A visible electric discharge in the form of a flash of light which results from charge separation in a thundercloud. There are two parts to the flash: the first is from the cloud to the ground or tall structure; the second is the return stroke from ground to cloud. The reflection of lightning on surrounding clouds, in which the illumination is diffused, is known as sheet lightning.
Lightning is an atmospheric discharge of electricity, usually, but not always, during a rain storm. William Wall (1708), Stephen Gray (1735), and Abbé Nollet) proposed that small-scale sparks shared some similarity with lightning.
Benjamin Franklin also invented the lightning rod, endeavouring to test this theory using a spire which was being erected in Philadelphia. However, experiments involving lightning are always risky and frequently fatal. While the experiment was under way, ball lightning appeared, collided with Richmann's head and killed him, leaving a red spot. (Discuss)
Although experiments from the time of Franklin showed that lightning was a discharge of static electricity, there was little improvement in theoretical understanding of lightning (in particular how it was generated) for more than 150 years. The impetus for new research came from the field of power engineering: as power transmission lines came into service, engineers needed to know much more about lightning in order to adequately protect lines and equipment.
An initial bipolar discharge, or path of ionized air, starts from a negatively charged region in the thundercloud. This initial phase involves a relatively small electric current (tens or hundreds of amperes), and the leader is almost invisible compared to the subsequent lightning channel.
As one of the rising streamers meets a stepped leader, the circuit is closed, and the main lightning stroke (often referred to as the return stroke) follows with much higher current.
In addition, negative lightning usually contains a number of restrikes along the same channel.
Positive lightning (a rarer form of lightning that originates from positively charged regions of a thundercloud) does not generally fit the above pattern.
NASA scientists have also found that the radio waves created by lightning clear a safe zone in the radiation belt surrounding the earth.
How it is formed
The first process in the generation of lightning is still a matter of debate: one common idea from scientists is that lightning forms from the ejection of charged particles from the sun, which reach Earth through the solar wind. Large quantities of ice in the clouds is suspected to enhance lightning development. This may assist in the forcible separation of positive and negative charge carriers within a cloud or air, and thus help in the formation of lightning.
Polarization mechanism theory
The mechanism by which charge separation happens is still the subject of research, but one theory is the polarization mechanism, which has two components:
Falling droplets of ice and rain become electrically polarized as they fall through the atmosphere's natural electric field; Charge separation and accumulation continue until the electrical potential becomes sufficient to initiate lightning discharges.The discharge
When sufficient negative and positive charges gather, and when the electric field becomes sufficiently strong, an electrical discharge (the bolt of lightning) occurs within clouds or between clouds and the ground.
A theory proposed by Alex Gurevich of the Lebedev Physical Institute in 1992 suggests that lightning strikes are triggered by cosmic rays which ionize atoms, releasing electrons that are accelerated by the electric fields, ionizing other air molecules and making the air conductive by a runaway breakdown, then starting a lightning strike.
As the cloud progresses over the Earth's surface, an equal but opposite charge is induced in the Earth below, and the induced ground charge follows the movement of the cloud. The region of high current propagates back up the positive stepped leader into the cloud with a "return stroke" that is the most luminous part of the lightning discharge. Lightning can also occur within the ash clouds from volcanic eruptions, or can be caused by violent forest fires which generate sufficient dust to create a static charge.
It has been seen using "stop action" movies of lightning strikes that most lightning strikes consist of several (up to 12) separate discharges of different intensities, causing the "flickering" effect commonly seen during a lightning discharge. The rolling and gradually dissipating rumble is caused by the heating and cooling of the discharge channel, by successive lightning strokes, and the time delay of sound coming from different portions of a long stroke.
An average bolt of negative lightning carries a current of 30-to-50 kiloamperes (kA), although some bolts can be up to 120kA, and transfers a charge of 5 coulombs and 500 megajoules (enough to light a 100 watt light bulb for 2 months). However, it has been observed from experiments that different locations in the US have different potentials (voltages) and currents, in an average lightning strike for that area. California, on the other hand, has fewer lightning strikes (being dryer). while Oklahoma, with cloud bases about 1,500-2,000 feet above ground level and fairly soft, clay-rich soil, has big, blue-white explosive lightning strikes, that are very hot (high current) and cause sudden, explosive noise when the discharge comes.
Gamma rays and the runaway breakdown theory
It has been discovered in the past 15 years that among the processes of lightning is some mechanism capable of generating gamma rays, which escape the atmosphere and are observed by orbiting spacecraft.
A 1996 study by Professor Umran Inan of Stanford University linked a TGF to an individual lightning stroke occurring within 1.5 ms of the TGF event, proving that the TGF was of atmospheric origin and associated with lightning strikes.
Scientists from Duke University have been studying the link between certain lightning events and the mysterious gamma ray emissions that emanate from the Earth's own atmosphere, in light of newer observations of TGFs made by the RHESSI spacecraft.
In the new study, Cummer and his co-researchers made what he termed "very careful and continuous recordings" of lightning emissions in a targeted area over a four month period of 2004. They identified lightning episodes they could link in time and place to TGFs recorded by RHESSI in the tropical Caribbean region. So we were able to say with very strong certainty whether lightning happened in the Caribbean at a specific time," said Cummer.
"If this were the operating mechanism, we should see enormous lightning strokes associated with every one of those TGFs," Cummer said. Instead, the lightning strokes his group analyzed were 50-500 times smaller than what should be required to create TGFs by runaway breakdown.
The analysis also disclosed that, on average, TGFs occurred 1.24 milliseconds before their associated lightning strokes. "But the coincidence between the lightning and the TGFs we found is too good to be random. So, even if the TGFs precede the lightning, they are in some way connected."
Questions linger, though, about whether the TGF really does occur prior to the lightning stroke, or whether it ccurs slightly after as an effect. For instance, the energy levels seen by the RHESSI spacecraft tend to drop in time over the duration of the TGF, suggesting a weakening effect rather than the strengthening buildup that would logically occur just before a lightning strike.
Furthermore, the accuracy of the RHESSI clock, and the reliant conclusion that TGFs precede lightning, has been called into question by a study of TGFs observed by the Compton Gamma Ray Observatory, the original spacecraft which "accidentally" discovered these TGFs. A recent study by Stanford University has made a similar analysis of TGFs from the original CGRO spacecraft, and found based on this dataset that TGFs in fact came after the lightning stroke by 1-3 ms.
A recent comparison of a similar event between RHESSI and other spacecrafts indicated an offset of roughly 3 ms, implying that TGFs thought to be 1.24 ms early were actually occurring ~1.5 ms late, in line with the Stanford study of CGRO events.
TGFs occurring in the time following a lightning stroke is more suggestive of TGFs being generated at higher altitudes, 10s of km above the thundercloud, where the gamma rays can easily escape the very thin atmosphere without being absorbed.
Positive lightning
Positive lightning makes up less than 5% of all lightning. Research carried out after the discovery of positive lightning in the 1970s showed that positive lightning bolts are typically six to ten times more powerful than negative bolts, last around ten times longer, and can strike tens of kilometres/miles from the clouds. The voltage difference for positive lightning must be considerably higher, due to the tens of thousands of additional metres/feet the strike must travel. During a positive lightning strike, huge quantities of ELF and VLF radio waves are generated.
As a result of their greater power, positive lightning strikes are considerably more dangerous.
Positive lightning is also now believed to have been responsible for the 1963 in-flight explosion and subsequent crash of Pan Am Flight 214, a Boeing 707. Subsequently, aircraft operating in U.S. airspace have been required to have lightning discharge wicks to reduce the chances of a similar occurrence.
Positive lightning has also been shown to trigger the occurrence of upper atmosphere lightning.
An average bolt of positive lightning carries a current of up to 300 kiloamperes (about ten times as much current as a bolt of negative lightning), transfers a charge of up to 300 coulombs, has a potential difference up to 1 gigavolt (a billion volts), and lasts for hundreds of milliseconds, with a discharge energy of up to 3x1011joule.
Types of lightning
Some lightning strikes take on particular characteristics; scientists and the public have given names to these various types of lightning.
Intracloud lightning, sheet lightning, anvil crawlers
Intracloud lightning is the most common type of lightning, and occurs completely inside one cumulonimbus cloud; it is termed sheet lightning because the bolt is not seen, instead one sees the whole cloud light up from inside. Lightning that appears to travels extensively along the cloud anvil or its base is commonly called a crawler, or sometimes 'spider lightning'.
Cloud-to-ground lightning, anvil-to-ground lightning
Cloud-to-ground lightning is a great lightning discharge between a cumulonimbus cloud and the ground initiated by the downward-moving leader stroke. This is the second most common type of lightning. One special type of cloud-to-ground lightning is anvil-to-ground lightning, a form of positive lightning, since it emanates from the anvil top of a cumulonimbus cloud where the ice crystals are positively charged. In anvil-to-ground lightning, the leader stroke issues forth in a nearly horizontal direction until it veers toward the ground.
Bead lightning, ribbon lightning, staccato lightning
Another special type of cloud-to-ground lightning is bead lightning. A third special type of cloud-to-ground lightning is ribbon lightning. The last special type of cloud-to-ground lightning is staccato lightning, which is nothing more than a leader stroke with only one return stroke.
Rare image of bead lightning by Marty Pouwelse
Cloud-to-cloud lightning
The bolt of lighting hits a clouds
Ground-to-cloud lightning
Ground-to-cloud lightning is a lightning discharge between the ground and a cumulonimbus cloud from an upward-moving leader stroke. Lightning occurs less frequently in the winter because there is not as much instability and moisture in the atmosphere as there is in the summer. These two ingredients work together to make convective storms that can produce lightning. Lightning originates around 15,000 to 25,000 feet above sea level when raindrops are carried upward until some of them convert to ice. For reasons that are not widely agreed upon, a cloud-to-ground lightning flash originates in this mixed water and ice region.
However, it has been proven by movies taken of typical lightning strikes and then, using single-frame examination (looking at each frame of a sequence), that a typical lightning strike is made up of anywhere from 8 to 12 or more individual discharges, with each successive discharge being less intense and farther apart in time.
Heat lightning or summer lightning
Heat lightning (or, in the UK, "summer lightning") is nothing more than the faint flashes of lightning on the horizon or other clouds from distant thunderstorms. Heat lightning was named because it often occurs on hot summer nights. Heat lightning can be an early warning sign that thunderstorms are approaching. In Florida, heat lightning is often seen out over the water at night, the remnants of storms that formed during the day along a seabreeze front coming in from the opposite coast.
Some cases of "heat lightning" can be explained by the refraction of light or sound by bodies of air with different densities. An observer may see nearby lightning, but the sound from the discharge is refracted over his head by a change in the temperature, and therefore the density, of the air around him. As a result, the lightning discharge seems to be silent.
Ball lightning
Ball lightning is described as a floating, illuminated ball that occurs during thunderstorms. Ball lightning has been described by eyewitnesses but rarely, if ever, recorded by meteorologists. There is some speculation that electrical breakdown and arcing of cotton and gutta-percha wire insulation used by Tesla may have been a contributing factor, since some theories of ball lightning require the involvement of carbonaceous materials.
Several theories have been advanced to describe ball lightning, with none being universally accepted. Any complete theory of ball lightning must be able to describe the wide range of reported properties, such as those described in Singer's book "The Nature of Ball Lightning" and also more contemporary research. Japanese research shows that ball lightning has been seen several times without any connection to stormy weather or lightning.
Ball lightning field properties are more extensive than realized by many scientists not working in this field. The typical diameter is usually standardized as 20–30 cm (8-12 inches), but ball lightning several meters in diameter has been reported (Singer).
Ball lightning has been seen in tornadoes, and has also been seen to split apart into two or more separate balls and recombine. Ball lightning has carved trenches in the peat swamps in Ireland.
Another very strong possibility is that ball lightning may be caused by plasma. It is not the same as ball lightning.
In 2000, students at Highland Park High School in Dallas Texas were sent home after ball lightning reportedly traveled through a school corridor.
Sprites, elves, jets, and other upper atmospheric lightning
Reports by scientists of strange lightning phenomena above storms date back to at least 1886.
Sprites are now well-documented electrical discharges that occur high above the cumulonimbus cloud of an active thunderstorm. They appear as luminous reddish-orange, neon-like flashes, last longer than normal lower stratospheric discharges (typically around 17 milliseconds), and cause the discharges of positive lightning between the cloud and the ground. Sprites can occur up to 50 km (30 miles) from the location of the lightning strike, and with a time delay of up to 100 milliseconds.
Recent research carried out at the University of Houston in 2002 indicates that some normal (negative) lightning discharges produce a sprite halo, the precursor of a sprite, and that every lightning bolt between cloud and ground attempts to produce a sprite or a sprite halo. More probably, as said before, they may be generated from interaction with the upper atmosphere's neutralizing a charge derived from the Earth's movement through the Solar Wind.
Blue jets differ from sprites in that they project from the top of the cumulonimbus above a thunderstorm, typically in a narrow cone, to the lowest levels of the ionosphere 40 to 50 km (25 to 30 miles) above the earth.
Elves often appear as a dim, flattened, expanding glow around 400 km (250 miles) in diameter that lasts for, typically, just one millisecond.
On September 14, 2001, scientists at the Arecibo Observatory photographed a huge jet double the height of those previously observed, reaching around 80 km (50 miles) into the atmosphere. Lightning was initially observed traveling up at around 50,000 m/s in a similar way to a typical blue jet, but then divided in two and sped at 250,000 m/s to the ionosphere, where they spread out in a bright burst of light.
On July 22, 2002, five gigantic jets between 60 and 70 km (35 to 45 miles) in length were observed over the South China Sea from Taiwan, reported in Nature.
Researchers have speculated that such forms of upper atmospheric lightning may play a role in the formation of the ozone layer.
Streak lightning
Most lightning is streak lightning. This is nothing more than the return stroke, the visible part of the lightning stroke.
Triggered lightning
Lightning has been triggered directly by human activity in several instances. Lightning struck the Apollo 12 soon after takeoff, and has struck soon after thermonuclear explosions. The wire unwinds as the rocket climbs, making a convenient path for the lightning to use.
Lightning during volcanic eruptions
Extremely large volcanic eruptions, which eject gases and solid material high into the atmosphere can trigger lightning, and this phenomenon was documented by Pliny The Elder during the AD79 eruption of Vesuvius in which he perished.
Rocket Lightning
A form of cloud discharge, generally horizontal and at cloud base, with a luminous channel appearing to advance through the air with visually resolvable speed, often intermittently.
Lightning throughout the Solar System
Lightning requires the electrical breakdown of gas, so it cannot exist in a visual form in the vacuum of space. However, lightning has been observed within the atmospheres of other planets, such as Venus and Jupiter. Lightning on Jupiter is estimated to be 100 times as powerful as, but fifteen times less frequent than, that which occurs on Earth. Lightning on Venus is still a controversial subject after decades of study. During the Soviet Venera and U.S. Pioneer missions of the 1970s and 80s, signals suggesting lightning may be present in the upper atmosphere were detected. However, recently the Cassini-Huygens mission fly-by of Venus detected no signs of lightning at all.
Lightning safety
Thunderstorms are the primary source of lightning. Because people have been struck many miles away from a storm, seeking immediate and effective shelter when thunderstorms approach is an important part of lightning safety.
Several different types of devices, including lightning rods and electrical charge dissipators, are used to prevent lightning damage and safely redirect lightning strikes.
Nearly 2000 people per year in the world are injured by lightning strikes, and between 25 to 33% of those struck die. Lightning injuries result from three factors: electrical damage, intense heat, and the mechanical energy which these generate. While sudden death is common because of the huge voltage of a lightning strike, survivors often fare better than victims of other electrical injuries caused by a more prolonged application of lesser voltage.
Lightning can incapacitate humans in four different ways:
Direct strike 'Splash' from nearby objects struck Ground strike near victim causing a difference of potential in the ground itself (due to resistance to current in the Earth), amounting to several thousand volts per foot, depending upon the composition of the earth that makes up the ground at that location. EMP or electromagnetic pulse from close strikes - especially during positive lightning dischargesIn a direct hit the electrical charge strikes the victim first. Splash hits occur when lightning prefers a victim (with lower resistance) over a nearby object that has more resistance, and strikes the victim on its way to ground. Many victims suffer immediate cardiac arrest and will not survive without prompt emergency care, which is safe to administer because the victim will not retain any electrical charge after the lightning has struck (of course, the helper could be struck by a separate bolt of lightning in the vicinity). The intense heat generated by a lightning strike can burn tissue, and cause lung damage, and the chest can be damaged by the mechanical force of rapidly expanding heated air.
The lightning often leaves skin burns in characteristic Lichtenberg figures, sometimes called lightning flowers; It is also speculated that the EMP created by a nearby lightning strike can cause cardiac arrest.
There is sometimes spectacular and unconventional lightning damage. Hot lightning (high-current lightning) which lasts for more than a second can deposit immense energy, melting or carbonizing large objects.
Facts and trivia
A bolt of lightning can reach temperatures approaching 28,000 degrees Celsius (50,000 degrees Fahrenheit) in a split second. The heat of lightning that strikes loose soil or sandy regions of the ground may fuse the soil or sand into glass channels called fulgurites. Fulgurites are evidence that lightning spreads out into branching channels when it strikes the ground.
Trees are frequent conductors of lightning to the ground . Since sap is a poor conductor, its electrical resistance causes it to be heated explosively into steam, which blows off the bark outside the lightning's path. It is commonly thought that a tree standing alone is more frequently struck, though in some forested areas, lightning scars can be seen on almost every tree.
Of all common trees the most frequently struck is the oak.
The odds of an average person living in the USA being struck by lightning once in his lifetime has been estimated to be 1:280,000. The odds of having a friend or family member struck by lightning in the USA in a lifetime has been estimated to 1:3000. Singapore has one of the highest rate of lightning activity in the world. The city of Teresina in northern Brazil has the third-highest rate of occurrences of lightning strikes in the world. The surrounding region is referred to as the Chapada do Corisco ("Flash Lightning Flatlands"). The United States is home to "Lightning Alley", a group of states in the American Southeast that collectively see more lightning strikes per year than any other place in the US. The most notable state in Lightning Alley is Florida. The saying "lightning never strikes twice in the same place" is false. The Empire State Building is struck by lightning on average 100 times each year, and was once struck 15 times in 15 minutes. Jim Caviezel, the actor who played Jesus in the film The Passion of the Christ, is reported to have been struck twice by lightning during shooting. Golfers Retief Goosen and Lee Trevino have both been struck by lightning while playing. Although commonly associated with close thunderstorms, lightning strikes can occur on a day that seems devoid of clouds. This occurrence is known as "A Bolt From the Blue" and is due to the fact that lightning can strike up to 10 miles from a cloud. Lightning interferes with AM (amplitude modulation) radio signals much more than FM (frequency modulation) signals, providing an easy way to gauge local lightning strike intensity. Roy Sullivan has the record for being the human who has been struck by lightning the most times. Colombian soccer player Herman Gaviria a.k.a Carepa, was struck by lightning during a training session in Cali, Colombia and died at the age of 37. Strangely, before starting the session, he said "Lightning is not going to kill me." On average, lightning strikes the earth about 100 times every second.In movies and comics of the contemporary U.S. and many other countries, lightning is often employed as an ominous, dramatic sign. This has often also been spoofed, with the uttering of certain words or phrases causing flashes of lightning to appear outside of windows (and often scaring or disturbing some characters). Various novels and role playing games with fantasy tint involves wizardry of lightning bolt, weapon embodying the power of lightning, etc. The comic book character Billy Batson changed into the superhero Captain Marvel by saying the word "Shazam!", which called down a bolt of magic lightning to make the change. Flash II (Barry Allen) and III (Wally West) were both granted their superspeed in accidents involving lightning. Also in the 1931 movie, Frankenstein, lightning is used as the catalyst for endowing life into the creature.
The bolt of lightning in heraldry is called a thunderbolt and is shown as a zigzag with non-pointed ends. It is also distinguished from the "fork of lightning". The lightning bolt shape was a symbol of male humans among the Native Americans such as the Apache (a rhombus shape being a symbol for females) in the American Old West.
The name of New Zealand's most celebrated thoroughbred horse, Phar Lap, derives from the shared Zhuang and Thai word for lightning.
Some European languages have a separate word for lightning which strikes the ground (as opposed to lightning in general).
Estimating distance of a lightning strike: The flash of a lightning strike and resulting thunder occur at roughly the same time. Sound travels at the slower speed of 330 m/s in the same time, so the flash of lightning is seen before thunder is heard. P., "Elves : Ionospheric Heating By the Electromagnetic Pulses from Lightning (A primer)". Wayt, "Sprites and Elves : Lightning's strange cousins flicker faster than light itself". Lucky snapshot: lightning strikes chemical mill in Germany Lightning detection system shows lightning activity in the Tampa Bay, Florida area Sprites, jets and TLE pictures and articles
User Comments Add a comment…