Lightning
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Lightning is a powerful natural electrostatic discharge produced during a thunderstorm. Lightning's abrupt electric discharge is accompanied by the emission of light. The electricity passing through the discharge channels rapidly heats and expands the air, producing lightning's characteristic thunder sound.
History of lightning research
During early investigations into electricity via Leyden jars and other instruments, a number of people proposed that small scale sparks shared some similarity with lightning. Benjamin Franklin endeavored to test this theory by using a spire which was being erected in Philadelphia but while waiting for its completion, he got the idea of using a flying object, such as a kite instead.
During the next thunderstorm, 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 enough 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.
Although some others (Dalibard and De Lors) had conducted similar experiments in France, Franklin had suggested the original high-object-with-some-spark-gap idea that they used, so he usually gets the credit.
As news of the experiment and its particulars spread, it was met with, attempts at replication. Experiments involving lightning are always risky to extremes, and frequently fatal. The most well known death during the spate of Franklin-imitators was Professor Richman, of Saint Petersburg, Russia. He had created a setup 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 underway, a large ball lightning showed up, collided with Richman's head, leaving a red spot, and killed him. 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.
Franklin also invented the lightning rod, likely because of the results of his experiment.
How lightning is formed
The first process in the generation of lightning is the forcible separation of positive and negative charges within a cloud or air. The mechanism by which this happens is still the subject of research, but one widely accepted theory is the polarization mechanism. This mechanism has two components: the first is that falling droplets of ice and rain become electrically polarized as they fall through the atmosphere's natural electric field, and the second is that colliding ice particles become charged by electrostatic induction. Once charged, by whatever mechanism, work is performed as the opposite charges are driven apart and energy is stored in the e-fields between them. The positively charged crystals tend to rise to the top, causing the cloud top to build up a positive charge, and the negatively charged crystals and hailstones drop to the middle and bottom layers of the cloud, building up a negative charge. Cloud-to-cloud lightning can appear at this point. Cloud-to-ground lightning is less common. Cumulonimbus clouds that do not produce enough ice crystals usually fail to produce enough charge separation to cause lightning.
When sufficient negatives and positives gather in this way, and when the e-field becomes sufficiently strong, and electrical discharge occurs within the clouds or between the clouds and the ground, producing the bolt. As for all air electric discharge, the electrons from the cosmic ray strike are accelerated by the electric fields, they ionize air molecules which then collide with making the air conductive and starting a lightning strike. 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 (stepped channels called step leaders). The conductive filament grows in length. At the same time, electrical energy stored in the e-field flows radially inward into the conductive filament.
When a charged step leader is near the ground opposite charges appear on the ground and enhance the electric field. The e-field is higher on trees and tall buildings. If the e-field is high enough a discharge can initiate from the ground. This discharge is called a positive streamer, and can eventually connect to the descending discharge from the cloud.
Lightning can also occur within the ash clouds from volcanic eruptionsTemplate:Ref,Template:Ref, or can be caused by violent forest fires which generate sufficient dust to create a static charge.
Negative lightning
A bolt of lightning usually begins when an invisible negatively charged stepped leader stroke is sent out from the cloud. As it does so, a positively charged stepped leader is usually sent out from the positively charged ground or cloud. 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. This "return stroke" is the most luminous part of the strike, and is the part that is really visible. Most lightning strikes usually last about a quarter of a second. Sometimes several strokes will travel up and down the same leader strike, causing a flickering effect. Thunder is caused when the discharge rapidly superheats the leader channel, causing a shock wave to be sent out.
It is possible for streamers to be sent out from several different objects simultaneously, with only one connecting with the leader and forming the discharge path. Photographs have been taken on which non-connected streamers are visible [1] (http://www.erh.noaa.gov/er/lwx/lightning/lgtng-hits-tree.jpg).
![[1]](http://www.erh.noaa.gov/er/lwx/lightning/lgtng-hits-tree.jpg){kind=link}
This type of lightning is known as negative lightning due to the discharge of negative charge from the cloud, and accounts for over 95% of all lightning.
Statistics: an average bolt of negative lightning carries a current of 30 kiloamperes, transfers a charge of 5 coulombs, has a potential difference of about 100 megavolts, dissipates 500 megajoules (enough to light a 100 watt lightbulb for 2 months), and lasts a few milliseconds.
Positive lightning
Positive lightning makes up less than 5 percent of all lightning. It occurs when the stepped 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 several miles or kilometers distant from the clouds. During a positive lightning strike, huge quantities of ELF and VLF radio waves are generated.
As a result of their 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 [2] (http://www.dft.gov.uk/stellent/groups/dft_avsafety/documents/page/dft_avsafety_500699.hcsp). It has since been suggested that it may have been positive lightning that caused the crash of Pan Am flight 214 in 1963. Positive lightning is now also thought to be responsible for many forest fires.
Positive lightning has also been shown to trigger the occurrence of upper atmospheric lightning. It tends to occur more frequently in winter storms and at the end of a thunderstorm.
Statistics (based on a small number of measurements): an average bolt of positive lightning carries a current of 300 kiloamperes, transfers a charge of up to 300 coulombs, has a potential difference up to 1 gigavolt (a thousand million volts), dissipates enough energy to light a 100 watt lightbulb for up to 95 years, and lasts for tens or hundreds of milliseconds.
Bipolar lightning
Heinz Kasemir first hypothesized that a lightning leader system actually develops in a bipolar fashion, with both a positive and a negative branching leader system connected at the system origin and containing a net zero charge. This process provides a means for the positive leader to conduct away the net negative charge collected during development, allowing the leader system to act as an extending polarized conductor. Such a polarized conductor would be able to maintain intense electric fields at its ends, supporting continued leader development in weak background electric fields.
During the eighties, implementation of flight test campaigns proved that an aircraft itself is triggering a bipolar stepped leader when crossing charged cloud areas. Many scientists think that positive and negative lightning in a cloud is actually bipolar lightning.
Types of lightning
Some lightning strikes take on particular characteristics, and 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 which occurs completely inside one cumulonimbus cloud, jumping between differently charged regions within the cloud. Intracloud lightning is commonly known as sheet lightning because it lights up the cloud and the surrounding sky with an apparent sheet of light. One special type of intracloud lightning is commonly called an anvil crawler. 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 lightning, bead lightning, ribbon lightning, staccato 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 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 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.
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 crosswinds and multiple return strokes. The winds will blow each successive return stroke slightly to one side of the previous return stoke, 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.
Cloud-to-cloud lightning
Cloud-to-cloud 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. Most ground-to-cloud lightning occurs from tall buildings, mountains and towers.
Heat or summer lightning
Heat lightning (or, in the UK, 'summer lightning') is nothing more than the faint flashes of lightning on the horizon 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.
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 in Electrical World and Engineer, March 5, 1904, "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.
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 are usually spawned by discharges of positive lightning between the cloud and the ground. Sprites can occur up to 50 km 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 due to heating and ionization less than 1 milisecond before the sprite. Sprites were first photographed on July 6, 1989, by scientists from the University of Minnesota and named after the mischievous sprites in the plays of Shakespeare.
Recent research [3] (http://www.uh.edu/admin/media/nr/102002/beringsprites100702.html) 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. [4] (http://dx.doi.org/10.1029/2004GL021943)
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 colour. They were first recorded on October 21, 1989 on a video taken from the space shuttle as it passed over Australia.
Elves often appear as a dim, flattened expanding glow around 400 km (250 miles) in diameter that lasts for, typically, just one millisecond [5] (http://hbar.stanford.edu/cpbl/elves). 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 having been energized by the electromagnetic pulse caused by a positive lightning bolt).
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 speeded 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 [6] (http://sprite.phys.ncku.edu.tw/new/news/0626_presss/nature01759_r.pdf). 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.
One theory about the cause of the Space Shuttle Columbia disaster is that the craft was struck by atmospheric lightning. [7] (http://www.thunderbolts.info/tpod/2005/arch05/050223columbia.htm)
Streak lightning
All 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. For more information, see triggered lightning.
Lightning throughout the Solar System
Lightning requires the electrical breakdown of gas, so lightning cannot exist in the vacuum of space. However, lightning has been observed within the atmospheres of other planets, such as Venus and Jupiter, and electrical discharges between Jupiter and Io often occur within the gas cloud sent out by Io's volcanos. Lightning on Jupiter is estimated to be 100 times as powerful, 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 '70s and '80s, signals suggesting lightning may be present in the upper atmosphere were detected [8] (http://www-ssc.igpp.ucla.edu/~strange/JATP_paper/JATP_title.html). However, recently the Cassini-Huygens mission flyby of Venus detected no signs of lightning at all.
Runaway breakdown theory
To spontaneously ionize air and conduct electricity across it, an electric field of approximately 2500 kilovolts per meter is required. However, measurements inside storm clouds to date have failed to locate fields this strong, with typical fields being between 100 and 400 kilovolts per meter. While there remains a possibility that researchers are failing to encounter the small high-strength regions of the large clouds, the odds of this are diminishing as further measurements continue to fall short.
A theory by Alex Gurevich of the Lebedev Physical Institute in 1992 proposes that cosmic rays may provide the beginnings of what he called a runaway breakdown. After striking an air molecule and rendering it extremely energetic, it strikes and ionizes other air molecules in the storm's electric field, forming a chain reaction until electricity can flow through the air. This was initially considered a fringe theory, but is now becoming mainstream due to the lack of other theories.
It has been recently revealed that most lightning emits an intense burst of X-rays and/or gamma-rays which seem to be produced during the stepped-leader and dart-leader phases just before the stroke becomes visible. The X-ray bursts typically have a total duration of less than 100 microseconds and have energies extending up to nearly a few hundred KeV. The presence of these high-energy events match and support the "runaway breakdown" theory, and were discovered through the examination of rocket triggered lightning, and from satellite monitoring of natural lightning.
NASA's RHESSI satellite typically reports 50 gamma-ray events per day, and many of these are strong enough to fit the theory. Additionally, low-frequency radio emissions detected at ground level can detect lightning bolts from upwards of 4000km away; combining these with gamma-ray burst events detected from above show overlapping positions and timing.
There are problems with the "runaway breakdown" theory, however. While there seems to be a strong correlation between gamma-ray events and lightning, there are insufficient events detected to account for the amount of lightning occurring across the planet. Another issue is the amount of energy the theory states is required to initiate the breakdown. Cosmic rays of sufficient energy strike the atmosphere on average only once per 50 seconds per square kilometer. Measured X-ray burst intensity also falls short, with results indicating particle energy 1/20th of the theory's value.
Lightning facts
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A bolt of lightning can reach temperatures approaching 28,000 kelvins (50,000 degrees Fahrenheit) in a split second. This is many times hotter than the surface of the sun. The heat of lightning which 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 (photo of a tree being struck by lightning (http://www.erh.noaa.gov/er/lwx/lightning/lgtng-hits-tree.jpg)). 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 forest areas, lightning scars can be seen on almost every tree.
Lightning injuries
Nearly 2000 persons per year in the world are injured by lightning strikes, and between 25 to 33 percent 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 due to the huge voltage of a lightning strike, survivors often fare better than victims of other electrical injuries which result in a more prolonged application of lesser voltage.
People may be hit in several different ways. 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 effectively bounces off a nearby object and strikes the victim en route to ground. Ground stikes, in which the bolt lands near the victim and is conducted through the victim via his grounded feet or other body part, 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, it is worth noting, is safe to administer, due to the fact that 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 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.
Lightning safety
Lightning is responsible for approximately 100 deaths a year in the United States alone. Lightning ranks second only to floods for storm related casualties in the U.S. every year. Many of these deaths could be prevented if basic precautions were taken when thunderstorms are expected in an area. Listening to a radio to keep up to date on storms in the area is the first step in lightning safety.
One way to prepare is to install a lightning conductor (or, lightning rod) for preventing lightning damage to a building. A lightning conductor is a metal spike that is connected to earth by a low-resistance path. Should lightning strike a building, the current will travel through the conductor rather than through the fabric of the building, causing less damage.
Electrical equipment can be protected from lightning by a lightning arrester, a device that contains one or more gas-filled spark gaps between the equipment's cables and earth. Should lightning strike one of the cables, the high voltage will cause the gas in the spark gap to break down and become a conductor, providing a path for the lightning to reach the ground without passing through the equipment.
Safer locations
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No place is 100% safe in a thunderstorm, but some are more safe than others. Larger, better constructed structures are better than smaller or more open structures. Fully enclosed metal vehicles with the windows rolled up are good shelters, providing that no contact is made with any exposed metal inside or outside the vehicle.
When outside, avoid the following:
- High places and open fields
- isolated trees
- unprotected gazebos
- rain or picnic shelters
- baseball dugouts
- communications towers
- flagpoles
- light poles
- bleachers (stadium seating) (metal or wood)
- metal fences
- open top vehicles such as convertibles, tractors (contrary to myth, rubber tires are not protective)
- golf carts
- water (ocean, lakes, swimming pools, rivers, etc.)
- metal-shafted or conductive umbrellas, golf clubs, lacrosse sticks, baseball bats, shovels, or fishing rods
If you find yourself trapped in an open area during a storm, position yourself close to the ground by squatting with your feet close together and on the balls of your feet. Crouch in a ditch if possible. Avoid proximity to other people (minimum 5 meters or 15 feet). Since lightning spreads when it hits the ground, you want to minimize as much contact area between you and the ground; consequently you should not lie on the ground in an attempt to minimise your height. Remember, humans are good conductors of electricity, better so than air, and lightning tends to strike the highest thing in an area, because electricity will always take the path of least resistance.
Lone tall trees are particularly dangerous; the tree being moist, the electricity generally passes down underneath the bark, splitting it in all directions, and the lightning will pass to the best conductor near it. Cattle often seek shelter under trees during a thunderstorm and are frequently killed by strikes.
When inside avoid the following:
- Use of the telephone (cellular and cordless telephone use is safe)
- taking a shower or bath
- washing your hands
- doing dishes
(basically anything to do with water)
- any contact with conductive surfaces with exposure to the outside such as metal door or window frames, electrical wiring, telephone wiring, cable TV wiring, plumbing, etc.
- using electrical appliances that plug into the wall
- being near external windows and doors in general
- using a radio or TV receiver with an antenna outside the house
Quick first aid vital:
Many apparently lifeless victims, especially those who received a side flash, or only a portion of the full discharge through their bodies, may suffer cardiac arrest but surprisingly little other damage. Quick administration of cardiopulmonary resuscitation (CPR) may revive them and save their lives. First responders to places where there are multiple victims are taught to practice reverse triage. Instead of helping first those who appear the most salvageable, they try to resuscitate the unconscious victims first, since those who survived the initial hit usually survive on their own. It is important to note that lightning strike victims carry no electrical charge as a result, and it is, therefore, safe to handle them immediately; although it should be considered if there is a significant risk of first aiders becoming victims to a subsequent lightning bolt.
Spectacular lightning damages
There were sometimes spectacular and unconventional lightning damages. One of them was the destruction of the basement insulator of the 250 metre high central mast of longwave transmitter Orlunda, which led to its collapse.
Lightning in contemporary culture
In movies and comics of the contemporary U.S. and many other countries, the lightning is often employed as an ominous, dramatic sign. It may herald a waking of a great evil or emergence of a crisis. Various novels and role playing games with fantasy tint involves wizardry of lightning bolt, weapon embodying the power of lightning, etc. The comic book character Billy Batson changed into the superhero Captain Marvel by saying the word, "Shazam!" which called down a bolt of magic lightning to make the change.
Lightning in heraldry
The bolt of lightning in heraldry is distinguished from the thunderbolt and is shown as a zigzag with nonpointed ends. It is also distinguished from the "fork of lightning."
Trivia
- The U.S. is home to "Lightning Alley", a group of states in the American Southeast that collectively see more lightning strikes per year than any other place on Earth. The most notable state in Lightning Alley is Florida.
- The saying "lightning never strikes twice in the same place" is frequently disproven. The Empire State Building is struck by lightning on average 25 times each year, and was once struck 15 times in 15 minutes.
- Some repeat lightning strike victims claim that lightning can choose its target, although this theory is entirely disregarded by the scientific community.