Lightning is one of the most beautiful displays in nature. It is also one of the most deadly natural phenomenon known to man. With bolt temperatures hotter than the surface of the sun and shockwaves beaming out in all directions, lightning is a lesson in physical science and humility.
Beyond its powerful beauty, lightning presents science with one of its greatest local mysteries - How does it work? It is common knowledge that lightning is generated in electrically charged storm systems, but the method of cloud charging still remains elusive.
In this edition of How Stuff Works we will look at lightning from the inside out, so that you can understand this amazing phenomena!
The Water Cycle
One aspect of lightning that is not a mystery is the water cycle. To fully understand how the water cycle works, we must first understand the principals of evaporation and condensation:
The Great Mystery
A cloud acts like a huge capacitor. The top and bottom of the cloud are like the two pieces of foil. Huge amounts of electricity can be stored inside this cloud capacitor.
In the process of the water cycle, moisture can accumulate in the atmosphere. This accumulation is what we see as a cloud. Interestingly, clouds can contain millions upon millions of water droplets and ice suspended in the air. As the process of evaporation and condensation continues, these droplets encounter many collisions with other moisture that is in the process of condensing as it rises. Also, the rising moisture may collide with ice or sleet that is in the process of falling to the earth or located in the lower portion of the cloud. The importance of these collisions is that electrons are knocked off of the rising moisture, thus creating a charge separation.
The newly knocked off electrons gather at the lower portion of the cloud giving it a negative charge. The rising moisture that has just lost an electron carries a positive charge to the top of the cloud. Beyond the collisions, "freezing" plays an important role. As the rising moisture encounters colder temperatures in the upper cloud regions and begins to freeze, the frozen portion becomes negatively charged and the unfrozen droplets become positively charged. At this point, rising air currents have the ability to remove the positively charged droplets from the ice and carry them to the top of the cloud. The remaining frozen portion would likely fall to the lower portion of the cloud or continue on to the ground. Combining the collisions with the freezing, we can begin to understand how a cloud may acquire the extreme charge separation that is required for a lightning stroke.
The Electric Field
When there is a charge separation in a cloud, there is also an electric field that is associated with the separation. Like the cloud, this field is negative in the lower region and positive in the upper region. The strength or intensity of the electric field is directly related to the amount of charge build up in the cloud. As the collisions and freezing continue to occur and the charges at the top and bottom of the cloud increase, the electric field becomes more and more intense. So intense, in fact, that the electrons at the earth's surface are repelled deeper into the earth by the strong negative charge at the lower portion of the cloud. This repulsion of electrons causes the earth's surface to acquire a strong positive charge. All that is needed now is a conductive path for the negative cloud bottom to contact the positive earth surface. The strong electric field, being somewhat self-sufficient, creates this path.
Follow the Leader
The following description is also exactly what occurs when operating a Van de Graaff generator. If you have a hankering to play with lightning, a VDG is definitely the safest way to go and can provide hours of entertainment.
The strong electric field causes the air around the cloud to "break down", allowing current to flow in an attempt to neutralize the charge separation. Simply stated, the air "breakdown" creates a path that short-circuits the cloud/earth as if there is a long metal rod connecting the cloud to the earth. Here's how this "breakdown" works…
When the electric field becomes very strong (on the order of tens of thousands of volts per inch), conditions are ripe for the air to begin breaking down. The electric field causes the surrounding air to become separated into positive ions and electrons; the air is "ionized". Keep in mind that the ionization does not mean there is more negative charge (electrons) or more positive charge (positive atomic nuclei - positive ions) than before. This ionization only means that the electrons and positive ions are farther apart than they were in their original molecular or atomic structure. Essentially, the electrons have been stripped from the molecular structure of the non-ionized air. The importance of this separation/stripping is that the electrons are now free to move much more easily than they could before the separation. Hence this ionized air (also known as plasma) is much more conductive than the previous non-ionized air. Incidentally, the ability or freedom of the electrons to move is what makes any material a good conductor of electricity. Often times, metals are referred to as positive atomic nuclei surrounded by a fluid-like cloud of electrons. That makes many metals good conductors of electricity.
These electrons have excellent mobility allowing for electrical current to flow. The ionization of air or gas creates plasma with conductive properties similar to that of metals. Plasma is the tool nature wields to neutralize charge separation in an electric field. Those readers who are familiar with the chemical reaction of fire will recall that oxidation plays an important role. Oxidation is the process by which an atom or molecule losses an electron when combined with oxygen. Simply, the atom or molecule is changed from a lower positive potential to a higher positive potential. Interestingly enough, the process of ionization, which creates plasma, also occurs through the loss of electrons (as discussed above). By this comparison, we can view the ionization process as burning a path through the air for the lightning to follow, much like digging a tunnel through a mountain for a train to follow.
Once the ionization process begins and plasma forms, a path is not created instantaneously. In fact, there are usually many separate paths of ionized air stemming from the cloud. These paths are typically referred to as "Step Leaders". The step leaders propagate toward the earth in stages, which do not have to result in a straight line to the earth. The air may not ionize equally as easy in all directions. Dust or impurities (any object) in the air may cause the air to breakdown more easily in one direction giving a better chance that the step leader will reach the earth faster. Also the shape of the electric field can greatly affect the ionization path. This shape depends on the location of the charged particles, which in this case are located at the bottom of the cloud and the earth's surface. If the cloud is parallel to the earth's surface and the area is small enough that the curvature of the earth is negligible, the two charge locations will behave as two charged parallel plates. The lines of force (electric flux) generated by the charge separation will be perpendicular to the cloud and earth. Flux lines always radiate perpendicularly from the charge surface before moving toward their destination (opposite charge location). Given this knowledge, we can say that if the lower surface of the cloud is not straight, the flux lines will not be uniform. Draw two points on opposite ends of a basketball. Next draw a line on the basketball that connects the two points. The curvature of the line is analogous to the flux lines in a non-uniform electric field. The lack of uniform force can cause the step leaders to follow a path that is not a straight line to the earth.
Considering these possibilities, it becomes obvious that there are various factors that effect the direction of the step leader. We are taught that the shortest distance between two points is a straight line, but in the case of electric fields, the lines of force (flux lines) may not follow the shortest distance. This distance does not always represent the path of least resistance.
So now we have an electrically charged cloud with ever growing step leaders stretching out toward the earth in stages. These leaders are faintly illuminated in a purplish glow and may sprout other leaders in areas where the original leaders bend or turn. Once begun, the leader will remain until the current flows, regardless of whether or not it is the leader that reaches the ground first. The leader basically has two possibilities, continue to grow in stages of growing plasma or wait patiently in its present plasma condition until another leader hits a target. The leader that reaches the earth first reaps the rewards of the journey by providing a conductive path between the cloud and the earth. This leader is not the lightning strike; it only maps out the course that the strike will follow. The strike is the sudden, massive, electrical current flow from the cloud to the ground.
Reaching for the Sky
Before we get ahead of ourselves, we have to consider what is happening with the surface of the earth and objects on the surface. As the step leaders approach the earth, objects on the surface begin responding to the strong electric field. The objects reach out to the cloud by "growing" positive streamers. These streamers also have a purplish color and appear to be more prominent on sharp edges. The human body can and does produce these positive streamers when subjected to a strong electric field as that of a storm cloud. In actuality, anything on the surface of the earth has the potential to send a streamer. Once produced, the streamers do not continue to grow towards the clouds; bridging the gap is the job of the step leaders as they stage their way down. The streamers wait patiently stretching upwards as the step leaders approach.
Let's Get Together
Next to occur is the actual meeting of a step leader and a streamer. As discussed earlier, the streamer that the step leader reaches is not necessarily the closest streamer to the cloud. It's very common for lightning to strike the ground even though there is a tree or a light pole or any other tall object in the vicinity. The fact that the step leader does not take the path of a straight line allows for this to occur. After the step leader and the streamer meet, the ionized air (plasma) has completed its journey to the earth, leaving a conductive path from the cloud to the earth. With this path complete, current flows between the earth and the cloud. This discharge of current is nature's way of trying to neutralize the charge separation. The flash we see when this discharge occurs is not the strike, it is the local effects of the strike.
Any time there is an electrical current, there is also heat associated with the current. Since there is an enormous amount of current in a lightning strike, there is also an enormous amount of heat. In fact, a bolt of lightning is hotter than the surface of the sun. This heat is the actual cause of the brilliant white/blue flash that we see. When a leader and a streamer meet and the current flows (the strike), the air around the strike becomes extremely hot. So hot that it actually explodes, because the heat causes the air to expand so rapidly. The explosion is soon followed by what we all know as thunder. Thunder is the shockwave radiating away from the strike path.
When the air heats up, it expands rapidly creating a compression wave that propagates through the surrounding air. This compression wave manifests itself in the form of a sound wave. That does not mean that thunder is harmless. On the contrary, if you are close enough, you can feel the shockwave as it shakes the surroundings. Keep in mind that when a nuclear explosion occurs, typically the most destruction is caused by the energy of the rapidly moving shockwave. In fact, the shockwave that produces the thunder from a lightning strike can most certainly damage structures and people. This danger is more prominent when you are close to the strike, because the shockwave is stronger there and will dampen (decrease) with distance. Physics teaches us that sound travels much slower than light, so we see the flash before we hear the thunder. In air, sound travels roughly 1 mile every 4.5 seconds. Light, on the other hand, travels at a blazing 186,000 miles per second.
You are sitting in your car and you see a flash from a lightning strike. The first thing you notice is that there were many other branches that flashed at the same time as the main strike. Next you notice that the main strike flickers or dims a few more times. The branches that you saw were actually the step leaders that were connected to the leader that made it to its target. When the first strike occurs, current flows in an attempt to neutralize the charge separation. This requires that the current associated with the energy in the other step leaders also flow to the ground. The electrons in the other step leaders, being free to move, flow through the leader to the strike path. So when the strike occurs, the other step leaders are providing current and exhibiting the same heat flash characteristics of the actual strike path. After the original stroke occurs, it is usually followed by a series of secondary strikes. These strikes follow only the path of the main strike; the other step leaders do not participate in this discharge. Often times in nature, what we see is not what is not what we get, and this is definitely the case with the secondary strikes. It is very possible that the main strike can be followed by 30-40 secondary strikes. Depending on the time delay between the strikes, we may see what looks like one long-in-duration main strike, or a main strike followed by other flashes along the path of the main stroke. These conditions are easy to understand if we realize that the secondary strike can occur while the flash from the main stroke is still visible. Obviously this would cause a viewer to think the main stroke flash lasted longer than it actually did. By the same token, the secondary strikes may occur after the flash from the main strike ends, appearing like the main strike is flickering.
Now you know the mechanics of a lightning strike. It's amazing to realize that all of the activity from the time the ionization begins to the time of the strike occurs in a fraction of a second. High-speed cameras used to take pictures of lightning have actually caught the positive streamers on film. If you would like to observe this phenomenon in a safe environment, build a Van de Graaff generator and run it in a dark room. As you approach the generator, your fingertips will begin to glow a purplish glow like that of a step leader or positive streamer.
Types of Strikes
Types of Lightning
Lightning rods were originally developed by Benjamin Franklin. They are very simple if you've ever seen one - it's a pointed metal rod attached to the roof of a building. The rod might be an inch (2 cm) in diameter. The rod connects to a huge piece of copper or aluminum wire that's also an inch or so in diameter. The wire is connected to a conductive grid buried in the ground nearby.
The purpose of lightning rods is often misunderstood. Many people believe that lightning rods "attract" lightning. If we take a moment to actually consider what this is saying, we will realize that to purposely attract lightning is pretty foolish. We know that lightning is deadly, destructive and unpredictable. Why would anyone purposely attempt to attract a phenomenon this dangerous to a location near the structure that they are trying to protect?
It is better stated to say that lightning rods provide a low resistance path to ground that can be used to conduct the enormous electrical currents when lightning strikes occur. If lightning strikes, the system attempts to carry the harmful electrical current away from the structure and safely to ground. The system has the ability to handle the enormous electrical current associated with the strike. If the strike contacts a material that is not a good conductor, the material will suffer massive heat damage. The lightning rod system, however, is an excellent conductor and thus allows the current to flow to the ground without causing any heat damage.
Lightning can "jump around" when it strikes. This "jumping" is associated with the electrical potential of the strike target with respect to the earth's potential. The lightning can strike and then "seek" a path of least resistance by jumping around to nearby objects that provide a better path to ground. If the strike occurs near the lightning rod system, the system will have a very low path of resistance and can then receive a "jump", diverting the strike current to ground before it can do any more damage. As you can see, the purpose of the lightning rod is not to attract lighting - it merely provides a safe option for the lightning strike to choose. This may sound a little picky, but its not if you consider that the lightning rods only become relevant when a strike occurs or immediately after a strike occurs. Regardless of whether or not a lightning rod system is present, the strike will still occur.
If the structure that you are attempting to protect is out in an open flat area, you often create a lightning protection system that uses a very tall lightning rod. This rod should be taller than the structure. If the area finds itself in a strong electric field, the tall rod can begin sending up positive streamers in an attempt to dissipate the electric field. While it is not a given the rod will always conduct the lightning discharged in the immediate area, it does have a better possibility than the structure. Again, the goal is to provide a low resistance path to ground in an area that has the possibility to receive a strike. This possibility arises from the strength of the electric field generated by the storm clouds.
Safety in a Storm
Over 1,000 people get struck by lightning every year in the U.S., and over 100 of them die as a result of the strike. Lightning is not something to be toyed with.
If you are caught outside in a storm always look for APPROPRIATE shelter. Do not take any chances, lightning can use you as a path to the earth just as easily as any other object. Appropriate shelter would be a building or a car. If you do not have anywhere to go, then you should avoid taking shelter under trees. Trees attract lightning. Put your feet as close together as possible and crouch down with your head as low as possible without touching the ground. Never lay down on the ground. After lightning strikes the ground, there is an electric potential that radiates outward from the point of contact. If your body is in this area, current can flow through you. You never want the current to have the ability to pass through your body. This could cause cardiac arrest, not to mention other organ damage and burns. By making your body as low to the ground as possible and minimizing the amount of your body in contact with the ground, you can lower the possibility of a lightning related injury. If a strike occurs near you, the current would have a much more difficult time flowing through your body in this position.
If you are indoors, stay off the phone. If you must call someone, use a cordless or cell phone. If lightning strikes the phone line the strike will travel to every phone on the line (and potentially to you if you are holding the phone). Stay away from plumbing pipes (bath tub, shower, etc). Lightning has the ability to strike a house or near a house and impart an electrical charge on the metal pipes used for plumbing. This threat is not as great as it used to be, because PVC is now used often for indoor plumbing. If you are not sure, wait it out.
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