How Lightning Works

By: John Zavisa & Jesslyn Shields  | 

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Flashes of lightning against a dark sky create an otherworldly effect. What is the science behind this beautiful — but dangerous — natural phenomenon? ClassicStock/Getty Images

Lightning is the most beautiful and dangerous aspect of a storm. According to the National Weather Service, between 1989 and 2018, an average of 43 lighting-related deaths were reported in the United States each year. Only about 10 percent of the victims of lightning strikes result in deaths — usually from cardiac arrest or irrevocable brain damage — which is somewhat surprising, considering a lightning bolt is about 50,000 degrees Fahrenheit (27,760 degrees Celsius) — about five times hotter than the surface of the sun.

Although a lightning bolt is only about an inch in diameter (2 to 3 centimeters), it can stretch for miles. While most lightning bolts are 2 to 3 miles (3.2 to 4.8 kilometers) long, the world record flash was observed to stretch 477.2 miles (768 kilometers) across three U.S. states — Mississippi, Louisiana and Texas — in 2020. So the expression "out of the blue" is completely accurate — if you can hear thunder, it's possible, if not very likely, that lightning from a nearby thunderstorm can make it to wherever you are.

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Something as powerful as lightning begins with a process that happens in your life all the time: static electricity. If you've ever shocked yourself on a doorknob after walking across a carpet in your socks, you experienced the pop of a tiny lightning bolt. Static electrical charge builds up whenever two objects rub together, and in the case of lightning, the objects are moisture molecules and ice crystals in clouds.

Clouds are one of the places moisture is held and collected in the water cycle. Clouds are formed gradually as terrestrial water absorbs heat from the sun and the surroundings until they collect enough energy to transform from liquid to vapor. A cloud you see is just a collection of water vapor, some dust and other impurities that hangs together in the sky, much like the water in a lake or ocean hangs together on the ground.

Precipitation like rain, snow and sleet occurs comes from clouds, obviously. But it happens as a result of water vapor rising higher and higher, and the temperature of the surrounding air becoming lower and lower. Eventually, the vapor loses enough heat to the surrounding air to allow it to turn back into a liquid. Earth's gravitational pull then causes the liquid to fall back down. If the temperatures in the surrounding air are low enough, the vapor can condense and then freeze into snow or sleet.

In the next section, we'll see what causes electrical storms.

Electrical Storms

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The brilliant white-blue flash of lightning is caused by its extreme heat. A lightning bolt is hotter than the surface of the sun. NurPhoto/Getty Images

Scientists have some pretty good guesses about what causes electrical storms, though they're learning more all the time. In the process of the water cycle, moisture accumulates in the atmosphere to form clouds. As the process of evaporation and condensation rolls along, condensing droplets collide with each other as they rise.

Storm clouds are made up of billions of dust borne water molecules and ice crystals that begin moving faster and faster as they rub together. As the water molecules collide, electrons are knocked off, creating a charge separation. The newly knocked-off electrons fall to the lower portion of the cloud, giving it a negative charge. As a result, clouds become charged like giant batteries in the sky — the upper portion of the cloud becomes more positively charged and the lower portion more negatively charged.

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Because positively and negatively charged objects attract each other while objects with the same charge repel each other — think magnets! — the negative end of the cloud that's closer to the ground pushes away the negatively charged molecules on the ground.This repulsion of electrons causes 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.

We'll look at the next stage of the lightning creation process, air ionization, next.

Air Ionization

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The tallest objects in a storm don't always get struck by lightning. Lightning can strike the ground at a close distance to a tall object. Gary Hershorn/Getty Images

As the difference between the charges in the bottom of the cloud and the earth below it builds up, the first stage of lightning begins.

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 that 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 atomic structure.

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The importance of this separation is that the electrons are now free to move much more easily than they could before the separation. So this ionized air (also known as plasma) is much more conductive than the previous non-ionized air.

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 loses an electron when combined with oxygen. Simply put, 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. 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.

After the ionization process, the path between the cloud and the earth begins to form. Learn about stepped leaders, or paths of ionized air, next.

Stepped Leaders

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Multiple cloud-to-ground and cloud-to-cloud lightning strikes are possible under certain conditions. NurPhoto/Getty Images

Most of the time we see negative lightning, which originated both on the ground and in the lower, and more negatively charged, portion of the cloud.

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 stepped leaders.

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The stepped leaders are mostly invisible, except to incredibly high speed cameras — they branch towards the ground unbelievably quickly — about 164 feet (50 meters) per microsecond!

They form branches because the air may not ionize equally in all directions. Dust or impurities in the air may cause the particles to stream more easily in one direction, giving a better chance that the stepped leader will reach the earth faster in that direction. 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 Earth's surface. If the cloud is parallel to Earth's surface, and the area is small enough that the curvature of 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 the earth.

Flux lines always radiate perpendicularly from the charge surface before moving toward the location of the opposite charge. Given that the lower surface of the cloud is not straight, the flux lines will not be uniform.

When one flux line reaches the ground first, it creates a conductive path that all other branches between the cloud and the earth will then follow. The path reaching the ground results in the strike — the sudden, massive, flow of electrical current moving from the cloud to the ground.

But while all this action is happening in the cloud, the surface of Earth is getting ready for the lightning strike as well. Next we'll look at positive streamers and what happens when these streamers meet stepped leaders.

Positive Streamers and Exploding Air

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Looking at this bolt that resembles some sort of celestial being, it's easy to see why lightning was once viewed as supernatural. DEA/V. GIANNELLA/Getty Images

As the stepped 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 such as that of a storm cloud. In actuality, anything on the surface of Earth has the potential to send a streamer. Once produced, the streamers do not continue to grow toward the clouds; bridging the gap is the job of the stepped leaders as they travel downward.

Next to occur is the actual meeting of a stepped leader and a streamer. The streamer that the stepped 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 stepped leader does not take the path of a straight line allows for this to occur.

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After the stepped 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 ground. The resulting flash is both bright and incredibly hot.

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 that causes a 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 (1.6 kilometers) every 4.5 seconds. Light travels at a blazing 186,000 miles (299,000 kilometers) per second.

Multiple Strikes

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Multiple cloud-to-ground and cloud-to-cloud lightning strikes are possible under certain conditions. NurPhoto/Getty Images

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 stepped 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 stepped leaders also flows to the ground. The electrons in the other stepped leaders, being free to move, flow through the leader to the strike path. So when the strike occurs, the other stepped leaders are providing current and exhibiting the same heat flash characteristics of the actual strike path. After the original strike occurs, it is usually followed by a series of secondary strikes. These strikes follow only the path of the main strike; the other stepped leaders do not participate in this discharge.

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In nature, what we see is often not what we get, and this is definitely the case with the secondary strikes. Depending on the time delay between the strikes, we may see what looks like one long-duration main strike, or a main strike followed by other flashes along the path of the main strike. 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 that 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, making it appear that the main strike is flickering.

Types of Strikes and Types of Lightning

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Wild cloud-to-cloud lightning illuminates the night sky. Gary Hershorn/Getty Images

Types of Lightning Strikes

There are several different types of lightning strikes:

  • Cloud to ground — Negatively charged particles are drawn toward the positive particles on the ground.
  • Ground to cloud — The same as cloud-to-ground lightning, except the strike is initiated by a usually a tall, earth-bound object rather than the cloud.
  • Cloud to cloud — Similar mechanics to ground-to-cloud lightning, except the strike travels from one cloud to another

Types of Lightning

There are several different types of lightning:

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  • Sheet lightning — This is normal lightning that is reflected in the clouds.
  • Heat lightning — This is normal lightning near the horizon that is reflected by high clouds.
  • Ball lightning — A phenomenon where lightning forms a slow, moving ball that can burn objects in its path before exploding or burning out.
  • Red sprite — A red burst of electricity that occurs about 50 miles (80 kilometers) above storm clouds. Shaped like carrot tops, crowns or even jellyfish, they're bright enough to see in daylight.
  • Blue jet — Occuring early in a thunderstorm, blue jets are bright, blue sprays of electricity that originate in the center of a storm cloud about 25 miles (40 kilometers) above the ground. Sometimes blue jets are observed closer to the ground, and the sprays are shorter. These are called blue starters.
  • Elves — Discovered in the early 1990s, elves are huge — 185-mile-wide (300-kilometer-wide) — halos of ring-shaped lightning, created in the upper, positively charged portion of the cloud.

In the next section, we'll learn about the purpose of lightning rods.

Lightning Rods

Lightning rods were originally developed by Benjamin Franklin. A lightning rod is very simple — it's a pointed metal rod attached to the roof of a building. The rod might be an inch (2 centimeters) in diameter. It 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. 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 is an excellent conductor and thus allows the current to flow to ground without causing any heat damage.

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Lightning can "jump around" when it strikes. This "jumping" is associated with the electrical potential of the strike target with respect to the ground'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-resistance path 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 lightning — it merely provides a safe option for the lightning strike to choose. This may sound a little picky, but it's 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 that 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.

Lightning Safety

Lightning is one of nature's most dramatic, and also one of its deadliest, meteorological events. According to the National Weather Service, in 2020 there were 17 direct lightning fatalities in the U.S., compared with 20 in 2019. From 2010 to 2019, on average, 26 people died each year from lightning strikes in the United States. Most fatalities happen when people venture outside or into the water before the danger of being struck has passed. A good rule for deciding whether it's safe to venture outside is to count the number of seconds between a lightning flash and a roll of thunder. Since it takes the sound of thunder about five seconds to travel a mile, if you hear thunder within 30 seconds of seeing the lightning flash, the storm is close enough to be dangerous. After the storm, remain indoors for at least 30 minutes after hearing the last clap of thunder.

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 earth just as easily as it can use any other object. Appropriate shelter would be a building or a car. If you do not have anywhere to go, then you should make for the lowest possible ground like a valley or ravine. Avoid taking shelter under trees or near metal objects like fences and poles, or huddling up with other people in a group — spread out from your friends as much as you can.

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If you are indoors, stay away from plumbing pipes (bath tub, shower). Lightning has the ability to strike a house or near a house and impart an electrical charge to the metal pipes used for plumbing. This threat is not as great as it used to be, because PVC (polyvinyl chloride) is often used for modern indoor plumbing. If you are not sure what your pipes are made of, wait it out.

For more information on lightning and related topics, check out the links on the next page.

Lightning in 275 Words or Fewer

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To avoid being struck by lightning, always seek shelter during an active electrical storm. Wild Horizon/Getty Images

Sometimes it's helpful to do a little recap, especially when you're learning about a phenomenon as complex as lightning. In that vein, here's our quick explanation of cloud-to-ground lightning strikes.

On Earth, lightning begins with the water cycle. As the sun heats the planet, moisture heads for the skies in the form of vapor. Given enough moisture, a cloud will form.

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Thanks to updrafts, downdrafts, freezing and particle collisions, storm clouds become positively charged at the top and negatively charged at the bottom.

This separation of charges goes hand in hand with an electric field. As the charge separation grows stronger, so does the associated electric field.

Eventually, an intense electric field can cause the air around the cloud to "break down," or become ionized, allowing current to flow through the ionized air (or plasma) and potentially neutralize the charge separation. The path of ionized air is called a stepped leader.

Meanwhile, the positive charge is getting bigger on Earth's surface below, and objects (including people) respond locally to this strong electric field by sending out positive streamers.

When a streamer and a stepped leader meet, they can form a complete path for lightning to travel from the cloud to the ground (other types of lightning follow a slightly different process). After this fateful meeting, the lightning strike occurs.

Lastly, the air around the strike heats up and expands so much that it causes a shockwave in the form of a sound wave to radiate away from the strike path. That's thunder.

Originally Published: Apr 1, 2000

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