the covers.
Don't believe the old adage that lightning never strikes the same place twice. Former Park Ranger Roy "Dooms" Sullivan never did. According to the Guinness Book of World Records, Sullivan bas the dubious distinction of being the most lightning-struck person ever recorded. Between 1942 and his death in 1983, Roy Sullivan was struck by lightning seven times. The first lightning strike shot through Sullivan's leg and knocked his big toenail off. In 1969, a second strike burned off his eyebrows and knocked him unconscious. Another strike just a year later, left his shoulder seared. In 1972 his hair was set on fire and Roy had to dump a bucket of water over his head to cool off. In 1973, another bolt ripped through his hat and hit him on the head, set his hair on fire again, threw him out of his truck and knocked his left shoe off. A sixth strike in 1976 left him with an injured ankle. The last lightning bolt to hit Roy Sullivan sent him to the hospital with chest and stomach burns in 1977. Sullivan could never offer any explanation for this strange and unwelcome electrical attraction.
In ancient Rome, members of the College of Augurs divined the will of the gods by observing the southern sky for lightning, birds, and shooting stars. A lightning bolt passing from left to right was a favorable omen; a lightning bolt passing from right to left was a sign that Jove did not approve of current political events. Furthermore, whenever the augurs reported any sign of lightning, the magistrates of Rome were required to cancel all public assemblies on the following day. The augurs' reports became politically useful to postpone unwanted meetings, delay the passage of laws, or prevent the election of certain magistrates by popular assemblies.
In medieval Europe and England, ringing the church bell could be a hazardous occupation. During thunderstorms, it was general practice to ring church bells violently in an effort to keep the lightning from striking the tall church spire. Some felt the clamor of the bells dispersed evil spirits that sought to destroy the church with fire; others claimed that the noise of the bells disrupted the lightning strokes. (The second reason explains the common inscription on medieval bells: Fulgura Frango. which means "I break up the lightning flashes."). During the years from 1753 to 1786, lightning struck 386 French church towers. Lightning running down the bell ropes killed 103 French bell ringers. In 1786, the French government finally outlawed the custom.
During the eighteenth century, church vaults were often used to store large quantities of gunpowder. The combination of a high steeple and explosive contents often proved dangerous. In 1769, a lightning bolt struck the tower of St. Nazaire in Brescia, where 100 tons of gunpowder were stored. The resulting explosion destroyed one-sixth of the city and killed 3000 people. Lightning-induced explosions of stored gunpowder continued through the 1800's. As late as 1856, lightning struck the church of St. Jean on the island of Rhodes, the powder stored in the vaults exploded, and 4000 were killed.
In 1753, Benjamin Franklin published a description of the first lightning rod in Poor Richard's Almanac. From that publication onward. many so-called Franklin rods were installed on buildings in the American colonies. In 1760, a Franklin rod saved a house in Philadelphia from damage from a direct lightning stroke. By 1764, the rods were quite common on houses and churches. The lightning rod was first used in England in 1760 on the Eddystone Lighthouse, a wooden structure that had been previously destroyed by lightning.
Despite the success of Franklin's device, some viewed it as a hazard, claiming that the pointed rods favored by Franklin actually attracted lightning strokes to a building. These scientists advocated blunt-ended lightning rods. which. they felt, would conduct away any lightning that did strike, yet would not attract lightning to the building.
The debate over pointed versus blunt rods became a question of politics, rather than science. King George III favored blunt-ended rods. identifying pointed rods with the rebellious American colonies. This political consideration caused the East India Company to remove the pointed rods from their powder magazines in Sumatra, one of which was subsequently destroyed by a lightning bolt.
Harris, Jack. "Fire of the Lord." New Scientist, 20/27 December 1984.
Lansford, Henry. "The Global Circuit." Mosaic. May/June 1983.
Viemeister, Peter E. "The Lightning Book." Cambridge, MA: MIT Press, 1972.
At the Exploratorium, exhibit s are grouped together in sections that are labeled by overhead signs. To help you find exhibits related to the articles in this issue, we have noted in parentheses the section name or, when an exhibit is not associated with a section, the exhibit location.
Similar electrical charges repel each other, forcing two loops of thread apart.
These devices separate the positive and negative charges in ordinary matter, generating useful electric power.
By rubbing a plastic paddle on wool, you can generate electrical charges that make styrofoam chips jump and dance.