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Dr. Joseph Dwyer is an Associate Professor of Physics and Space Sciences at the Florida Institute of Technology. After receiving his Ph.D. in Physics from the University of Chicago in 1994, he worked in such fields as cosmic-ray physics and gamma-ray astronomy before shifting his interests to thunderstorms and lightning. In 2002, Dwyer and collaborators discovered that rocket-triggered lightning produces large quantities of X-rays, allowing for the first time the detailed study of a mysterious atmospheric phenomenon known as runaway breakdown. |
Joe Dwyer answered selected viewer questions about lightning on October 21, 2005. Please note we are no longer accepting questions, but please see our links and books section for additional information on lightning. Q: Lightning discharge releases a tremendous about of energy. In the story on NOVA scienceNOW, scientists are using land-launched rockets to trigger precise lightning strikes to the ground. In today's atmosphere of sky-high oil prices, is there a way to capture this lightning energy as it strikes the ground and storing it for commercial or home uses, maybe with massive arrays of capacitors or large batteries, as an alternative energy resource? I believe that lightning can definitely be an alternative energy source. What are your thoughts on such an idea? A: Dear Tuan, The energy liberated by lightning is indeed impressive. For example, during one lightning strike, tens of thousands of amps of electrical current flow between hundreds of millions of volts, releasing several billion joules of energy in just a fraction of a second. Furthermore, there are about four million lightning strikes on our planet every single day, resulting in a total worldwide power of more than 100 billion watts. If this large amount of energy could somehow be harnessed it would supply a significant fraction of the power needs of the United States. Unfortunately, lightning strikes are also distributed over a very large area, and the density of strikes at any one location is relatively small. At our 250-acre lightning center in north-central Florida, we get about six natural lightning strikes per year. Using rockets trailing wires we can trigger an additional dozen or so. The result is that even on a pretty big piece of land in the lightning capital of the United States the energy of the lightning would only power a few 100-watt light bulbs—and that assumes that somehow we could extract all the energy out of the lightning, which would be nearly impossible. To put this in perspective, if all the solar energy that falls upon the same area were captured then it would power several million 100-watt light bulbs. As a result, despite its impressive show of power, lightning is not in fact a practical source of energy. Q: Why is Florida the "lightning capital" of the U.S.? Why does it get so many lightning strikes? A: Dear Coral Gables Senior High, As you know, Florida is a long peninsula with nice warm oceans on each side. During hot summer days, moist sea breezes are drawn in from both sides of the peninsula. When these sea breezes collide, some of the warm, moist air is pushed upwards, which allows the energy stored in the air to be released. More specifically, humid air contains a lot of latent heat, which can be converted into other forms of energy. The result is what's called an air-mass thunderstorm. These thunderstorms have very powerful updrafts that shoot the clouds up to over eight miles above the ground and make a whole bunch of lightning. The highest concentration of lightning strikes occurs near Tampa, but we also get our fair share on the east coast where I live. Q: You hear meteorologists talk a lot about positive and negative lightning strikes. What is the difference between the two, and do you stand a greater chance of being hit by one or the other? A: Dear David, A positive cloud-to-ground lightning stroke brings down positive charge from the cloud to the ground, and a negative lightning stroke brings down negative charge to the ground. As in most electrical circuits, negatively charged electrons are what actually move when electrical currents flow in lightning. This is true for both positive and negative strokes. Consequently, during a negative stroke electrons flow downward, and during a positive stroke electrons flow upward, leaving behind a net positive charge. If you think of a thunderstorm as a big battery with one terminal up in the cloud and the other on the ground, the direction that the current flows depends upon which side has the more positive charge. Some parts of thunderstorms are more positive than the ground and some parts are more negative. On average worldwide, negative lightning flashes make up the vast majority, about 90 percent of all strikes. However, it is possible to be under a region of a thunderstorm where most of the flashes are positive. By the way, positive lightning strikes are believed to be the most dangerous, since they can produce very large currents, up to 300,000 amps! Q: Within the NOVA Web site I read that lightning heats the surrounding air of a lightning bolt to ~50,000°F, or hotter than the sun. The sun, as I understand it, generates heat through fusion reactions. So why don't we see fusion reactions taking place within the surrounding air of a bolt of lightning? A: Dear Casey, The surface of the sun is about 10,000°F, which is much cooler than the hottest part of lightning. However, the nuclear fusion that powers the sun occurs only near the center where the temperatures are much higher (30 million°F) and the pressures are very large. In comparison, lightning is downright chilly. As a result, no nuclear reactions are expected to take place during lightning. Having said all this, several independent research groups have recently measured nuclear by-products associated with lightning, which according to our standard picture doesn't seem possible. If these results are correct, then something very unusual is happening with lightning—so stay tuned. Q: My dad fought forest and range fires and said that he could see lightning in flames of fast-moving fires along a broad front. Is that true? How does that happen? A: Dear Joseph, Electrical sparks can be produced in turbulent media that contain lots of small particles. Charges become separated as the particles collide, resulting in a large enough electric field to cause a discharge. For example, sparks can be produced this way by sandstorms and volcanic clouds. Perhaps the lightning discharges that your dad saw were large sparks generated by the rising smoke particles. In addition, rising smoke from forest fires can produce clouds that can then make lightning. Q: Once, while I was in my bed listening to a series of small lightning/thunder discharges outside, a blue flash in the middle of the room and a sharp sound like a pistol shot terrified me. This happened on a windy winter night on an island off the coast of northern Norway. The Norwegian Meteorological Institute could not explain the phenomenon. A: Dear Ole, Hmmm. One possibility is that a nearby lightning strike generated a high voltage on the electrical wiring inside your house. Maybe this high voltage then produced a big spark called a side flash inside your room. Q: Are cosmic rays well distributed? If so, how does one explain the concentration of lightning in certain areas? A: Dear Kerry, Cosmic rays that enter the near-Earth environment are very isotropic, which means they arrive uniformly from all directions. In our atmosphere there are some differences in the numbers of cosmic rays, depending upon on what part of the globe you are and what energy of cosmic rays you are measuring. However, the highest energy cosmic rays, which according to one model are what trigger lightning inside thunderclouds, are very uniformly distributed around the world. Even if cosmic rays are involved in lightning initiation, a thunderstorm is still a necessary ingredient, and the occurrence of thunderstorms is highly dependent upon weather conditions, which vary with the location, the season, and the time of day. So it is the thunderstorms that determine the patterns and not the cosmic rays. Q: What should you not be touching if lightning strikes your vehicle? Your home? A: Dear Judy, If your car or your house were perfect conducting enclosures, then it wouldn't matter what you touched as long as you stayed inside. But because they don't completely shield us, during a thunderstorm, we shouldn't touch or go near objects that conduct electricity and are electrically connected to the outside world. For example, don't do the dishes during a thunderstorm. Electricity can flow along the plumbing and spark to you if you are near, even if you are not actually touching anything. Fires are also a concern if lightning strikes a house. In a car, the windows should be up and don't touch any metal parts or the radio. Q: It's always puzzled me how some people can be struck by lightning and survive, but others do not. Why is this so? One other question: how can a bolt of lightning appear to strike from a cloudless sky? I've read reports that state people have been struck by a bolt even though the nearest cloud was over 20 miles away. A: Dear Blake, Oftentimes people who are hurt by lightning are not directly struck. Lightning can strike some distance away and still produce significant injuries and even death from currents flowing through the ground or from sparks jumping between objects. Also, when people are directly struck, not all the current from the lightning flows through their bodies. As the lightning current begins to flow through a person, his or her electrical resistance will cause a large voltage difference to develop across his or her body. Eventually this high voltage will cause an arc to flash across the skin, which will then carry most of the current. This current flowing over the surface can cause severe burns and other injuries. How all this takes place is very complicated, and whether or not someone survives will depend upon the particulars of the lightning strike and the person being struck and whether or not first aid was administered. It is sometimes tempting to assign significance to why someone dies and why someone standing nearby survives, but it may come down to something as simple as whether or not they had their feet together or apart at the time of the strike (together is safer). To answer your second question, lightning can indeed travel many miles through the air, beyond the range of thunder, to locations where there is blue sky overhead. Such lightning strikes are the so-called "bolts from the blue." As a result, it is possible to be outside, not hear any thunder or see any clouds, and still be struck by lightning. It's scary stuff. Q: What other gaseous planets' atmospheres produce lightning, and are the processes similar in terms of electron charges? What elemental gases move electrons to produce such activity? A: Dear Pat, Lightning has definitely been detected on Jupiter and probably has been detected on Saturn, Uranus, and Neptune. Because the compositions of these planetary atmospheres are different from Earth's, the mechanism for electrification may be different. One problem is we still don't know exactly how charge is separated in Earth's thunderstorms, so it is hard to make detailed comparisons when we don't understand what is going on here at home. On the other hand, runaway breakdown should also work in those atmospheres like it does for the Earth, and so the ideas of how runaway breakdown and cosmic rays initiate lightning may also apply to lightning on these planets as well. However, research is just beginning in this area, so it will take some more work before we can decide if these ideas are correct or not. Q: What causes the funny smell as a lightning storm approaches? I've heard it's ozone, but I have not be able to verify that anywhere. Thanks. A: Dear Kenneth, Yep, it's ozone. Ozone, or O3, can be created during electrical discharges, which abound during thunderstorms. Q: What happens when lightning strikes a body of water? If you are swimming in the water, how far from a strike would be dangerous? Why are fish not electrocuted in large numbers? A: Dear Norm, Electrical currents like to flow over surfaces, so most of the current from the lightning flows over the surface of the water. Since fish usually swim below the surface they should be fairly safe from the effects of lightning. I am not actually sure how close to a strike you need to be for it to be dangerous when swimming. Since I don't want to be the guinea pig that finds out, I don't go anywhere near the water when there is lightning in the area. Q: Is there really such a thing as ball lightning? And if so, can you explain how it forms? I've heard both myths and realities of its existence. Thank you. A: Dear Mike, I don't know the answer. Most of the evidence for ball lightning is anecdotal, and so it is hard to know what to make of it. A scientist needs to remain skeptical and yet keep an open mind. In other words, I am willing to be convinced, but I am not convinced yet. Regarding possible explanations, ball lightning has too many explanations, nearly all untested. Who knows which if any are correct? Q: What makes thunder?? A: Dear Angela, Lightning does! The brightest part of the lightning flash is produced by large electrical currents that quickly raise the temperature of the air inside the narrow lightning channel to about 50,000°F. The hot air in the channel rapidly expands and then as it cools contracts back. The rapid expansion and subsequent contraction of the channel pushes the air back and forth, kind of like a big drum, making the sound waves that we call thunder. Q: In recent years lightning has been detected on Venus, Jupiter, Mars—in fact, throughout the solar system. A similar source and mechanism therefore appears likely. We also have comets producing X-rays. I wonder if you are aware that when the two Soviet Vega probes crossed the sunward side of Halley's in 1986 they detected a powerful electron beam?? It's true! And the fact that both the X-rays and the electron beam were on the sunward side indicates that these two phenomena are related. Clearly, the electron beam was producing the cometary X-rays, just as lightning produces X-rays on Earth. The facts also suggest that the common source of both is the sun—not cosmic rays. Have you considered this? A: Dear Mark, Energetic electrons indeed make X-rays when they interact with matter. This happens during lightning, inside X-ray tubes, and at comets. In space, there are a lot of ways of accelerating electrons that do not work in a dense atmosphere like on Earth. Indeed, energetic electrons are observed to be produced in a large number of places in our solar system. On the other hand, for thunderstorms and lightning, the only way that we have figured out to accelerate electrons through air is with runaway breakdown. Because the kind of runaway breakdown that occurs inside thunderstorms and uses cosmic rays does not work so well in space where the densities are very low, the two phenomena may not be closely related. Nevertheless, I agree that it is worth looking into, and so thanks for the info. Q: I remember reading that Ben Franklin concluded that lightning rods with a sharp point will work better then ones with a round point. Why? A: Dear Anonymous, As a lightning discharge approaches the ground on its trip down from the thundercloud, the electric field on the ground increases, and discharges (actually hot channels called leaders) from objects on the ground race up towards the lightning. Which discharge channel intercepts the lightning determines which object gets struck. In order to produce a discharge, the electric field needs to reach the breakdown field of 70,000 volts per inch. This is most easily accomplished if the object on the ground is tall and pointy. A pointy shape enhances the field at the tip and helps initiate a discharge. Therefore, the reasoning goes, if you are the tallest, pointiest thing around you should have a good chance of making the upward discharge that attaches to the lightning and brings it down to you. Indeed, pointy lightning rods have been used for centuries with great success. However, lightning is never simple, and it has been observed that sometimes the tallest, pointiest thing is not always what gets struck. For example, there are cases in which tall TV antennae have been struck not at the top but partway down. One explanation is that a pointy end works too well at making discharges and produces so much charge around it that it actually shields itself from the lightning, in effect making it appear extremely dull to the lightning. For this reason, some researchers think that perhaps a not so sharp tip on the lightning rod might be the way to go, and there is some experimental evidence to support this. It is amusing that scientists are still debating this more that 250 years after the lightning rod was invented. Q: Are lightning rods necessary on barns and outbuildings in a rural but modern area? I have asked electricians about this and have received about 10 different answers. Is not lightning attracted to ground or "potential" that is built into the power infratructure and not to the hightest point (which used to be large barns). This is a question that has been plaguing me for years. Thanks. A: Dear Bruce, Yes, lightning rods are a good idea. Due to a layer of charge emitted by coronal discharges on the ground, lightning usually cannot detect objects on the ground until it is practically on top of them. The electric field produced by the lightning then triggers leaders (hot discharge channels) from objects on the ground that shoot up to meet the downward propagating lightning. Tall buildings are not always what get struck by lightning, but they often are because they have a greater chance of producing a leader that attaches to the lightning. A lightning rod, while not 100 percent effective, will increase the odds that the structure will still be there after the lightning strikes. Q: Years ago I saw a segment about a person who I believe had MS and was struck by lightning while bathing, and all her MS symptoms were gone! From totally wheelchair-bound and a lack of sensations in her legs to walking on her own and feeling a pin prick on her legs. It stumped the doctors, of course. Never heard anything more about it. Do you know of this happening? Are the effects of lightning on humans being studied it terms of sparking neurons that once were malfunctioning or something like that? Or is it impossible to recreate and/or unethical? Either way, it makes you wonder. A: Dear Karen, I am not familiar with the details of this case, and furthermore, my training is in physics not in medicine, which makes me doubly unqualified to answer. I do know that how lightning affects the human body is only beginning to be understood, especially neurologically. Q: Is there any possibility that lightning travels to, or should I say, is attracted to the Earth due to our planet's high iron content, especially the core? It is strong enough to produce magnetism anywhere on the planet. I realize that the magma core is very far from the surface, but iron particles are distributed throughout the many layers, and these particles can be dense enough to draw on the positive charged storm clouds to be conductive. True or false? A: Dear Dan, There is an interesting electrical effect that conductors hide information about their internal structure from the outside universe. The surface of the Earth and even the oceans are reasonably good conductors, which means that, electrically, it doesn't matter what the inside of the Earth is made of. In other words, all that matters is what's on the surface. The inside of the Earth could be filled with golf balls and it wouldn't change how lightning behaves. So the answer is false! |
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© | Created October 2005 |
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