Lightning

• Type of physical science: Lightning, Electromagnetism, Classical physics
• Field of study: Electromagnetism

A large discharge of electrical energy in the atmosphere is called lightning. This discharge can produce brilliant flashes of light and thunder, although the thunder may not be heard if the distance to the event is greater than a few kilometers.

Overview

Lightning is a visible, large-scale atmospheric discharge of electricity that is usually, but not always, accompanied by violent rainstorms. This discharge of electricity is similar to, but much greater than, the electrical shock one may receive when touching a metal object on a very dry day.

During thunderstorms, positive and negative charges in clouds become separated. Why this happens is not fully understood, and no completely acceptable theory has been made. It is believed, though, that the presence of electrical charge is important in the formation of raindrops and ice crystals. This theory is supported by the fact that lightning usually comes from the largest storm clouds after they have reached heights where ice crystals begin to form. The fact that these clouds form mainly in the summer explains why lightning is mainly a summer phenomenon. What all this means is that for some reason, when drops form within the clouds, some will have a positive charge and some will have a negative charge.

The negative charges tend to cluster in the bottom of the cloud, while the positive charges form groups in the top of the clouds. Because of the large negative charge in the bottom of the cloud, the ground underneath will become positively charged (because opposite charges attract). Natural forces work to try and make these charges come together, but air is a very poor conductor of electricity and prevents this from happening until the charge difference (the electric potential, or voltage) between the charged regions becomes very high. The voltage between the bottom of the cloud and the ground, or the cloud bottom and cloud top, can get as high as 100 million volts. When this occurs, the air will become a conductor, and the sudden flow of electricity is so great that the brilliant flash of light known as "lightning" results. However, the apparently single stroke of lightning visible to the eye is actually multiple strokes.

It is believed that the lightning stroke begins when the large charge in a region of the cloud separates the electrons in the air from their atoms, ionizing the air. Ionized air is a conductor, and a small tube of such air with a radius of roughly 10 centimeters and roughly 50 meters long will form. This is called a "leader," and electrons in the cloud will begin to move into the leader. This will increase the voltage at the end of the leader and cause it to extend even further. Some leaders come to an end at this point and go away, but others will continue to grow. In this case, the conducting path continues to increase in length in very rapid bursts and is called a "step leader."

When the step leader has grown long enough to get close to a region that is positively charged, the voltage between the end of the step leader and the positively charged region will grow so large that the remaining region of air between them will become ionized. A step leader will then leap out of the positively charged region and close the path. If the positively charged region is the ground, the step leaders will tend to leap from high, more pointed objects such as trees, telephone poles, lightning rods, hilltops, and golf clubs.

The electrons at the end of the path will be the first to flow into the positively charged region, and the electrons behind them will then be able to rush forward at about one-sixth the speed of light. This causes the path to expand from the positive region toward the negative region, an effect called the "return stroke." It is this return stroke that illuminates the conducting path and is the familiar visible event. A single such stroke is capable of discharging a region of the cloud a kilometer wide.

This first stroke is then followed by additional strokes that discharge an even larger region. Each of these strokes begins with a "dart leader" that works in the same way that a step leader works but that it is more continuous and has fewer branches. If a dart leader does not occur within about one-tenth of a second after the return stroke, the conducting path will go away, and a new step leader will be required for additional strokes.

This tremendous event heats the air around it to temperatures several times hotter than the surface of the Sun and causes it to expand explosively. This expansion is what causes thunder. Since the speed of sound is approximately one-third of a kilometer per second, and the speed of light is nearly instantaneous, it is possible to estimate the distance to the lightning by observing the lightning flash and then counting the seconds until one hears the thunder. For every three seconds one counts, the lightning is approximately one kilometer away. (The speed of sound is also about one-fifth of a mile per second, so lightning will be about one mile away for every five seconds counted.)

Thunder that sounds like a long rumble is caused by lightning strokes with one end closer to the observer than the other. The sound coming from the closest end will arrive sooner than the farthest end. The greater the difference in distance between the observer and the two ends, the longer the rumble will be.

Three main kinds of lightning have been observed and documented, with several variations and subtypes. Lightning that goes between the cloud bottom and the ground is known as a cloud-to-ground flash (CG lightning); it is the most familiar type of lightning to humans and the best studied. While this is also the most dangerous kind of lightning, the most common is the lightning that occurs within a cloud, known as inside-cloud or intra-cloud (IC) lightning. Additionally, lightning will occur between different clouds; this is called cloud-to-cloud (CC) lightning. A rare but particularly dangerous kind of lightning occurs between the cloud tops and the ground. Because the thunderstorm causes the ground beneath it to be positively charged, the region outside of this area will be negatively charged. This creates a situation in which the positive charges on the top of the cloud can combine with the negative charges in the ground outside of the storm and result in powerful lightning bolts that strike the ground far from the storm cloud. This kind of lightning is called positive lightning, alluding to the positive charges in the tops of the clouds. Because the ground area where this lightning strikes can be far from the storm, with clear skies, these strokes are sometimes called "bolts from out of the blue."

Heat lightning is the same kind of lightning discussed here, except that it is so far away that the thunder cannot be heard. Thunder cannot be heard for more than a few kilometers, and even the strongest storms can seldom be heard more than 20 kilometers away. Lightning, however, can be seen for many tens of kilometers.

Two other types of atmospheric electrical discharge that are often associated with lightning are the mysterious sprites and jets. Sprites are weak but massive red flashes that appear directly over thunderstorms and extend upward to about 95 kilometers, lasting only a few thousandths of a second. Sprites were first documented in 1989 and were then observed from the space shuttle in 1990, but they have actually been reported for many years, and some historical documents indicate they have been observed for centuries.

Jets are another type of upward electrical discharge but are distinct from sprites. Jets are blue in color and shaped like large narrow cones, shooting upward from the tops of thunderstorms at about 100 kilometers per second and reaching altitudes of 40 to 50 kilometers.

Sprites and jets may be a feature of every active thunderstorm and may play an important role in the earth's global electrical environment. Both sprites and jets can be seen with the naked eye from dark areas at night above thunderstorms near the horizon, but because they are so faint and last only a small fraction of a second, they are difficult to see. Although these and other phenomena are often called upper-atmospheric lightning, many researchers believe sprites and jets are not as closely related to other forms of lightning as previously believed, and prefer the term transient luminous event (TLE).

While the instantaneous amount of energy in a lightning stroke may reach a trillion watts, the event is very short, so the total amount of electrical energy expended by a lightning stroke will average to several hundred kilowatt hours. This would be enough electrical energy to supply a typical house for about a month.

Clearly, lightning is a global event, and the individual strokes are just pieces of a much larger picture. In any given second, there are about 100 million lightning flashes worldwide, for a total of about 8.6 billion strokes per day. This is enough electrical energy to supply the electrical needs of about 700,000 average sized homes for a year. Various experiments and proposals have been suggested to attempt to harvest the energy from lightning for human use. However, many challenges in the process—including the unpredictable nature of lightning, the great amount of equipment needed, and the difficulty in storing the energy—have prevented the development of any successful systems for capturing the energy of lightning bolts.

Applications

Lightning is among the deadliest of severe weather phenomena, though the number of lightning deaths in the United States significantly decreased throughout the latter half of the twentieth century and the early twenty-first century. According to the National Weather Service, in 2006 and 2007, there were more than forty deaths per year, but from 2009 to 2018, there was an average of twenty-seven deaths per year due to lightning strikes in the United States. In 2022, nineteen deaths were reported, followed by fourteen deaths in 2023 and twelve deaths in 2024. Global estimates of the average number of people killed by direct lightning strikes each year range from about six thousand to nearly twenty-five thousand. Surprisingly, about two-thirds of all people struck by lightning make a complete recovery. Many people struck by lightning who appear to be dead can even be revived with proper first-aid treatment.

The largest group of people struck by lightning is composed of people seeking shelter underneath trees that were then struck by lightning. Golfers and people in, on, or near open water also account for large numbers of victims.

Most lightning strikes occur outdoors, but a few people are killed indoors. The largest source of indoor deaths occur to people talking on the phone during a thunderstorm. If the telephone lines are struck by lightning, the current can flow through the lines to anyone talking on the phone. Most of the remaining indoor deaths occur to people near, or in contact with, some metal fixture connected to the metal structure of the house (such as plumbing) or plugged into house wiring systems. Lightning can also strike outdoor television antennae, with the resulting current flowing through and striking someone too close to the television. It is possible to protect a house from these kinds of events by placing a lightning rod on the house or lightning-protector devices on the electrical system of the house.

A common myth about lightning is that it "never strikes the same spot twice." The Empire State Building in New York City provides dramatic evidence that this is not true; it is hit by an average of twenty-three lightning strikes per year. In fact, the properties of a particular location that lead it to be struck in the first place may make it susceptible to being hit again.

There are no locations completely safe from lightning, but some are certainly safer than others. Enclosed buildings and automobiles are among the safest locations; however, the rubber tires on a car do not provide protection if the car is struck. The electrical charge will go along the wet surface of the tire, or even through the tire itself. This last case is particularly more evident with steel-belted tires. The few inches of rubber in a tire will not stop an electrical current of the magnitude that is carried by lightning. This is also true of rubber-soled shoes, which do not provide protection from lightning.

A device that will help to protect a structure is the lightning rod. Lightning rods do not provide protection by preventing lightning from striking a structure and may even increase the odds that it will be struck. They provide protection by supplying a convenient path for the electrical current. Lightning rods are placed at high elevations on structures and are connected to the ground with conducting wires. Lightning tends to strike high, pointed objects, and so the lightning rod is more likely to be struck than the structure on which it is located. The current then flows through the lightning rod, down the wire, and safely into the ground.

In the event that one is outside when a thunderstorm develops, the most important things to do are not to make a lightning rod of oneself and to avoid things that might attract lightning. Do not project above the surrounding landscape, and do not get under isolated trees or isolated shelters. Avoid standing near any metal structure such as a fence or pipe, because these objects, if struck by lightning, will conduct the electricity into the ground where it could then strike a bystander. Likewise, avoid getting in water such as swimming pools and lakes, because the impurities in the water will also conduct electricity.

It is possible to purchase lightning detectors for relatively small amounts, but an AM radio will work well for most applications. Lightning generates radio waves in the AM band, so tune the radio to a channel without a broadcast station and listen. If static is audible, there is lightning in the area, and appropriate precautions should be taken.

Lightning is capable of doing widespread damage that can lead to great loss of life. On December 8, 1963, a Boeing 707 jetliner exploded after being hit by lightning near Elkton, Maryland, killing everyone on board. While this extreme example is very rare (but not isolated), aircraft are struck by lightning frequently and require protection.

Lightning strikes are also capable of killing livestock. On July 22, 1918, 504 sheep were killed by a single lightning strike in Utah. Hundreds of livestock animals are killed annually in the United States by lightning.

Fire is another source of damage caused by lightning. Every year, hundreds of homes are destroyed by fires caused by lightning. While it is possible to protect these structures with lightning rods, brush and forest fires cannot be prevented this way, and these fires are responsible for burning tens of thousands of acres every year.

Additional damage from lightning includes power outages caused by strikes on transformers, power lines, and generating stations. This damage not only is dangerous but also increases the cost of doing business.

The total amount of electricity generated by thunderstorms is approximately equal to two times the total electrical generating capacity of the United States. This is a significant amount of electricity. Work has been done to try to find a way to harness this source of energy, but the problem is that this energy is spread over the entire globe, and any one area will experience only a small percentage of the total. However, it may be possible in the future to find a way to collect enough of this energy to make it economically feasible.

Context

Because of the dramatic nature of lightning, it has been observed for thousands of years, and all peoples throughout history have developed a mythology concerning it. Yet it was not until the mid-1700s that scientists began to understand this phenomenon. While many scientists believed lightning was electrical in nature, it was Benjamin Franklin who was the first to prove it. His first experiment, designed in 1750, described a room on top of a house with an iron rod going from inside the room through a door and then rising 7 to 10 meters. An experimenter would stand inside, and if the rod became electrified, the experimenter would be able to detect it. This experiment was performed in May, 1752, in France by Thomas-Francois D'Alibard. It was successfully repeated in France, Belgium, and England that same year and in Sweden the following year.

Before he heard of these results, Benjamin Franklin devised his more famous experiment of flying a kite in a thunderstorm with a key on the end of the string. Franklin observed sparks jumping from the key to his knuckles, thus proving the presence of electricity. These experiments are very dangerous, however, and many experimenters have died attempting to duplicate both of them. If lightning had struck Franklin's kite, he almost certainly would have been killed.

Today, researchers send large weather balloons equipped with instruments into thunderstorms or fly specially equipped aircraft right into the worst of the storm. Similar to Franklin's kite experiment are special rockets that are launched into thunderstorms with wires connecting them to the ground. If the rocket or trailing wire is struck by lightning, the current flows dramatically down the wire to instruments on, or in, the ground. Lightning detectors are located all over the world to provide scientists with large-scale views of the world's thunderstorm environment, and lightning is even studied with satellites from space.

Like all things in nature, lightning has a function, and scientists seek to understand this function fully. What role does lightning play in the thunderstorm? Can analysis of lightning help with weather forecasting? Is there a relationship between the amount of lightning over a period of time and events such as droughts and floods? Lightning is part of the earth's global electrodynamic circuit and must be telling us something about the planet's atmosphere. Lightning researchers thus seek to understand what it is saying.

Principal terms

DART LEADER: A continuous leader that leads to successive flashes of a lightning stroke

LEADER: The conductive path of ionized air that forms prior to a lightning stroke

LIGHTNING: A sudden flash of light generated by the rapid flow of charged particles between oppositely charged regions of a cloud or a cloud and the ground

RETURN STROKE: The electric discharge of large amounts of electrons that results in the brilliant flash of lightning

STEP LEADER: The initial leader that forms the first full conducting connection between oppositely charged regions

THUNDER: A sound emitted by the explosively expanding gases along a stroke of lightning

Bibliography

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