Lightning strike

A lightning strike is an electric discharge between the atmosphere and an earth-bound object. They mostly originate in a cumulonimbus cloud and terminate on the ground, called cloud to ground (CG) lightning. A less common type of strike, called ground to cloud (GC), is upward propagating lightning initiated from a tall grounded object and reaches into the clouds. About 25% of all lightning events worldwide are strikes between the atmosphere and earth-bound objects. The bulk of lightning events are intra-cloud (IC) or cloud to cloud (CC), where discharges only occur high in the atmosphere.[1][2]

A single lightning event is a "flash", which is a complex, multi-stage process, some parts of which are not fully understood. Most cloud to ground flashes only "strike" one physical location, referred to as a "termination". The primary conducting channel, the bright coursing light that may be seen and is called a "strike", is only about one inch in diameter, but because of its extreme brilliance, it often looks much larger to the human eye and in photographs. Lightning discharges are typically miles long, but certain types of horizontal discharges can be upwards of tens of miles in length. The entire flash lasts only a fraction of a second. Most of the early formative and propagation stages are much dimmer and not visible to the human eye.

Panorama photography taken during a lightning storm over Bucharest, Romania

Strikes

Lightning strikes can injure humans in several different ways:[3]

  1. Direct
    • Direct strike – the person is part of the flash channel. Enormous quantities of energy pass through the body very quickly and this can result in internal burns and organ damage, explosions of flesh and bone, and a damaged nervous system. Depending on the flash strength and access to medical services, it may be instantaneously fatal or cause permanent injuries and impairments.
    • Contact injury – the person was touching an object, generally a conductor, that is electrified by the strike.
    • Side splash – branches form "jumping" from the primary flash channel, electrifying the person.
    • Blast injuries – being thrown and suffering blunt force trauma from the shock wave (if very close) and possible hearing damage from the thunder.
  2. Indirect
    • Ground current or "step potential" – Earth surface charges race towards the flash channel during discharge. Due to the high impedance of the ground, the current "chooses" a better conductor, often a person's legs, passing through the body. The near instantaneous rate of discharge causes a potential (difference) over distance, which may amount to several thousand volts per linear foot. This phenomenon is responsible for more injuries and deaths than the above three combined, with reports such as "hundreds of reindeer killed by a lightning storm..." being a classic example.[4]
    • EMPs – the discharge process produces an electromagnetic pulse (EMP) which may damage an artificial pacemaker, or otherwise affect normal biological processes.
  3. Secondary or resultant
    • Explosions
    • Fires
    • Accidents

Injuries

Main article: Lightning injuries

Lightning strikes can produce severe injuries, and have a mortality rate of between 10% and 30%, with up to 80% of survivors sustaining long-term injuries.[3] These severe injuries are not usually caused by thermal burns, since the current is too brief to greatly heat up tissues; instead, nerves and muscles may be directly damaged by the high voltage producing holes in their cell membranes, a process called electroporation.

In a direct strike, the electrical currents in the flash channel pass directly through the victim. The relatively high voltage drop around poorer electrical conductors (such as a human being), causes the surrounding air to ionize and break down, and the external flashover diverts most of the main discharge current so that it passes "around" the body, reducing injury.

Metallic objects in contact with the skin may "concentrate" the lightning's energy, given it is a better natural conductor and the preferred pathway, resulting in more serious injuries, such as burns from molten or evaporating metal. At least two cases have been reported where a strike victim wearing an iPod suffered more serious injuries as a result.[5]

However, during a flash, the current flowing through the channel and around the body will generate large electromagnetic fields and EMPs, which may induce electrical transients (surges) within the nervous system or pacemaker of the heart, upsetting normal operations. This effect might explain cases where cardiac arrest or seizures followed a lightning strike that produced no external injuries. It may also point to the victim not being directly struck at all, but just being very close to the strike termination.[3]

Another effect of lightning on bystanders is to their hearing. The resulting shock wave of thunder can damage the ears. Also, electrical interference to telephones or headphones may result in damaging acoustic noise.

Epidemiology

Memorial to a lightning victim in 1787 in London in a window

One estimate is that 24,000 people are killed by lightning strikes around the world each year and about 240,000 are injured.[6] Another estimate is that the annual global death toll is 6,000.[7]

According to the NOAA, over the last 20 years, the United States averaged 51 annual lightning strike fatalities, placing it in the second position, just behind floods for deadly weather.[8][9] In the US, between 9% and 10% of those struck die,[10] for an average of 40 to 50 deaths per year (28 in 2008).[11] The chance of an average person living in the US being struck by lightning in a given year is estimated at 1 in 960,000.[12]

In Kisii in western Kenya, some 30 people die each year from lightning strikes. Kisii's high rate of lightning fatalities occurs because of the frequency of thunderstorms and because many of the area's structures have metal roofs.[13]

These statistics do not reflect the difference between direct strikes, where the victim was part of the lightning pathway; indirect effects of being close to the termination point, like ground currents; and resultant, where the casualty arose from subsequent events, such as fires or explosions. Even the most knowledgeable first responders may not recognize a lightning related injury, let alone particulars, which a medical examiner, police investigator or on the rare occasion a trained lightning expert may have difficulty identifying to record accurately. This ignores the reality that lightning, as the first event, may assume responsibility for the overall and resulting accident.

Direct strike casualties could be much higher than reported numbers.[14]

Effect on nature

Impact on vegetation

A green tree which was struck by lightning, exploding the trunk.
A eucalyptus tree that was struck by lightning, while two nearby pine trees were untouched, Darwin, Northern Territory, Australia.

Trees are frequent conductors of lightning to the ground.[15] Since sap is a relatively 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.

In sparsely populated areas such as the Russian Far East and Siberia, lightning strikes are one of the major causes of forest fires.[16] The smoke and mist expelled by a very large forest fire can cause electric charges, starting additional fires many kilometers downwind.[16]

Shattering of rocks

When water in fractured rock is rapidly heated by a lightning strike, the resulting steam explosion can cause rock disintegration and shift boulders. It may be a significant factor in erosion of tropical and subtropical mountains that have never been glaciated. Evidence of lightning strikes includes erratic magnetic fields.[17][18]

Electrical and structural damage

A sculpture damaged by lightning in Wellington, New Zealand
The Eiffel Tower as a colossal lightning conductor. Photograph taken 1902-06-03 21:02

Telephones, modems, computers and other electronic devices can be damaged by lightning, as harmful overcurrent can reach them through the phone jack, Ethernet cable, or electricity outlet.[19] Close strikes can also generate electromagnetic pulses (EMPs) – especially during "positive" lightning discharges.

Lightning currents have a very fast rise time, 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 outer surface of the conductor.[20]

In addition to electrical wiring damage, the other types of possible damage to consider include structural, fire, and property damage.

Prevention and mitigations

The field of lightning protection systems is an enormous industry worldwide due to the impacts lightning can have on the constructs and activities of humankind. Lightning, as varied in properties measured across orders of magnitude as it is, can cause direct effects or have secondary impacts; lead to the complete destruction of a facility or process or simply cause the failure of a remote electronic sensor; it can result in outdoor activities being halted for safety concerns to employees as a thunderstorm nears an area and until it has sufficiently passed; it can ignite volatile commodities stored in large quantities or interfere with the normal operation of a piece of equipment at critical periods of time. The impacts of a lightning event are as varied and far reaching as the nearly infinite products and systems devised to mitigate the effects of lightning on our lives.

Most lightning protection devices and systems protect physical structures on the earth, aircraft in flight being the notable exception, however some attention has been paid to attempting to control lightning in the atmosphere, however all the attempts proved extremely limited in success. Chaff and silver iodide crystal concepts were devised to deal directly with the cloud cells and were dispensed directly into the clouds from an overflying aircraft. The chaff was devised to deal with the electrical manifestations of the storm from within, while the silver iodide salting technique was devised to deal with the mechanical forces of the storm.

Lightning protection systems

An example of a standard, pointed-tip, air terminal.
Main article: Lightning rod

Hundreds of devices, including lightning rods and charge transfer systems, are used to mitigate lightning damage and influence the path of a lightning flash.

A lightning rod (or lightning protector) is a metal strip or rod connected to earth through conductors and a grounding system, used to provide a preferred pathway to ground if lightning terminates on a structure. The class of these products are often called a "finial" or "air terminal". A lightning rod or "Franklin rod" in honor of its famous inventor, Benjamin Franklin, is simply a metal rod, and without being connected to the lightning protection system, as was sometimes the case in the old days, will provide no added protection to a structure. Other names include "lightning conductor", "arrester", and "discharger"; however, over the years these names have been incorporated into other products or industries with a stake in lightning protection. Lightning arrester, for example, often refers to fused links that explode when a strike occurs to a high voltage overhead power line to protect the more expensive transformers down the line by opening the circuit. In reality, it was an early form of a heavy duty surge protection device (SPD). Modern arresters, constructed with metal oxides, are capable of safely shunting abnormally high voltage surges to ground while preventing normal system voltages from being shorted to ground.

Monitoring and warning systems

The exact location of a lightning strike or when it will occur is still impossible to predict. However, products and systems have been designed of varying complexities to alert people as the probability of a strike increases above a set level determined by a risk assessment for the location's conditions and circumstances. One significant improvement has been in the area of detection of flashes through both ground and satellite-based observation devices. The strikes and atmospheric flashes are not predicted, however the level of detail recorded by these technologies has vastly improved in the past 20 years.

Although commonly associated with thunderstorms at close range, lightning strikes can occur on a day that seems devoid of clouds. This occurrence is known as "A Bolt From the Blue";[21] 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.[22] To do so, one should tune a standard AM medium wave receiver to a frequency with no transmitting stations, and listen for crackles amongst the static. Stronger or nearby lightning strikes will also cause cracking if the receiver is tuned to a station. As lower frequencies propagate further along the ground than higher ones, the lower medium wave (MW) band frequencies (in the 500–600 kHz range) can detect lightning strikes at longer distances; if the longwave band (153–279 kHz) is available, using it can increase this range even further.

Lightning detection 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.

Personal safety

The U.S. National Lightning Safety Institute[23] advises American citizens to have a plan for their safety when a thunderstorm occurs and to commence it as soon as the first lightning or thunder is observed. This is important as lightning can strike without rain actually falling. If thunder can be heard at all, then there is a risk of lightning. The safest place is inside a building or a vehicle. Risk remains for up to 30 minutes after the last observed lightning or thunder.

The National Lightning Safety Institute recommends using the F-B (flash to boom) method to gauge distance to a lightning strike. The flash of a lightning strike and resulting thunder occur at roughly the same time. But light travels 300,000 kilometers in a second, almost a million times the speed of sound. Sound travels at the slower speed of 344 m/s, so the flash of lightning is seen before thunder is heard. To use the method, count the seconds between the lightning flash and thunder. Divide by three to determine the distance in kilometers, or by five for miles. Immediate precautions against lightning should be taken if the F-B time is 25 seconds or less, that is, if the lightning is closer than 8 km (5.0 mi).

Reports differ regarding what to do if caught outside during a storm. One study shows that prostration is safer than lying down flat when there are no other alternatives.[24] A contrasting report suggested that it did not matter whether a person was standing up, squatting, or lying down when outside during a thunderstorm, because lightning can travel along the ground; this report suggested it was safest to be inside a solid structure or vehicle.[25] In the United States, the average annual death toll from lightning is 51 deaths per year, although there were only 23 deaths in 2013, which was a record low; the riskiest activities include fishing, boating, camping, and golf.[25] A person injured by lightning does not carry an electrical charge, and can be safely handled to apply first aid before emergency services arrive. Lightning can affect the brainstem, which controls breathing.[26]

Several studies conducted in South Asia and Africa suggest that the dangers of lightning are not taken sufficiently seriously there. A research team from the University of Colombo found that even in neighborhoods which had experienced deaths from lightning, no precautions were taken against future storms. An expert forum convened in 2007 to address how to raise awareness of lightning and improve lightning protection standards, and expressed concern that many countries had no official standards for the installation of lightning rods.[27]

Notable incidents

All events associated or suspected of causing damage are called "lightning incidents" due to four important factors.

As such it is often inconclusive, albeit highly probably a lightning flash was involved, hence categorizing it as a "lightning incident" covers all bases.

Earth-bound

In-flight

Most-stricken human

See also

References

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  2. "GHRC: Lightning Characteristics".
  3. 1 2 3 Ritenour AE, Morton MJ, McManus JG, Barillo DJ, Cancio LC; Morton; McManus; Barillo; Cancio (2008). "Lightning injury: a review". Burns. 34 (5): 585–94. doi:10.1016/j.burns.2007.11.006. PMID 18395987.
  4. Reindeer killed in Norway lightning storm, United Kingdom: BBC News, 29 August 2016
  5. Vastag B (2007). "fryPod: Lightning strikes iPod users". Science News. 172 (3).
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  11. "2008 Lightning Fatalities" (PDF). light08.pdf. NOAA. 2009-04-22. Retrieved 7 October 2009.
  12. "Lightning – Frequently Asked Questions". National Weather Service. Retrieved 17 June 2015.
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  14. Monheim, MS Sgt. (Ret.), Tony. "THE SHOCKING TRUTH ABOUT LIGHTNING DEATHS". Public Agency Training Council. Retrieved 17 March 2013.
  15. National Oceanic & Atmospheric Administration. "Image of lightning hitting a tree". National Oceanic & Atmospheric Administration. Archived from the original (.jpg) on October 20, 2006. Retrieved September 24, 2007.
  16. 1 2 "Lightning as a source of forest fires". Combustion, Explosion, and Shock Waves. Springer New York. 32 (5): 134–142. September 1996. ISSN 0010-5082. Retrieved 2009-07-25.
  17. Foss, Kanina, New evidence on lightning strikes University of the Witwatersrand, Johannesburg, Press release, 15 October 2013
  18. Knight, Jasper and Stefan W. Grab, Lightning as a geomorphic agent on mountain summits: Evidence from southern Africa, Geomorphology, Volume 204, 1 January 2014, Pages 61–70 abstract
  19. "Summer tips for telecom users". Blog.anta.net. 2008-06-17. ISSN 1797-1993. Retrieved 2008-06-18.
  20. Nair, Z., Aparna K.M., Khandagale R.S., Gopalan T.V. (2005). "Failure of 220 kV double circuit transmission line tower due to lightning". Journal of Performance of Constructed Facilities. 19 (2): 132. doi:10.1061/(ASCE)0887-3828(2005)19:2(132).
  21. NWS Pueblo Lightning Page – Bolts From The Blue. Crh.noaa.gov. Retrieved on 2011-11-08.
  22. Joni Jantunen et al. "Detection of lightning" U.S. Patent 7,254,484 Issue date: August 7, 2007
  23. Personal Lightning Safety Tips National Lightning Safety Institute . Accessed July 2008
  24. Adel, Miah M (2012). "Superiority of Prostration as a Protection from Lightning Strike". Physics International. 3 (1): 9–21. doi:10.3844/pisp.2012.9.21.
  25. 1 2 JONEL ALECCIA, June 20, 2014, NBC News, Debunked: 5 Lightning Myths That Could Kill You, Accessed June 20, 2014
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  27. "Striking back: lightning in the developing world". SciDev.Net.
  28. Rakov and Uman, p. 2
  29. La Tour Eiffel – The Eiffel Tower – Paris Things To Do – www.paris-things-to-do.co.uk. Paris-things-to-do.co.uk (2007-01-16). Retrieved on 2012-06-23.
  30. Evans, D. "An appraisal of underground gas storage technologies and incidents, for the development of risk assessment methodology" (PDF). British Geological Survey. Health and Safety Executive: 121. Retrieved 2008-08-14.
  31. Samantha Williams, Lightning kills 106 cows. news.com.au (2005-11-03)
  32. "Lightning kills 30 people in Pakistan's north". Reuters. 2007-07-20. Retrieved July 27, 2007.
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  39. "Roy Sullivan". The New York Times Archives (from UPI). 1983-09-30. Retrieved July 28, 2007.
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External links

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