Lightning is an atmospheric discharge of electricity, 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 60,000 m/s, and can reach temperatures approaching 30,000 °C (54,000 °F), hot enough to fuse silica sand into petrified lightning, known scientifically as glass channels or 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 576,000 to 1 and the chance of actually being killed by lightning is approximately 2,320,000 to 1.
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.
Benjamin Franklin (1706-1790) endeavored to test the theory that sparks shared some similarity with lightning using a spire which was being erected in Philadelphia. While waiting for completion of the spire, he got the idea of instead using 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. 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.
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 past 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.
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 The voltage depends on the length of the bolt, with the dielectric breakdown of air being three million volts per metre; this works out to approximately one gigavolt (one thousand million volts) for a 300 m (1000 ft) lightning bolt. With an electric current of 100 kA, this gives a power of 100 terawatts. However, lightning leader development is not a simple matter of dielectric breakdown, and the ambient electric fields required for lightning leader propagation can be a few orders of magnitude less than dielectric breakdown strength. Further, the potential gradient inside a well-developed return-stroke channel is on the order of hundreds of volts per metre or less due to intense channel ionisation, resulting in a true power output on the order of megawatts per metre for a vigorous return-stroke current of 100 kA.
Lightning heats nearby air to about nearly instantly, which is almost twice the temperature of the Sun’s surface. The air around a lightning strike is the hottest place on earth. The heating creates a shock wave that is heard as thunder.
The return stroke of a lightning bolt follows a charge channel only about a centimetre (0.4-in) wide. Most lightning bolts are about 1.6 kilometres (1 mi) long. The longest recorded length was 190 kilometres (118 mi), sighted near Dallas, Texas.
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. Arizona, which has very dry, sandy soil and a very dry air, has cloud bases as high as 1800-2100 m (6,000-7,000 ft) above ground level, and gets very long and thin purplish discharges which crackle; while Oklahoma, with cloud bases about 450-600 m (1,500-2,000 ft) 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. The difference in each case may consist of differences in voltage levels between clouds and ground. Research on this is still ongoing.
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.
Ice and supercooled water are the keys to the process. Violent winds buffet tiny hailstones as they form, causing them to collide. When the hailstones hit ice crystals, some negative ions transfer from one particle to another. The smaller, lighter particles lose negative ions and become positive; the larger, more massive particles gain negative ions and become negative.
There are several additional hypotheses for the origin of charge separation.
An initial bipolar discharge, or path of ionized air, starts from a negatively charged mixed water and ice region in the thundercloud. The discharge ionized channels are called leaders. The negative charged leaders, called a "stepped leader", proceed generally downward in a number of quick jumps, each up to 50 metres long. Along the way, the stepped leader may branch into a number of paths as it continues to descend. 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 compared to the subsequent lightning channel.
When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the electric field. The electric field is highest on 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. When the two leaders meet, the electric current greatly increases. 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.
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.
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, 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, 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.
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.
Each successive stroke is preceded by intermediate dart leader strokes again to, but weaker than, the initial stepped leader. The stroke usually re-uses the discharge channel taken by the previous stroke.
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.
Some lightning strikes take on particular characteristics; scientists and the public have given names to these various types of lightning. Most lightning is streak lightning. This is nothing more than the return stroke, the visible part of the lightning stroke. Because most of these strokes occur inside a cloud, we do not see many of the individual return strokes in a thunderstorm.
The return stroke of a lightning bolt, which is the visible bolt itself, follows a charge channel only about a half-inch (1.3 cm) wide. Most lightning bolts are about a mile (1.6 km) long.
Positive lightning, also known colloquially as "bolts from the blue", makes up less than 5% of all lightning. 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. 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. 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.
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."
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.
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).
Lightning discharges may occur between areas of cloud having different potentials without contacting the ground. These are most common between the anvil 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 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 you or begins to decay. The most vivid crawler behavior occurs in well developed thunderstorms that feature extensive rear anvil shearing.
The engineer Nikola Tesla wrote, "I have succeeded in determining the mode of their formation and producing them artificially". 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. Some later experimenters have been able to briefly produce small luminous balls by igniting carbon-containing materials atop sparking Tesla Coils.
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 several instances have been reported of ball lightning without any connection to stormy weather or lightning.
Ball lightning is typically 20 – 30 cm (8-12 inches) in diameter, but ball lightning several metres in diameter has been reported. Ball lightning has been seen in tornadoes, and has also been seen to split apart into two or more separate balls and recombine, and vertically-linked fireballs have been reported. 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').
Ball lightning apparently is created when lightning strikes silicon in soil, and has been created in a lab in this manner.
Sprites may be horizontally displaced by up to from the location of the underlying lightning strike, with a time delay following the lightning that is typically a few milliseconds, but on rare occasions may be up to 100 milliseconds. Sprites are sometimes, but not always, preceded by a sprite halo, a broad, pancake-like region of transient optical emission centred at an altitude of about above lightning. Sprite halos are produced by weak ionization from transient electric fields of the same type that causes sprites, but which are insufficiently intense to exceed the threshold needed for sprites. Sprites were first photographed on July 6, 1989 by scientists from the University of Minnesota. Several years after their discovery they were named after the mischievous sprite (air spirit) Puck in Shakespeare's Midsummer Night's Dream.
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. Research in 2004 by scientists from Tohoku University found that very low frequency emissions occur at the same time as the sprite, indicating that a discharge within the cloud may generate the sprites.
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.
In 2001, the Arecibo scientists modeled the blue-jet phenomenon to better understand how it works. It is like an electron avalanche that can flood up toward the ionosphere or slide earthward, depending on the electric field direction. Intense hail may trigger the avalanche. The field accelerates the electrons and slams them into air molecules. The molecules break down into ions and free electrons and emit light. The newly generated electrons also accelerate.
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.
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. 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.
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 . 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 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.
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. 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.
If the lightning strike gets closer, it will quickly turn into a louder crackle of thunder, sounding like a big "bom!" of thunder, quickly telling the hearer that the lightning strike was very close.
The lightning's thunder can give the hearer a warning that lightning strikes may be produced in that area.
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 but sometimes vertically, where faults, ore bodies, or ground water offers a less resistive path. Lightning-induced magnetic anomalies can be mapped in the ground, and analysis of magnetized materials can confirm lightning was the source of the magnetization and provide an estimate of the peak current of the lightning discharge.
In July 2007, lightning killed up to 30 people when it struck a remote mountain village Ushari Dara in northwestern Pakistan.
Lightning can also strike indoor pools, directed into the pump by electrical circuits from outdoor power poles. Such strikes could potentially kill people who are swimming or walking on wet floors around a pool. In 2000, lightning killed two boys in an outdoor pool in Florida.
A single lightning strike can have a potential of a billion volts and deliver 100,000 amperes of current. If a bolt directly hits a marine animal swimming on the surface, it will undoubtedly hurt or kill the animal. Lightning strikes have killed or injured people on the surface more than 30 yards away.
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.
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, for example, has roughly as many lightning strikes as the southern plains of the USA.
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.
In French and Italian, the expression for "Love at first sight" is Coup de foudre and Colpo di fulmine, respectively, which literally translated means "Bolt of lightning". 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 in Sanskrit is called Vajra and is shown as a zigzag with non-pointed ends. This symbol usually represents power and speed; and thus has been used to represent the Hindu god Indra, as well as many advertisements which use such symbol to describe their product. It is also distinguished from the "fork of lightning".