Lightning Safety and Physics

Personal Safety During Electrical Storms

It is possible to greatly minimize the chance of danger in lightning storms. Principles of Physics are used here to understand the situation better that most media present. This analysis then suggests certain logical precautions.

Every second, about 100 lightning strokes occur somewhere in the Earth's atmosphere. Every year, between 100 and 200 people in the U.S. die as result of lightning strikes, with quite a few more hospitalized.

There is quite a bit of incorrect understanding about lightning. If more people understood the subject better, greater public safety can be had.

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TV talk shows often show people who "were hit by lightning." This is (virtually always) slightly incorrect. They certainly were hospitalized and suffered burns and trauma due to the lightning, but they were not actually hit!

Lightning discharges generally carry 50,000 amperes of current at hundreds of thousands or even millions of volts of voltage. It is medically well established that if a current of over about 0.002 amps passes through the chest cavity of a person, the (bio-electrical) heart triggering system is disturbed, and it is possible that a person might die. If even 0.02 amps of electrical current passes through the chest cavity, the person almost certainly immediately dies. If more than a million times that current passed through the chest cavity the person would certainly instantly die! Actually, the person would be instantly incinerated, and a lot of the water in the body would instantly boil to steam, and the person would explode! For a LOT of different reasons, there is NO CHANCE that any person (or tree!) could actually be hit by a direct bolt of lightning and survive!

If anywhere near the current of an actual lightning bolt passed through any living body, the moisture inside all of the cells would instantly boil and the person or animal would EXPLODE. When trees get hit, this generally happens, and the exploding water vapor inside the many cells of the tree causes even a strong tree to suddenly split down the middle (with a VERY loud sound of the water boiling and essentially exploding). The lightning itself doesn't split the tree, the instantly boiling water in the cells of the tree does. Clearly, a human, filled with electrically conducting blood and salt water, couldn't possibly survive anything like this.

We actually are amazingly sensitive to electricity. Our brain and nervous system can only operate with some natural body electricity. It has been very well established that even a few milli-amperes (.001 ampere) passing through the chest cavity of any person or animal will stop the heart from getting its triggering signals, and the person can die. This fatal situation involves a small fraction of an ampere, far less than the 50,000 amperes and up to 1,000,000,000 volts of a lightning bolt. Even the extremely high resulting temperatures, as high as 54,000°F, would cause all the water in a person to boil and explode, which is what happens when a tree gets hit by lightning.

So, how does someone "get hit by lightning" and live? Well, they actually didn't get hit! It turns out that Physics and electromagnetism has the explanation for us. In a nutshell, often, the person became part of an "air-core transformer"

Whenever electricity is passing through a conductor, (whether a water-filled tree, or any metal conductor, or even moisture-filled air) it creates a temporary magnetic field in the area surrounding the conductor. This is called electromagnetism.

If a second parallel conductor is nearby, that changing magnetic field causes an electric current to be "induced" in the second conductor, called an EMF. This is also electromagnetism. This is actually the principle of all electrical transformers and many other modern electrical devices. This particular arrangement is called an air-core transformer. In the situation we are considering here, the lightning bolt is passing through a low resistance path between the ground and the cloud (the first conductor, such as including a water-filled tree). A standing person, who is also mostly (salt) water, is a pretty good conductor of electricity, represents the second conductor. IF the person's body is relatively parallel to the first conductor, an INDUCED CURRENT is created inside the body of the person, by that transformer effect. This current can be quite substantial, and can still be plenty high enough to cause death.

There have been many cases where many people in a crowd have all "been hit by lightning" at the same moment. A well-documented case was from a 1998 soccer match that was being televised. A lightning stroke hit the field, not actually where anyone was standing, but many of the players immediately collapsed to the ground. They were all victims of this air-core transformer phenomenon. The severity of injuries in such situations depends on several things. The most important is the distance between where the lightning stroke hit and the person, because the effectiveness of the air-core transformer phenomenon depends tremendously on the spacing between the two conductors. The second most important is the orientation of the person. If a player had been lying down on the ground that day, he likely would have not had any serious injuries at all! Of course, a soccer player who is laying down gets replaced rather quickly, so they were all standing up. There are many other possible variables, such as whether a player was is good electrical contact with wet ground through sweaty (electrically conductive) wet shoes. Fortunately, in the case of that soccer match, none of the players died, although some had severe injuries due to electrical burns. It seems strange that they could have electrical burns without actually having been hit by the lightning, but that is an indication of how effective an air-core transformer can be. It was not reported, but it is certain that every person in the stands watching that game would also have felt an intense electrical effect due to an air-core transformer effect. They were each farther away from the lightning stroke, they were not sweat covered, and they were probably fairly dry, so it is far less likely that any would have sustained any significant injuries. They were probably sitting down, where the "effective length of the second conductor" (their bodies) was shorter than the STANDING players on the field. It is extremely likely that they each person in the stadium felt an intense tingling throughout their bodies. It could easily have been that the STANDING peanut vendors might have sustained more serious injuries.

If the person has a good electrical path to ground, such as standing in water, that current can pass quickly out of the body and create serious burns. This explanation also clarifies why people holding a golf club or metal umbrella up above their head increases the chance of them "being hit." What is actually happening is that their actions just make the "second conductor" referred to above to be extended in both height and conductivity by holding such things. More of the created magnetic field lines intercept the body and umbrella, so the induced current and voltage are therefore much greater and more damaging in the body of the person. This all means that, even though the person was not actually hit by the lightning bolt, scorched shoes, skin burn marks, and other physical damage are very real. Since a metal golf club can intercept a lot of the electromagnetic effects and then conduct the resulting electricity extremely well, a golfer can receive very serious burns on the hands as a result.

Of course, if that person who is holding a metal golf club above his head is standing alone at the top of a hill, where he is the highest object around, there IS the possibility that he might actually be hit by a lightning stroke, but that would instantly stop his heart, boil all the water in his body, and kill him instantly. Actually, under those really unfortunate circumstances, there could conceivably be very little left to identify as a human.

I have seen references that as many as 250 golfers "get hit by lightning" every year. Not exactly! Yes, that many people HAD THE EFFECTS of having a lot of electricity go through some part of their bodies, but virtually all were actually affected by this air-core transformer effect and not actually being hit by a lightning bolt! There are people who brag about "being hit by lightning several times". It is a certainty that those people were NEVER actually HIT by lightning, but they sustained the effects of the air-core transformer effect.

Many of us have experienced a momentary tingling feeling during electrical storms. This is actually a more tame and distant example of the same air-core transformer phenomenon. (You were "HIT" by lightning!) Since you were much farther from the lightning's path, (possibly miles away) the current induced in you was far less, only enough that your nervous system noticed some extra signals to deal with. In a more technical sense, the greater distance between the lightning stroke and you made for a larger air-core in the transformer, which makes the transformer effect much less effective.

Several aspects of lightning lore now make sense.

"Don't stand near or under a tree in a lightning storm." TRUE. If the tree would get hit, the induced current in you would be so high that you would probably die. By being very close to any object that is actually hit by lightning, the air-core of the transformer is physically small, making the transformer effect much greater. That induces a much greater voltage and amperage in the person. In addition, if a person is actually standing UNDER the branches of a tree (to stay out of the rain), an even worse danger is present. When the tree is hit by the lightning stroke, there is a LOT of electricity that is trying to find its way to the ground. If you (a bag of conductive salt-water) are standing under a big branch, near the trunk of the tree, it is possible that part of that 50,000 amperes of electricity will branch off and use YOU as a path to ground. Remember that even 0.001 ampere of electricity going through your chest could kill you. That means that this possibility of such electrical branching would be fatal. Remember how much branching of lightning strokes happens in the sky, when lightning occurs between clouds.

"Don't stand near a window during a lightning storm." TRUE. If the building was hit, and the outer surfaces were wet, most of the discharge would be along all of the outer surfaces of the building (called the Faraday effect, where virtually none of the current actually passes through the inside of the wet house). Windows of mostly glass are good electrical insulators and represent gaps in that path, and your conductive body could represent an effective alternate path for the discharge. The discharge could jump to your body to get past the window space. In addition, your standing near a large window diminishes the Faraday effect of protecting you within the house, so that the air-core transformer effect mentioned above might induce dangerous currents in your body even if a nearby tree was hit.

"Don't hold a golf club or umbrella up during an electrical storm. " TRUE. By holding a metal (conductive) object above your head, you are actually making for a better "transformer" with a longer and better conductor, so even more current would be induced in you. In addition, the contact with the metal conductor creates a specific path for the induced current to discharge, increasing the chance of getting electrical burns.

"Don't be on the phone or near your TV." Basically TRUE. The elevated phone line or the antenna wire or cable line, on a utility pole, could actually be hit by the lightning directly, carrying a lot of current along those lines to your house. All such lines have a special grounding provision as they enter your house, but some current could still enter your house along those electrical cables. If your body is nearby or touching such a conducting wire, you could feel an effect. More likely, however, is that those wires might have picked up induced currents (as the second conductor from lightning that hit some distance away, by the air-core transformer method). Then the momentary high voltage discharge in that wire could induce a third current in your nearby body, by a second application of the air-core transformer effect. (Still with us?) People using the phone during an electrical storm might therefore feel a tingling or even feel their hand being "zapped" while talking. This situation is certainly unsettling and uncomfortable, but probably less dangerous than the others above. The secondary inductance generally carries far less power and therefore causes much reduced effects. (If you are using a portable or cellular telephone, these dangers do NOT exist at all.)

O.K. Now that we have some understanding of the situation, what about this new suggestion for additional safety? Well, the electromagnetic air-core transformer phenomenon has a strong dependency on orientation. That is, the greatest induced current happen if the first conductor and the second conductor are PARALLEL. Transformers are designed to take advantage of this, since they want the greatest possible induced current in the secondary circuit. This would give the most efficient transfer of power. In our situation, would NOT want to be standing vertically near a vertical tree which might get hit by lightning! Instead, we would like to ensure the least induced current possible. This means that we should try to have the secondary conductor (YOU) at 90 degrees to the first conductor (the lightning strike). Since the lightning strikes are usually pretty vertical, this suggests that if a person was lying down, very little induced current would or could occur, and the person would be quite safe, whether indoors or outdoors.

This suggests that if a person was really scared of being electrocuted by lightning, they should just lie down in bed for the duration. If a person was outdoors, a horizontal position would be safest. Of course, the problem here is whether anyone would lie down on muddy, wet ground to deal with the remote possibility of a lightning strike! With a tornado, people see the danger and jump down in a gully. The rapidity of a lightning stroke doesn't allow such preparation. Still, if a dry place to lie down exists, it might be worth considering. But even just squatting down, to represent a physically shorter electrical conductor, can be very safe.

In the foregoing comments, there is one aspect that might be technically incorrect, although it does not affect any of the safety considerations. Science has found that most lightning strokes that hit the ground begin with an initial stroke that goes from a cloud to the ground, that seems to open up (ionize) a path for the full current of the lightning stroke to actually flow from the ground up to the cloud, just an instant later. These two happen almost instantaneously, and either or both carry far more current that that necessary to injure or kill anyone. I just wanted to clear up this technical detail that the main stroke of most lightning strokes actually flows upward from the ground to a cloud.

Other Dangers: There are an assortment of other dangers associated with lightning. There have been very rare cases where a person had been standing, with feet somewhat apart, in water, relatively near where a tree got hit by lightning. Once the current passed through the tree, the Faraday effect operates again, keeping much of the current in or near the surface of the ground (and not immediately penetrating into the Earth.) This means that current can sort of radiate outward along the ground from the tree. In the case of our unfortunate person, one foot was nearest the tree, and again, being mostly conductive saltwater, our bodies are a good electrical conductor, so electricity could travel UP that leg, and then down the other leg back to the ground (but farther from the tree.) The high electrical conductivity of the saltwater within us makes this longer path sometimes electrically preferable over going straight across on the ground. Such a person can sustain very serious burns and other injuries, but tends to survive because the current did not pass through the region of the heart or brain.

There is an especially good place to go if you are really concerned and you are outdoors.

Any car or truck (except a convertible or a non-metal Corvette) is a great place to stay safe! The reasons that this is true are two: The rubber tires of the vehicle somewhat insulate the vehicle from the ground, so the amount of induced current is necessarily far less; and the metal of the vehicle represents a good Faraday cage! A Faraday cage is the Physics term for a metal conductor, where all of the current passes along its surface AND NONE PENETRATES INSIDE OF IT! There have been cases where vehicles have been seen to have actually gotten directly hit by lightning, and the occupants survived. Usually, they were deaf for a while, due to the incredibly loud sound of the thunder, but otherwise fairly well. It is also true that airliners sometimes get hit by lightning while in the air, without anyone inside being hurt, for the same reason of the airplane being a Faraday cage!.


Since we're discussing lots of things about lightning, we might as well add some comments about thunder. As the amazing amount of power of a lightning stroke is passing through the atmosphere, some of that power is used up in the process. Technically, the air has some electrical resistance, and a certain amount of heat (in watts) is therefore generated in the air that related to that resistance and the current flowing, according to simple electrical formulas. This heat is formed extremely quickly, right along the path of the lightning stroke. The effect of this sudden heat is to heat that air up to extremely high temperatures very quickly. Technically, any gas heated to that high a temperature is called a plasma.

There is a rule in science, called the Ideal Gas Law, that says that any gas (like air) gets larger (increases in volume) when it is heated up. Therefore, as the air right along the path of a lightning stroke suddenly heats from normal temperatures to maybe 50,000°F, it violently and explosively expands in all directions. This creates a shock wave in the surrounding air, which moves outward at the speed of sound, around 1,100 feet per second.

Whenever a lightning stroke occurs, whether from cloud to cloud or from cloud to ground, the flash of light and this shock wave in the air are created at the exact same time. If that occurred two miles from you, the light will get to you in 2/186000 seconds, since the speed of light is 186,000 miles per second. That means that the light gets to you in far less than one thousandth of a second, effectively instantly. The shock wave in the air, which has the effect of a sound wave, takes around 10 seconds to travel the 10,560 feet of the two miles.

Therefore, we see a bright lightning stroke, and then, ten seconds later, hear the rumble of thunder, which is the effect of the shock wave. That's why people sometimes count seconds after a lightning stroke to estimate the distance away. For each five seconds until the thunder, it means the lightning was another mile further away.

Sometimes, we see something called sheet lightning. Usually, this is seen at night when there is no storm around. No thunder is usually heard with sheet lightning. That doesn't mean that the thunder does not exist. This phenomenon is actually normal lightning, associated with a storm that is more than about 30 miles away. We see the effect of the light of many lightning strokes, but the sound of the thunder does not carry for the 30 or 50 or 100 miles, so we do not notice any thunder.

Thunder generally has a very low-pitched sound, because the shock wave produced by the rapidly expanding air created one very large shock wave. The physically large dimensions of that wave cause the sound to have such a rumbling low pitch. There are some variations on the sound of thunder. If a lightning stroke was from side to side (from where you see it), it might all have been at about the same distance from you, so the sound of the shock waves created all along its length would all get to you at about the same time. The sound would be a very brief but loud boom! If a lightning stroke of a mile long occurred sort of toward and away from you (in the sky), the sound from the shock waves along different parts of it (all created at the same time) would get to you spread out over about five seconds, making it sound like an extended rumbling sound, that's usually not as loud as the boom kind.

When lightning hits a tree, there is often a loud crraacck sound at the very start of the thunder, but usually only if the tree was nearby. This higher pitched sound is not actually from the lightning stroke itself, but is due to the sound of parts of the tree exploding as the water within it suddenly boiled.

Modern science is still studying lightning in order to better understand it. Much more still remains to be learned about it. It was only a few decades ago that a statistical analysis was done regarding the time of day when lightning strokes occur. Scientists were surprised that a tremendous majority of lightning strokes occur between around noon and 4 p.m. in nearly all locations. It is thought that this is due to there being far more energy available in the atmosphere during those hours due to bright sunshine providing that energy. It seems likely that heating of the ground with such solar heating causes convection of the atmosphere above it so that air is rapidly rising. Such rapid vertical air motion can aid in segregating positive and negative charges in clouds at the upper and lower altitudes. This seems like a logical method where the amazingly high voltages and currents that cause lightning strokes could develop.

First Published on the Web: Aug 28, 1997

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago