Lightning Explained

Lightning should not be confused with Lighting.

For other uses see Lightning (disambiguation).

Lightning is an atmospheric discharge of electricity usually accompanied by thunder, which typically occurs during thunderstorms, and sometimes during volcanic eruptions or dust storms. In the atmospheric electrical discharge, a leader of a bolt of lightning can travel at speeds of 60000abbr=onNaNabbr=on, and can reach temperatures approaching 30000abbr=onNaNabbr=on, hot enough to fuse silica sand into glass channels known as fulgurites which are normally hollow and can extend some distance into the ground. There are some 16 million lightning storms in the world every year. For an American, the chance of being struck by lightning is approximately 1 in 576,000 and the chance of actually being killed by lightning is approximately 1 in 2,320,000.[1]

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.

How lightning initially forms is still a matter of debate: Scientists have studied root causes ranging from atmospheric perturbations (wind, humidity, friction, and atmospheric pressure) to the impact of solar wind and accumulation of charged solar particles. Ice inside a cloud is thought to be a key element in lightning development, and may cause a forcible separation of positive and negative charges within the cloud, thus assisting in the formation of lightning.

Historical scientific research

Benjamin Franklin (1706-1790) endeavored to test the theory that sparks shared some similarity with lightning by using a spire which was being erected in Philadelphia. While waiting for completion of the spire, he got the idea to use a flying object such as a kite. During the next thunderstorm, which was in June 1752, it was reported that he raised a kite, accompanied by his son as an assistant. On his end of the string he attached a key, and he tied it to a post with a silk thread. As time passed, Franklin noticed the loose fibers on the string stretching out; he then brought his hand close to the key and a spark jumped the gap. The rain which had fallen during the storm had soaked the line and made it conductive.

Franklin was not the first to perform the kite experiment. Thomas-François Dalibard and De Lors conducted it at Marly-la-Ville in France, a few weeks before Franklin's experiment.[2] In his autobiography (written 1771-1788, first published 1790), Franklin clearly states that he performed this experiment after those in France, which occurred weeks before his own experiment, without his prior knowledge as of 1752.[3]

As news of the experiment and its particulars spread, others attempted to replicate it. However, experiments involving lightning are always risky and frequently fatal. One of the most well-known deaths during the spate of Franklin imitators was that of Professor George Richmann of Saint Petersburg, Russia. He created a set-up similar to Franklin's, and was attending a meeting of the Academy of Sciences when he heard thunder. He ran home with his engraver to capture the event for posterity. According to reports, while the experiment was under way, ball lightning appeared and collided with Richmann's head, killing him.

Although experiments from the time of Benjamin 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. In 1916, Charles Steinmetz was the first to demonstrate artificial lightning by using a generator capable of producing 10,000 amperes and more than 100,000 volts.

Properties of lightning

An average bolt of lightning carries an electric current of 40 kiloamperes (kA), and transfers a charge of five coulombs and 500 MJ. Large bolts of lightning can carry up to 120 kA and 350 coulombs[https://www.llnl.gov/str/pdfs/05_96.1.pdf]. The voltage is proportional to the length of the bolt.

Lightning leader development is not just a matter of the dielectric breakdown air, which is about three million volts per metre. The ambient electric fields required for lightning leader propagation can be one or two orders of magnitude less than dielectric breakdown strength. The potential gradient inside a well-developed return-stroke channel is in the order of hundreds of volts per metre due to intense channel ionisation, resulting in a true power output in the order of a megawatt per metre for a vigorous return stroke current of 100 kA.[4] The average power output of a single lightning stroke is about a terawatt (1012 W) and it lasts for around 30 microseconds.[5]

Lightning rapidly heats the air in its immediate vicinity to around 20,000 °C (36,000 °F) - about three times the temperature of the surface of the Sun. This compresses the surrounding clear air and creates a supersonic shock wave which decays to an acoustic wave that is heard as thunder.[5]

The return stroke of a lightning bolt follows a charge channel about a centimetre (0.4-in) wide.

Different locations have different potentials (voltages) and currents for an average lightning strike. For example, Florida, with the United States' largest number of recorded strikes in a given period during the summer season, has very sandy ground in some areas and conductive saturated mucky soil in others. As much of Florida lies on a peninsula, it is bordered by the ocean on three sides. The result is the daily development of sea and lake breeze boundaries that collide and produce thunderstorms. The difference in each case may consist of differences in voltage levels between clouds and ground. NASA scientists have found the radio waves created by lightning clear a safe zone in the radiation belt surrounding the earth. This zone, known as the Van Allen Belt slot, can potentially be a safe haven for satellites, offering them protection from the Sun's radiation.[6]

Formation

Note:Positive lightning (a rarer form of lightning that originates from positively charged regions of the thundercloud) does not generally fit the following pattern.

Electrostatic induction hypothesis

According to the electrostatic induction hypothesis charges are driven apart by as-yet uncertain processes. Charge separation appears to require strong updrafts which carry water droplets upward, supercooling them to between -10 and -20 °C. These collide with ice crystals to form a soft ice-water mixture called graupel. The collisions result in a slight positive charge being transferred to ice crystals, and a slight negative charge to the graupel. Updrafts drive the less heavy ice crystals upwards, causing the cloud top to accumulate increasing positive charge. Gravity causes the heavier negatively charged graupel to fall toward the middle and lower portions of the cloud, building up an increasing negative charge. Charge separation and accumulation continue until the electrical potential becomes sufficient to initiate a lightning discharge, which occurs when the distribution of positive and negative charges forms a sufficiently strong electric field.[5]

Polarization mechanism hypothesis

The mechanism by which charge separation happens is still the subject of research. Another hypothesis is the polarization mechanism, which has two components:[7]

  1. Falling droplets of ice and rain become electrically polarized as they fall through the Earth's natural electric field;
  2. Colliding ice particles become charged by electrostatic induction (see above).

There are several additional hypotheses for the origin of charge separation.[8]

According to one such hypothesis, charge separation is initiated by the ionisation of an air molecule by an incoming cosmic ray. [9]

Leader formation and the return stroke

As a thundercloud moves over the surface of the Earth, an electric charge equal to but opposite the charge of the base of the thundercloud is induced in the Earth below the cloud. The induced ground charge follows the movement of the cloud, remaining underneath it.

An initial bipolar discharge, or path of ionized air, starts from a negatively charged mixed water and ice region in the thundercloud. Discharge ionized channels are known as leaders. The negatively charged leaders, generally a "stepped leader", proceed downward in a number of quick jumps (steps). Each step is on the order of 50 to 100 ft (15 to 30 metres) long but may be up to 165 ft (50 m).[10] As it continues to descend, the stepped leader may branch into a number of paths. The progression of stepped leaders takes a comparatively long time (hundreds of milliseconds) to approach the ground. This initial phase involves a relatively small electric current (tens or hundreds of amperes), and the leader is almost invisible when compared with the subsequent lightning channel.

When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the strength of the electric field. The electric field is strongest on ground-connected objects whose tops are closest to the base of the thundercloud, such as trees and tall buildings. If the electric field is strong enough, a conductive discharge (called a positive streamer) can develop from these points. This was first theorized by Heinz Kasemir. As the field increases, the positive streamer may evolve into a hotter, higher current leader which eventually connects to the descending stepped leader from the cloud. It is also possible for many streamers to develop from many different objects simultaneously, with only one connecting with the leader and forming the main discharge path. Photographs have been taken on which non-connected streamers are clearly visible.[11] When the down and up leaders meet, the flow of electric current greatly increases.

Once a channel of ionized air is established between the cloud and ground this becomes a path of least resistance and allows for a much greater current to propagate from the Earth back up the leader into the cloud. This is the return stroke and it is the most luminous and noticeable part of the lightning discharge.

Discharge

When the electric field becomes strong enough, an electrical discharge (the bolt of lightning) occurs within clouds or between clouds and the ground. During the strike, successive portions of air become a conductive discharge channel as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions.

The electrical discharge rapidly superheats the discharge channel, causing the air to expand rapidly and produce a shock wave heard as thunder. The rolling and gradually dissipating rumble of thunder is caused by the time delay of sound coming from different portions of a long stroke.[12]

Gurevich's runaway breakdown theory

See main article: Runaway breakdown. A theory of lightning initiation, known as the "runaway breakdown theory", proposed by Aleksandr Gurevich[13] 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 "seeding" a lightning strike.[14] [15]

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. Brought to light by NASA's Gerald Fishman in 1994 in an article in Science,[16] these so-called Terrestrial Gamma-Ray Flashes (TGFs) were observed by accident, while he was documenting instances of extraterrestrial gamma ray bursts observed by the Compton Gamma Ray Observatory (CGRO). TGFs are much shorter in duration, however, lasting only about 1 ms.

Professor Umran Inan of Stanford University linked a TGF to an individual lightning stroke occurring within 1.5 ms of the TGF event,[17] proving for the first time that the TGF was of atmospheric origin and associated with lightning strikes.

CGRO recorded only about 77 events in 10 years; however, more recently the RHESSI spacecraft, as reported by David Smith of UC Santa Cruz, has been observing TGFs at a much higher rate, indicating that these occur about 50 times per day globally (still a very small fraction of the total lightning on the planet). The voltage levels recorded exceed 20 MeV.

Scientists from Duke University have also 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 RHESSI. Their study suggests that this gamma radiation fountains upward from starting points at surprisingly low altitudes in thunderclouds.

Steven Cummer, from Duke University's Pratt School of Engineering, said, "These are higher energy gamma rays than come from the sun. And yet here they are coming from the kind of terrestrial thunderstorm that we see here all the time."

Early hypotheses of this pointed to lightning generating high electric fields at altitudes well above the cloud, where the thin atmosphere allows gamma rays to easily escape into space, known as "relativistic runaway breakdown", similar to the way sprites are generated. Subsequent evidence has cast doubt, though, and suggested instead that TGFs may be produced at the tops of high thunderclouds. Though hindered by atmospheric absorption of the escaping gamma rays, these theories do not require the exceptionally high electric fields that high altitude theories of TGF generation rely on.

The role of TGFs and their relationship to lightning remains a subject of ongoing scientific study.

Re-strike

High speed videos (examined frame-by frame) show that most lightning strikes are made up of multiple individual strokes. A typical strike is made of 3 to 4 strokes. There may be more.[18]

Each re-strike is separated by a relatively large amount of time, typically 40 to 50 milliseconds. Re-strikes can cause a noticeable "strobe light" effect.[12]

Each successive stroke is preceded by intermediate dart leader strokes akin to, but weaker than, the initial stepped leader. The stroke usually re-uses the discharge channel taken by the previous stroke.[19]

The variations in successive discharges are the result of smaller regions of charge within the cloud being depleted by successive strokes.

The sound of thunder from a lightning strike is prolonged by successive strokes.

Types of lightning

Some lightning strikes exhibit particular characteristics; scientists and the general public have given names to these various types of lightning. The lightning that is most-commonly observed is streak lightning. This is nothing more than the return stroke, the visible part of the lightning stroke. The majority of strokes occur inside a cloud so we do not see most of the individual return strokes during a thunderstorm.

Cloud-to-ground lightning

This is the best known and second most common type of lightning. Of all the different types of lightning, it poses the greatest threat to life and property. Cloud-to-ground lightning is a lightning discharge between a cumulonimbus cloud and the ground. It is initiated by a leader stroke moving down from the cloud (see Leader formation above).

Bead lightning

Bead lightning is a type of cloud-to-ground lightning which appears to break up into a string of short, bright sections, which last longer than the usual discharge channel. It is relatively rare. Several theories have been proposed to explain it; one is that the observer sees portions of the lightning channel end on, and that these portions appear especially bright. Another is that, in bead lightning, the width of the lightning channel varies; as the lightning channel cools and fades, the wider sections cool more slowly and remain visible longer, appearing as a string of beads.[20] [21]

Ribbon lightning

Ribbon lightning occurs in thunderstorms with high cross winds and multiple return strokes. The wind will blow each successive return stroke slightly to one side of the previous return stroke, causing a ribbon effect.

Staccato lightning

Staccato lightning is a cloud to ground lightning strike which is a short-duration stroke that appears as a single very bright flash and often has considerable branching.[22]

Fork lightning

Another name for cloud-to-ground lightning. Not in formal usage. Refers to its branching (forking) nature. Compare sheet lightning below.

Ground-to-cloud lightning

Ground-to-cloud lightning is a lightning discharge between the ground and a cumulonimbus cloud initiated by an upward-moving leader stroke. It is much rarer than cloud-to-ground lightning.

Cloud-to-cloud lightning

Lightning discharges may occur between areas of cloud without contacting the ground. When it occurs between two separate clouds it is known as inter-cloud lightning and when it occurs between areas of differing electric potential within a single cloud, it is known as intra-cloud lightning. Intra-cloud lightning is the most frequently occurring type.[5]

These are most common between the upper anvil portion and lower reaches of a given thunderstorm. This lightning can sometimes be observed at great distances at night as so-called "heat lightning". In such instances, the observer may see only a flash of light without hearing any thunder. The "heat" portion of the term is a folk association between locally experienced warmth and the distant lightning flashes.

Another terminology used for cloud-cloud or cloud-cloud-ground lightning is "Anvil Crawler", due to the habit of the charge typically originating from beneath or within the anvil and scrambling through the upper cloud layers of a thunderstorm, normally generating multiple branch strokes which are dramatic to witness. These are usually seen as a thunderstorm passes over the observer or begins to decay. The most vivid crawler behavior occurs in well developed thunderstorms that feature extensive rear anvil shearing.

Sheet lightning

Another name for cloud-to-cloud lightning. Not in formal usage. Refers to the diffuse brightening of the surface of a cloud, because the actual discharge path is hidden. Compare fork lightning above.

Heat lightning

See main article: Heat lightning. Sheet lightning that occurs too far away for the thunder to be heard.

Dry lightning

See main article: Dry lightning. Dry lightning is a term in the United States for lightning that occurs with no precipitation at the surface. This type of lightning is the most common natural cause of wildfires.

Rocket lightning

It is 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.[23]

Positive lightning

Positive lightning, also known colloquially as "bolts from the blue", makes up less than 5% of all lightning.[24] It occurs when the leader forms at the positively charged cloud tops, with the consequence that a negatively charged streamer issues from the ground. The overall effect is a discharge of positive charges to the ground. 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.[25] 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.[26]

As a result of their greater power, positive lightning strikes are considerably more dangerous. At the present time, aircraft are not designed to withstand such strikes, since their existence was unknown at the time standards were set, and the dangers unappreciated until the destruction of a glider in 1999.[27]

One type of positive lightning is anvil-to-ground, since it emanates from the anvil top of a cumulonimbus cloud where the ice crystals are positively charged. The leader stroke of lightning issues forth in a nearly horizontal direction until it veers toward the ground. These usually occur kilometers/miles from (and often ahead of) the main storm and will sometimes strike without warning on a sunny day. An anvil-to-ground lightning bolt is a sign of an approaching storm, and if one occurs in a largely clear sky, it is known colloquially as a "Bolt from the blue."[28]

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.[29] Due to the dangers of lightning, aircraft operating in U.S. airspace have been required to have lightning discharge wicks to reduce the damage by a lightning strike, but these measures may be insufficient for positive lightning.[30]

Positive lightning has also been shown to trigger the occurrence of upper atmosphere lightning. It tends to occur more frequently in winter storms, as with thundersnow, and at the end of a thunderstorm.[5]

An average bolt of positive lightning carries a current of up to 300 kA (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 (one billion volts), and lasts for hundreds of milliseconds, with a discharge energy of up to 300 GJ (gigajoules) (a billion joules).

Ball lightning

See main article: Ball lightning. Ball lightning may be an atmospheric electrical phenomenon, the physical nature of which is still controversial. The term refers to reports of luminous, usually spherical objects which vary from pea-sized to several meters in diameter.[31] It is sometimes associated with thunderstorms, but unlike lightning flashes, which last only a fraction of a second, ball lightning reportedly lasts many seconds. Ball lightning has been described by eyewitnesses but rarely recorded by meteorologists. Scientific data on natural ball lightning is scarce owing to its infrequency and unpredictability. The presumption of its existence is based on reported public sightings, and has therefore produced somewhat inconsistent findings.

Laboratory experiments have produced effects that are visually similar to reports of ball lightning, but it is presently unknown whether these are actually related to any naturally occurring phenomenon. Ball lightning apparently is created when lightning strikes silicon in soil, and has been created in a lab in this manner.[32] Given inconsistencies and the lack of reliable data, the true nature of ball lightning is still unknown.[33] Until recently, ball lightning was often regarded as a fantasy or a hoax.[34] Reports of the phenomenon were dismissed for lack of physical evidence, and were often regarded the same way as UFO sightings.[33]

Ball lightning has carved trenches in the peat swamps in Ireland. Because of its strange behaviour, ball lightning has been mistaken for alien spacecraft by many witnesses, which often spawns UFO reports. One theory that may account for this wider spectrum of observational evidence is the idea of combustion inside the low-velocity region of axisymmetric (spherical) vortex breakdown of a natural vortex (e.g., the 'Hill's spherical vortex').[35] Natural ball lightning appears infrequently and unpredictably, and is therefore rarely (if ever truly) photographed. However, several purported photos and videos exist. Perhaps the most famous story of ball lightning unfolded when 18th-century physicist Georg Wilhelm Richmann installed a lightning rod in his home and was struck in the head - and killed - by a "pale blue ball of fire."[36]

Upper-atmospheric lightning

See main article: Upper-atmospheric lightning. Reports by scientists of strange lightning phenomena above storms date back to at least 1886. However, it is only in recent years that fuller investigations have been made. This has sometimes been called megalightning.

Sprites

See main article: article and Sprites (lightning). Sprites are large scale electrical discharges which occur high above a thunderstorm cloud, or cumulonimbus, giving rise to a quite varied range of visual shapes. They are triggered by the discharges of positive lightning between the thundercloud and the ground.[37] The phenomena were named after the mischievous sprite (air spirit) Puck in Shakespeare's Midsummer Night's Dream. They normally are colored reddish-orange or greenish-blue, with hanging tendrils below, and arcing branches above, their location, and can be preceded by a reddish halo. They often occur in clusters, lying to above the Earth's surface. Sprites were first photographed on July 6, 1989 by scientists from the University of Minnesota and have since been witnessed tens of thousands of times.[38] Sprites have erroneously been held responsible for otherwise unexplained accidents involving high altitude vehicular operations above thunderstorms.[39]

Blue jets

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 to above the earth. They are also brighter than sprites and, as implied by their name, are blue in colour. They were first recorded on October 21 1989, on a video taken from the space shuttle as it passed over Australia, and subsequently extensively documented in 1994 during aircraft research flights by the University of Alaska.

On September 14 2001, scientists at the Arecibo Observatory photographed a huge jet double the height of those previously observed, reaching around into the atmosphere. The jet was located above a thunderstorm over the ocean, and lasted under a second. 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. The jets lasted under a second, with shapes likened by the researchers to giant trees and carrots.

Elves

Elves often appear as dim, flattened, expanding glows around in diameter that last for, typically, just one millisecond. They occur in the ionosphere above the ground over thunderstorms. Their color was a puzzle for some time, but is now believed to be a red hue. Elves were first recorded on another shuttle mission, this time recorded off French Guiana on October 7, 1990. Elves is a frivolous acronym for Emissions of Light and Very Low Frequency Perturbations from Electromagnetic Pulse Sources.[40] This refers to the process by which the light is generated; the excitation of nitrogen molecules due to electron collisions (the electrons possibly having been energized by the electromagnetic pulse caused by a discharge from the Ionosphere).

Triggered lightning

Rocket-triggered

Lightning has been triggered directly by human activity in several instances. Lightning struck Apollo 12 soon after takeoff, and has struck soon after thermonuclear explosions. It has also been triggered by launching lightning rockets carrying spools of wire into thunderstorms. The wire unwinds as the rocket ascends, providing a path for lightning. These bolts are typically very straight due to the path created by the wire.

Flying aircraft can also trigger lightning.[41]

Volcanically triggered

Extremely large volcanic eruptions, which eject gases and material high into the atmosphere, can trigger lightning. This phenomenon was documented by Pliny The Elder during the AD79 eruption of Vesuvius, in which he perished.[42]

Laser-triggered

Since the 1970s,[43] [44] [45] [46] researchers have attempted to trigger lightning strikes by means of infrared or ultraviolet lasers, which create a channel of ionized gas through which the lightning would be conducted to ground. Such triggered lightning is intended to protect rocket launching pads, electric power facilities, and other sensitive targets.[47]

In New Mexico, U.S., scientists tested a new terawatt laser which provoked lightning. Scientists fired ultra-fast pulses from an extremely powerful laser thus sending several terawatts into the clouds to call down electrical discharges in storm clouds over the region. The laser beams sent from the laser make channels of ionized molecules known as "filaments". Before the lightning strikes earth, the filaments lead electricity through the clouds, playing the role of lightning rods. Researchers generated filaments that lived too short a period to trigger a real lightning strike. Nevertheless, a boost in electrical activity within the clouds was registered. According to the French and German scientists, who ran the experiment, the fast pulses sent from the laser will be able to provoke lightning strikes on demand.[48] Statistical analysis showed that their laser pulses indeed enhanced the electrical activity in the thundercloud where it was aimed—in effect they generated small local discharges located at the position of the plasma channels.[49]

Extraterrestrial lightning

Lightning requires the electrical breakdown of a 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, Jupiter and Saturn. 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. Despite this, it has been suggested that radio pulses recorded by the spacecraft Venus Express may originate from lightning on Venus.(S&T, Mar. 2008)

Trees and lightning

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. In following seasons trees overgrow the damaged area and may cover it completely, leaving only a vertical scar. If the damage is severe, the tree may not be able to recover, and decay sets in, eventually killing the tree. 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.

The two most frequently struck tree types are the oak and the elm. Pine trees are also quite often hit by lightning. Unlike the oak, which has a relatively shallow root structure, pine trees have a deep central root system that goes down into the water table. Pine trees usually stand taller than other species, which also makes them a likely target. Factors which lead to its being targeted are a high resin content, loftiness, and its needles which lend themselves to a high electrical discharge during a thunderstorm.

Trees are natural lightning conductors, and are known to provide protection against lightning damages to the nearby buildings. Tall trees with high biomass for the root system provide good lightning protection. An example is the teak tree (Tectona grandis), which grows to a height of 45m (148feet). It has a spread root system with a spread of 5 m and a biomass of 4 times that of the trunk; its penetration into the soil is 1.25m (04.1feet) and has no tap root. When planted near a building, its height helps in catching the oncoming lightning leader, and the high biomass of the root system helps in dissipation of the lightning charges.[50]

Lightning currents have a very fast risetime, on the order of 40 kA per microsecond. Hence, conductors of such currents exhibit marked skin effect, causing most of the currents to flow through the conductor skin.[51] The effective resistance of the conductor is consequently very high and therefore, the conductor skin gets heated up much more than the conductor core. When a tree acts as a natural lightning conductor, due to skin effect most of the lightning currents flow through the skin of the tree and the sap wood. As a result, the skin gets burnt and may even peel off. The moisture in the skin and the sap wood evaporates instantaneously and may get split. If the tree struck by lightning is a teak tree (single stemmed with branches) it may not be completely destroyed since only the tree skin and a branch may be affected; the major parts of the tree may be saved from complete destruction due to lightning currents. But if the tree involved is a coconut tree it may be completely destroyed by the lightning currents.

Fulgurites

See main article: article and Fulgurite. Lightning strikes on sandy soil can produce fulgurites. These root-shaped tubes of melted and fused sand grains are sometimes called petrified lightning.

X-rays and lightning

The production of X-rays by a bolt of lightning was theoretically predicted as early as 1925 but no evidence was found until 2001/2002[52], when researchers at the New Mexico Institute of Mining and Technology, detected x-ray emissions from an induced lightning strike along a wire trailed behind a rocket shot into a storm cloud. In the same year University of Florida and Florida Tech researchers used an array of electric field and X-ray detectors at a lightning research facility in North Florida to confirm that natural lightning makes X-rays in large quantities. The cause of the X-ray emissions is still a matter for research, as the temperature of lightning is too cold to account for the X-rays observed.[53]

Sound

See also: Thunder.

Because the electrostatic discharge of terrestrial lightning superheats the air to plasma temperatures along the length of the discharge channel in a short duration, kinetic theory dictates gaseous molecules undergo a rapid increase in pressure and thus expand outward from the lightning creating a shock wave audible as thunder. Since the sound waves propagate not from a single point source but along the length of the lightning's path, the sound origin's varying distances from the observer can generate a rolling or rumbling effect. Perception of the sonic characteristics are further complicated by factors such as the irregular and possibly branching geometry of the lightning channel, by acoustic echoing from terrain, and by the typically multiple-stroke characteristic of the lightning strike.

Since light travels at a significantly greater speed than sound through air, an observer can approximate the distance to the strike by timing the interval between the visible lightning and the audible thunder it generates. At standard atmospheric temperature and pressures near ground level, sound will travel at roughly 343m/s (1125 ft/sec); a lightning flash preceding its thunder by five seconds would be about one mile distant. A flash preceding thunder by three seconds is about one kilometer distant.

Lightning-induced magnetism

The movement of electrical charges produces a magnetic field (see electromagnetism). The intense currents of a lightning discharge create a fleeting but very strong magnetic field. Where the lightning current path passes through rock, soil, or metal these materials can become permanently magnetized. This effect is known as lightning-induced remanent magnetism, or LIRM. These currents follow the least resistive path, often horizontally near the surface[54] but sometimes vertically, where faults, ore bodies, or ground water offers a less resistive path.[55] Lightning-induced magnetic anomalies can be mapped in the ground,[56] [57] and analysis of magnetized materials can confirm lightning was the source of the magnetization[58] and provide an estimate of the peak current of the lightning discharge.[59]

Records and locations

An old estimate of the frequency of lightning on Earth was 100 times a second. Now that there are satellites that can detect lightning, including in places where there is nobody to observe it, it is known to occur on average 44 ± 5 times a second, for a total of nearly 1.4 billion flashes per year. 80% of these flashes are in-cloud and 20% are cloud-to-ground.

The map at right shows that lightning is not distributed evenly around the planet. Approximately 70% of lightning occurs in the tropics where the majority of thunderstorms occur. The place where lightning occurs most often is near the small village of Kifuka in the mountains of eastern Democratic Republic of the Congo, where the elevation is around 975m (3,199feet). On average this region receives 158 lightning strikes per square kilometre (approx. 0.4 square mile) a year. Singapore has one of the highest rates 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"). In the US, Central Florida sees more lightning than any other area. For example, in what is called "Lightning Alley", an area from Tampa, to Orlando, there are as many as 50 strikes per square mile (about 20 per km²) per year. The Empire State Building is struck by lightning on average 23 times each year, and was once struck 8 times in 24 minutes.[60] Roy Sullivan held a Guinness World Record after surviving 7 different lightning strikes across 35 years.[61]

In July 2007, lightning killed up to 30 people when it struck a remote mountain village Ushari Dara in northwestern Pakistan.[62]

On 31 October 2005, sixty-eight dairy cows, all in full milk, died on a farm at Fernbrook on the Waterfall Way near Dorrigo, New South Wales after being struck by lightning. Three others were paralysed for several hours but they later made a full recovery. The cows were sheltering under a tree when it was struck by lightning and the electricity spread onto the surrounding soil killing the animals.[63]

Lightning rarely strikes the open ocean, although some sea regions are lightning "hot spots". Winter storms passing off the east coast of the United States often erupt with electrical activity when they cross the warm waters of the Gulf Stream. The Gulf Stream endures about the same number of lightning strikes as the southern plains of the USA.

Lightning detection

The earliest detector invented to warn of the approach of a thunder storm was the lightning bell. Benjamin Franklin installed one such device in his house.[64] The detector was based on an electrostatic device called the 'electric chimes' invented by Andrew Gordon in 1742.

Lightning discharges generate a wide range of electromagnetic radiations, including radio-frequency pulses. The times at which a pulse from a given lightning discharge arrive at several receivers can be used to locate the source of the discharge. The United States federal government has constructed a nation-wide grid of such lightning detectors, allowing lightning discharges to be tracked in real time throughout the continental U.S.[65] [66]

In addition to ground-based lightning detection, several instruments aboard satellites have been constructed to observe lightning distribution. These include the Optical Transient Detector (OTD), aboard OrbView-1 satellite launched on April 3, 1995, and the subsequent Lightning Imaging Sensor (LIS) aboard TRMM launched on November 28, 1997.

Notable lightning strikes

Some lightning strikes have caused either numerous fatalities or great damage. The following is a partial list:

In culture

As expressions and symbols

The expression "Lightning never strikes twice (in the same place)" is similar to "Opportunity never knocks twice" in the vein of a "once in a lifetime" opportunity, i.e., something that is generally considered improbable. Lightning occurs frequently and more so in specific areas. Since various factors alter the probability of strikes at any given location, repeat lightning strikes have a very low probability (but are not impossible).[60] [75] Similarly, "A bolt from the blue" refers to something totally unexpected.

In French and Italian, the expression for "Love at first sight" is Coup de foudre and Colpo di fulmine, respectively, which literally translated means "lightning strike". Some European languages have a separate word for lightning which strikes the ground (as opposed to lightning in general); often it is a cognate of the English word "rays". The name of New Zealand's most celebrated thoroughbred horse, Phar Lap, derives from the shared Zhuang and Thai word for lightning. The bolt of lightning in heraldry is called a thunderbolt and is shown as a zigzag with non-pointed ends. This symbol usually represents power and speed. In Hindu mythology the thunderbolt (Sanskrit Vajra) is an attribute of the Hindu god Indra. The lightning bolt or thunderbolt appears also as a heraldic charge.

Ceraunoscopy

This is divination by observing lightning or by listening to thunder. It is a type of aeromancy. People have also sought to control lightning by conducting rituals or casting spells.

See also

Sources

External links

Jets, sprites & elves

Notes and References

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  2. Pages 3-4.
  3. Book: Wåhlin, Lars; Wh̄lin, Lars. Atmosphere electrostatics. Research Studies Press. Forest Grove, Ore. 1986. 0-86380-042-4. 2007-09-24.
  4. Rakov, V; Uman, M, Lightning: Physics and Effects, Cambridge University Press, 2003
  5. Web site: A Lightning Primer - Characteristics of a Storm. 2009-02-08. Dr. Hugh J. Christian. Melanie A. McCook. NASA.
  6. Demirkol. M. K.. Inan, Umran S.; Bell, T.F.; Kanekal, S.G.; and Wilkinson, D.C.. (abstract) Ionospheric effects of relativistic electron enhancement events. Geophysical Research Letters. Vol. 26. No. 23. 3557–3560. December. 1999.
  7. Web site: Electric Ice. 2007-07-05. NASA.
  8. Web site: Frazier. Alicia. THEORIES OF LIGHTNING FORMATION. Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder. December 12 2005. 2007-07-29.
  9. New Scientist, 7 may, 2005, p. 30.
  10. Spectacular slow-motion videos of stepped-leader formation are available on YouTube.com. See: "Coolest Lightning in Slow Motion" and "Slow Motion Video of a Lightning Negative Ground Flash".
  11. http://www.erh.noaa.gov/er/lwx/lightning/lgtng-hits-tree.jpg
  12. Book: Martin A. Uman. All About Lightning. Dover Publications, Inc.. 1986. 103–110. 0-486-25237-X.
  13. A. V. Gurevich, G. M. Milikh, and R. Roussel-Dupre (1992) "Runaway electron mechanism of air breakdown and preconditioning during a thunderstorm," Physics Letters A, vol. 165, pages 463-468.
  14. Dwyer, Joseph R., "A bolt out of the blue," Scientific American, vol. 292, no. 5, pages 64-71 (May 2005)
  15. Shrope. Mark. Lightning research: The bolt catchers. Nature. 431. 120–121. September 9 2004. 10.1038/431120a. 2007-07-27. .
  16. G. J. Fishman, P. N. Bhat, R. Malozzi, J. M. Horack, T. Koshut, C. Kouvelioton, G. N. Pendleton, C. A. Meegan, R. B. Wilson, W. S. Paciesas, S. J. Goodman, and H. J. Christian (1994) "Discovery of intense gamma-ray flashes of atmospheric origin," Science, vol. 264, pages 1313-1316.
  17. U.S. Inan, S.C. Reising, G.J. Fishman, and J.M. Horack. On the association of terrestrial gamma-ray bursts with lightning and implications for sprites. Geophysical Research Letters, 23(9):1017-20, May 1996. As quoted by http://elf.gi.alaska.edu/spr20010406.html#InanUS:theatg Retrieved 2007-03-06.
  18. Uman, 1986. Chapter 5, page 41.
  19. Uman, 1986. Chapter 9, page 78.
  20. Web site: Beaded Lightning. Glossary of Meteorology, 2nd edition. American Meteorological Society (AMS). 2000. 2007-07-31.
  21. Uman (1986) Chapter 16, pages 139-143
  22. Web site: Glossary. National Oceanic and Atmospheric Administration. National Weather Service. 2008-09-02.
  23. Web site: Definition of Rocket Lightning, AMS Glossary of Meteorology. 2007-07-05.
  24. Web site: NWS JetStream - The Positive and Negative Side of Lightning. NOAA. 2007-09-25.
  25. http://www.crh.noaa.gov/gid/Web_Stories/2004/other/lightningsafety/intro/introduction.php Bolt from the Blue
  26. Boccippio. D. J., et al.. Sprites, ELF Transients, and Positive Ground Strokes. Science. 269. 1088–1091. August. 1995. 10.1126/science.269.5227.1088. 17755531.
  27. Web site: Air Accidents Investigation Branch (AAIB) Bulletins 1999 December: Schleicher ASK 21 two seat glider.
  28. Web site: Bolt from the Blue. 2007-07-05. Lawrence. David. National Oceanic and Atmospheric Administration.
  29. Web site: Aviation Safety Network. 2006-06-12.
  30. http://www.aerospaceweb.org/question/design/q0234.shtml
  31. Book: Singer, Stanley. The Nature of Ball Lightning. Plenum Press. 1971. New York. 0306304945.
  32. Web site: Lightning balls created in the lab. New Scientist. 2007-12-08.
  33. http://www.abc.net.au/science/articles/2008/03/20/2194630.htm?site=science&topic=latest ABC.net.edu: Ball lightning bamboozles physicist
  34. Web site: Ball lightning scientists remain in the dark. 20 December 2001. New Scientist.
  35. The scientist Coleman was the first to propose this theory in 1993 in Weather, a publication of the Royal Meteorological Society.
  36. Web site: Great balls of fire!. 27 March 2008. The Economist.
  37. Boccippio. D. J., et al.. Sprites, ELF Transients, and Positive Ground Strokes. Science. 269. 1088–1091. August. 1995. 10.1126/science.269.5227.1088. 17755531.
  38. Walter A. Lyons and Michey D. Schmidt (2003). P1.39 The Discovery of Red Sprites as an Opportunity For Informal Science Education. American Meteorological Society. Retrieved on 2009-02-18.
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  41. Uman (1986), chapter 4, pages 26-34
  42. Web site: Pliny the Younger's Observations. 2007-07-05. Pliny the Younger. Behind us were frightening dark clouds, rent by lightning twisted and hurled, opening to reveal huge figures of flame..
  43. David W. Koopman and T.D. Wilkerson (1971) "Channeling of an ionizing streamer by a laser beam," Journal of Applied Physics, vol. 42, pages 1883-1886. See also: K. A. Saum and David W. Koopman (November 1972) "Discharges guided by laser-induced rarefication channels," Physics of Fluids, vol. 15, pages 2077-2079.
  44. C. W. Schubert (December 1977) "The laser lightning rod: A feasibility study" Technical report AFFDL-TR-78-60, ADA063847, [U.S.] Air Force Flight Dynamics Laboratory, Wright-Patterson AFB [Air Force Base] Ohio. For abstract, see: http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA063847 .
  45. Charles W. Schubert and Jack R. Lippert(1979) "Investigation into triggering lightning with a pulsed laser" in: A. H. Guenther and M. Kristiansen, ed.s, Proceedings of the 2nd IEEE International Pulse Power Conference, Lubbock, Texas, 1979 (Piscataway, N.J.: IEEE, 1979), pages 132-135. Available on-line at: ftp://ftp.pppl.gov/pub/neumeyer/Pulsed_Power_Conf/data/papers/1979/1979_025.PDF .
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  51. Nair. Zinnia. Aparna K.M., Khandagale R.S., Gopalan T.V.. Failure of 220 kV double circuit transmission line tower due to lightning. Journal of Performance of Constructed Facilities. Vol.19. No.2. May 1, 2005.
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  57. http://www.archaeophysics.com/pubs/LIRM.html Archaeo-Physics, LLC | Lightning-induced magnetic anomalies on archaeological sites
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  60. Uman (1986), chapter 6, page 47
  61. News: ROY SULLIVAN. The New York Times Archives (from UPI). September 30 1983. 2007-07-28.
  62. News: Lightning kills 30 people in Pakistan's north. Reuters. July 20 2007. 2007-07-27.
  63. http://www.news.com.au/story/0,10117,17117877-13762,00.html?from=rss Lightning kills 106 cows
  64. The Franklin Institute.Ben Franklin's Lightning Bells. Accessed 2008-12-14
  65. NOAA page on how the U.S. national lightning detection system operates
  66. Real-time map of lightning discharges in U.S.
  67. Rakov, A., Vladimir (2003). Page 2 of Lightning: Physics and Effects. Publisher: Cambridge University Press. Limited preview available at http://books.google.ca/books?id=NviMsvVOHJ4C&printsec=frontcover#PPA2,M1.
  68. http://www.paris-things-to-do.co.uk/paris-attraction-la-tour-eiffel-tower.html La Tour Eiffel - The Eiffel Tower - Paris Things To Do - www.paris-things-to-do.co.uk
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