Lightning

Lightning is an atmospheric discharge of electricity, which usually, but not always, occurs during rain storms, and frequently during volcanic eruptions or dust storms.

Early research
During early investigations into electricity via Leyden jars and other instruments, a number of people (D. 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. While he was waiting for the spire completion, some others (Thomas-François Dalibard and De Lors) conducted at Marly in France what became known as the Philadelphia Experiment that Franklin had suggested in his book.

Franklin usually gets the credit, as he was the first to suggest this experiment. The Franklin experiment is as follows:

''Whilste waiting for completion of the spire, he got the idea of using a flying object, such as a kite, instead. During the next thunderstorm, which was in June 1752, he raised a kite, accompanied by his son as an assistant. On his end of the string he attached a key and 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.''

However, 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, the experiment was met with attempts at replication. However, experiments involving lightning are always risky and frequently fatal. The most well-known death during the spate of Franklin imitators was that of Professor Georg Richmann, of Saint Petersburg, Russia. He had 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. While the experiment was under way, ball lightning appeared, collided with Richmann's head and killed him, leaving a red spot. His shoes were blown open, parts of his clothes singed, the engraver knocked out, the doorframe of the room split, and the door itself torn off its hinges.

Modern research


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. The discharge ionized channels are called leaders. The negative charged leaders, called a "stepped leader", proceed generally downward in a large 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 the downward leader is quite close to the ground, one or more smaller discharges (called positive streamers) arise from nearby, usually tall, grounded objects due to the intense electric field created by the approaching leaders.

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. The main stroke travels at about 0.1 c (30 million meters/second or 100 million feet/second) and the peak current lasts for tens of microseconds or so. After the peak, the current typically decays over tens or hundreds of microseconds.

In addition, negative lightning usually contains a number of restrikes along the same channel. Each restrike is separated by a much larger amount of time, typically 30 milliseconds or so. Additional return strokes are punctuated by intermediate dart leader strokes akin to, but weaker than, the initial stepped leader. This rapid restrike effect was probably known in antiquity, and the "strobe light" effect is often quite noticeable.

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. 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.

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. These charged particles cause the Earth to acquire an electric charge in its outer atmospheric layers, especially the ionosphere. Large quantities of ice in the clouds are suspected to enhance lightning development. This charge will neutralize itself through any available path. 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:
 * 1) Falling droplets of ice and rain become electrically polarized as they fall through the atmosphere's natural electric field;
 * 2) Colliding ice particles become charged by electrostatic induction.

Source: science.nasa.gov: Electric Ice, 2006-09-13

Electrostatic induction theory
Another theory is that opposite charges are driven apart by the above mechanism and energy is stored in the electric fields between them. Cloud electrification appears to require strong updrafts which carry water droplets upward, supercooling them to -10 to -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 lighter ice crystals upwards, causing the cloud top to accumulate increasing positive charge. The heavier negatively charged graupel falls towards 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 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. During the strike, successive portions of air become conductive as the electrons and positive ions of air molecules are pulled away from each other and forced to flow in opposite directions.

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. When a step 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. 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. Each successive stroke re-uses the heated path taken by the previous stroke. The electrical discharge rapidly superheats the leader channel, causing the air to expand rapidly and produce a shock wave heard as thunder. 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. The variations in successive discharges are the result of smaller regions of charge within the cloud being depleted by successive strokes.

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. For example, Florida, with the largest number of recorded strikes in a given period, has a very sandy ground saturated with salt water, and is surrounded by water. California, on the other hand, has fewer lightning strikes (being dryer). Arizona, which has very dry, sandy soil and a very dry air, has cloud bases as high as 6,000-7,000 feet above ground level, and gets very long, thin, purplish discharges, which crackle; 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. Potentially, the difference in each case may consist of differences in voltage levels between clouds and ground. Research on this is still ongoing.

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 Nature in 1994, 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 much shorter in duration, however, lasting only ~1ms.

A 1996 study in the journal Geophysical Research Letters 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 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 ~50 times per day globally (still a very small fraction of the total lightning on the planet). The energy levels recorded exceed 20 MeV, implying that they came from particles (likely electrons) at the speed of light.

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 theories 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 "relativisitic 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.



Positive lightning
Positive lightning 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. 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. It tends to occur more frequently in winter storms and at the end of a thunderstorm.

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. That's enough to power that same 100-watt light bulb for 100 years.



Kinds 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 travel extensively along the cloud anvil or its base is commonly called a crawler, or sometimes 'spider lightning'. Discharges of electricity in anvil crawlers travel up the sides of the cumulonimbus cloud branching out at the anvil top.

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. These usually occur miles ahead of the main storm and will strike without warning on a sunny day. They are signs of an approaching storm and are known colloquially as "bolts out of the blue".

Bead lightning, ribbon lightning, staccato lightning
Another special type of cloud-to-ground lightning is bead lightning. This is a regular cloud-to-ground stroke that contains a higher intensity of luminosity. When the discharge fades it leaves behind a string of beads effect for a brief moment in the leader channel. A third special type of cloud-to-ground lightning is ribbon lightning. These occur in thunderstorms where there are high cross winds and multiple return strokes. The winds will blow each successive return stroke slightly to one side of the previous return stroke, causing a ribbon effect. 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
Cloud-to-cloud or intercloud lightning is a somewhat rare type of discharge lightning between two or more completely separate cumulonimbus 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. These thunderstorm clouds are formed wherever there is enough upward motion, instability in the vertical, and moisture to produce a deep cloud that reaches up to levels somewhat colder than freezing. These conditions are most often met in summer. 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. Without instability and moisture, strong thunderstorms are unlikely. 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. The charge then moves downward in 50-yard sections called step leaders. It keeps moving toward the ground in these steps and produces a channel along which charge is deposited. Eventually it encounters something on the ground that is a good connection. The circuit is complete at that time, and the charge is lowered from cloud-to-ground. The return stroke is a flow of charge (current) which produces luminosity much brighter than the part that came down. This entire event usually takes less than half a second.

However, it has been shown in high speed videos examined frame-by frame, that a typical lightning strike is made up of anywhere from 8 to 12 or more individual discharges.

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. In some cases, the Thunderstorm may be too distant to hear the associated thunder from the lightning discharge.

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. They can be fast moving, slow moving or nearly stationary. Some make hissing or crackling noises or no noise at all. Some have been known to pass through windows and even dissipate with a bang. Ball lightning has been described by eyewitnesses but rarely, if ever, recorded by meteorologists.

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 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). A recent photograph by a Queensland ranger, Brett Porter, showed a fireball that was estimated to be 100 meters (330 feet) in diameter. The photograph has appeared in the scientific journal Transactions of the Royal Society. The object was a glowing globular zone (possibly the breakdown zone) with a long, twisting, rope-like projection (possibly the funnel).

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. Vertically linked fireballs have been reported. 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'). The scientist Coleman was the first to propose this theory in 1993 in Weather, a publication of the Royal Meteorological Society.

St Elmo's fire was correctly identified by Benjamin Franklin as electrical in nature. 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.

A fireball of lightning occurred in the Ashbourne/Sponden area of Derbyshire, England in about 1907. An eyewitness described it rushing towards herself and her brother quickly, as they crossed a field on their walk home from school at around 4.00pm. She said that it was about a yard in diameter and resembled a spinning ball of fire. It hit her brother, then aged about five or six, completing a circuit of him and leaving him shocked but otherwise unharmed, continuing onwards in the direction in which it had started. She did not mention a thunderstorm in progress at the time, though the ground was wet from recent rain.

Sprites, elves, jets, and other 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 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. Sprites usually occur in clusters of two or more simultaneous vertical discharges, typically extending from 65 to 75 km (40 to 47 miles) above the earth, with or without less intense filaments reaching above and below. Sprites are preceded by a sprite halo that forms because of heating and ionization less than 1 millisecond before the sprite. Sprites were first photographed on July 6, 1989, by scientists from the University of Minnesota and named after the mischievous sprite (air spirit) Ariel in Shakespeare's "The Tempest". These Sprites may be the result of the neutralization of accumulated charge from the Earth sweeping up particles from the Solar Wind, as described at the beginning of this article.

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. 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. They are also brighter than sprites and, as implied by their name, are blue in color. They were first recorded on October 21, 1989, on a video taken from the space shuttle as it passed over Australia. Again, this could be currents being generated from potential differences in the upper atmosphere caused by the same derivation of charge from the Solar Wind.

Elves often appear as a dim, flattened, expanding glow around 400 km (250 miles) in diameter that lasts for, typically, just one millisecond. They occur in the ionosphere 100 km (60 miles) 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. 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).

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. 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.

Researchers have speculated that such forms of upper atmospheric lightning may play a role in the formation of the ozone layer. Alternatively, they may be due to differences in potential that result in current from 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. Because most of these strokes occur inside a cloud, we do not see many of the individual return strokes in a thunderstorm.

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. It has also been triggered by launching rockets carrying spools of wire into thunderstorms. The wire unwinds as the rocket climbs, making a convenient path for the lightning to use. These bolts are typically very straight, due to the path created by the wire.

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
This 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. The movement resembles the movement of a rocket, hence its name. It is also one of the rarest of cloud discharges.

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. Contrary to popular notion, there is no 'safe' location outdoors. People have been struck in sheds and makeshift shelters. A better alternative is to seek shelter within an enclosure of conductive material (such as an automobile which is an example of a crude type of Faraday Cage). Once inside the enclosure, it is advisable to keep away from any attached metallic components (such as keys in an ignition, etc...). If thunder can be heard, there is a risk of being struck.

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 (sand being a fair insulator and wet, salty and spongy earth being more conductive).
 * EMP or electromagnetic pulse from close strikes - especially during positive lightning discharges

In a direct hit the electrical charge strikes the victim first. Counterintuitively, if the victim's skin resistance is high enough, much of the current will flash around the skin or clothing to the ground, resulting in a surprisingly benign outcome. 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. Ground strikes, in which the bolt lands near the victim and is conducted through the victim and his or her connection to the ground (such as through the feet, due to the voltage gradient in the earth, as discussed above), can cause great damage.

The most critical injuries are to the circulatory system, the lungs, and the central nervous system. 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). Others incur myocardial infarction and various cardiac arrhythmias, either of which can be rapidly fatal as well. 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. Either the electrical or the mechanical force can result in loss of consciousness, which is very common immediately after a strike. Amnesia and confusion of varying duration often result as well. A complete physical examination by paramedics or physicians may reveal ruptured eardrums, and ocular cataracts may develop, sometimes more than a year after an otherwise uneventful recovery.

The lightning often leaves skin burns in characteristic Lichtenberg figures, sometimes called lightning flowers; they may persist for hours or days, and are a useful indicator for medical examiners when trying to determine the cause of death. They are thought to be caused by the rupture of small capillaries under the skin, either from the current or from the shock wave. 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. One such example is the destruction of the basement insulator of the 250-metre-high central mast of longwave transmitter Orlunda, which led to its collapse.

Lightning prediction systems have been developed and may be deployed in locations where lightning strikes present special risks, such as public parks. Such systems are designed to detect the conditions which are believed to favor lightning strikes and provide a warning to those in the vicinity to allow them to take appropriate cover.

Facts and trivia
A bolt of lightning can reach temperatures approaching 28,000 degrees Celsius (50,000 degrees Fahrenheit) in a split second. This is about five times hotter than the surface of the sun. 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. These are sometimes found under the sandy surfaces of beaches and golf courses, or in desert regions. 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. 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. Occasionally, a tree may explode completely, as in this Giant Sequoia struck in Geneva. 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.

A frequently struck tree is the oak. It has a deep central root that goes beneath the tree, as well as hollow water-filled cells that run up and down the wood of the oak's trunk. These two qualities make oak trees better grounded and more conductive than trees with shallow roots and closed cells. In Johannesburg - one of the places with a very high incidence of lightning strikes - the most commonly struck tree is Cedrus deodara, locally known as the Xmas tree. Factors which lead to its being targeted, are a high resin content, its loftiness and its needles which lend themselves to a high electrical discharge during a thunderstorm.


 * The science of lightning is termed fulminology and a person studying it, a fulminologist.


 * The odds of an average person living in the USA being struck by lightning at least once in his lifetime is approximately 1:3000.


 * The odds of having a friend or family member killed by lightning in the USA in a lifetime is approximately 1:3000.


 * 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").


 * "Lightning Alley", referring to Interstate 4 between Orlando and St. Petersburg FL, 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. The assistant director Jan Michelini was struck as well.


 * 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. Working as a park ranger, Roy was struck seven times over the course of his 35 year career. He lost a nail on his big toe, and suffered multiple injuries to the rest of his body.


 * 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. Ironically, before starting the session, he said "Lightning is not going to kill me."


 * On average, lightning strikes the earth about 100 times every second.

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). Often it's a cognate of the English word "rays."

Estimating distance of a lightning strike: The flash of a lightning strike and resulting thunder occur at roughly the same time. But light travels at 300,000 kilometers in a second, almost a million times the speed of sound. 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. By counting the seconds between the flash and the thunder and dividing by 3, you can estimate your distance from the strike and initially the actual storm cell (in kilometers). Similarly, by dividing by 5, you can estimate the distance in miles.

Jets, sprites & elves

 * Homepage of the Eurosprite campaign, itself part of the CAL (Coupled Atmospheric Layers research group
 * March 2, 1999, University of Houston: UH Physicists Pursue Lightning-Like Mysteries Quote: "...Red sprites and blue jets are brief but powerful lightning-like flashes that appear at altitudes of 40-100 km (25-60 miles) above thunderstorms..."
 * Ground and Balloon-Borne Observations of Sprites and Jets
 * Barrington-Leigh, C. P., "Elves : Ionospheric Heating By the Electromagnetic Pulses from Lightning (A primer)". Space Science Lab, Berkeley.
 * "Darwin Sprites '97". Space Physics Group, University of Otago.
 * Gibbs, W. Wayt, "Sprites and Elves : Lightning's strange cousins flicker faster than light itself". San Francisco. ScientificAmerican.com.
 * Barrington-Leigh, Christopher, "VLF Research at Palmer Station".
 * Heavenly light show caught on film (Nature) - Requires subscription to news@nature.com.
 * Sprites, jets and TLE pictures and articles

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