Editing Lightning (section)

=== Electrical discharge as flashes and strikes ===

The charge carrier in lightning is mainly electrons in a plasma.<ref>{{Cite book|last=Uman|first=Martin|title=All About Lightning|publisher=Dover|year=1986|isbn=978-0-486-25237-7|location=New York|pages=74}}</ref> The process of going from charge as ions (positive hydrogen ion and negative hydroxide ion) associated with liquid water or solid water to charge as electrons associated with lightning must involve some form of electro-chemistry, that is, the oxidation and/or the reduction of chemical species.<ref>{{cite journal |last1=Witzke |first1=Megan |last2=Rumbach |first2=Paul |last3=Go |first3=David B |last4=Sankaran |first4=R Mohan |title=Evidence for the electrolysis of water by atmospheric-pressure plasmas formed at the surface of aqueous solutions |journal=Journal of Physics D |date=November 7, 2012 |volume=45 |issue=44 |pages=442001 |doi=10.1088/0022-3727/45/44/442001|bibcode=2012JPhD...45R2001W |s2cid=98547405 }}</ref>

The best-studied and understood form of lightning is cloud to ground (CG) lightning. Although more common, intra-cloud (IC) and cloud-to-cloud (CC) flashes are very difficult to study given there are no "physical" points to monitor inside the clouds. Also, given the very low probability of lightning striking the same point repeatedly and consistently, scientific inquiry is difficult even in areas of high CG frequency.

==== Lightning leaders ====
[[File: Lightning formation.gif|thumb|A downward leader travels towards earth, branching as it goes.]]
[[File:Leaderlightnig.gif|thumbnail|Lightning strike caused by the connection of two leaders, positive shown in blue and negative in red]]

In a process not well understood, a bidirectional channel of [[ionized]] air, called a "[[leader (spark)|leader]]", is initiated between oppositely-charged regions in a thundercloud. Leaders are electrically conductive channels of ionized gas that propagate through, or are otherwise attracted to, regions with a charge opposite of that of the leader tip. The negative end of the bidirectional leader fills a positive charge region, also called a well, inside the cloud while the positive end fills a negative charge well. Leaders often split, forming branches in a tree-like pattern.<ref>Ultraslow-motion video of stepped leader propagation: [http://www.ztresearch.com/ ztresearch.com] {{webarchive|url=https://web.archive.org/web/20100413125231/http://www.ztresearch.com/ |date=April 13, 2010 }}</ref> In addition, negative and some positive leaders travel in a discontinuous fashion, in a process called "stepping". The resulting jerky movement of the leaders can be readily observed in slow-motion videos of lightning flashes.

It is possible for one end of the leader to fill the oppositely-charged well entirely while the other end is still active. When this happens, the leader end which filled the well may propagate outside of the thundercloud and result in either a cloud-to-air flash or a cloud-to-ground flash. In a typical cloud-to-ground flash, a bidirectional leader initiates between the main negative and lower positive charge regions in a thundercloud. The weaker positive charge region is filled quickly by the negative leader which then propagates toward the inductively-charged ground.

The positively and negatively charged leaders proceed in opposite directions, positive upwards within the cloud and [[Electric charge|negative]] towards the earth. Both ionic channels proceed, in their respective directions, in a number of successive spurts. Each leader "pools" ions at the leading tips, shooting out one or more new leaders, momentarily pooling again to concentrate charged ions, then shooting out another leader. The negative leader continues to propagate and split as it heads downward, often speeding up as it gets closer to the Earth's surface.

About 90% of ionic channel lengths between "pools" are approximately {{convert|45|m|ft|abbr=on}} in length.<ref>Goulde, R.H. (1977) "The lightning conductor", pp. 545–576 in ''Lightning Protection'', R.H. Golde, Ed., ''Lightning, Vol. 2'', Academic Press.</ref> The establishment of the ionic channel takes a comparatively long amount of time (hundreds of [[millisecond]]s) in comparison to the resulting discharge, which occurs within a few dozen microseconds. The [[electric current]] needed to establish the channel, measured in the tens or hundreds of [[ampere]]s, is dwarfed by subsequent currents during the actual discharge.

Initiation of the lightning leader is not well understood. The electric field strength within the thundercloud is not typically large enough to initiate this process by itself.<ref>{{cite journal|doi=10.1007/s11214-008-9338-z|title=Charge Structure and Dynamics in Thunderstorms|date=2008|last1=Stolzenburg|first1=Maribeth|last2=Marshall|first2=Thomas C.|journal=Space Science Reviews|volume=137|issue=1–4|page=355|bibcode = 2008SSRv..137..355S |s2cid=119997418}}</ref> Many hypotheses have been proposed. One hypothesis postulates that showers of relativistic electrons are created by [[cosmic rays]] and are then accelerated to higher velocities via a process called [[runaway breakdown]]. As these relativistic electrons collide and ionize neutral air molecules, they initiate leader formation. Another hypothesis involves locally enhanced electric fields being formed near elongated water droplets or ice crystals.<ref>{{cite journal|doi=10.1029/2007JD009036|title=A brief review of the problem of lightning initiation and a hypothesis of initial lightning leader formation|date=2008|last1=Petersen|first1=Danyal|last2=Bailey|first2=Matthew|last3=Beasley|first3=William H.|last4=Hallett|first4=John|journal=Journal of Geophysical Research|volume=113|issue=D17|pages=D17205|bibcode = 2008JGRD..11317205P }}</ref> [[Percolation theory]], especially for the case of biased percolation,<ref>{{cite journal|doi=10.1103/PhysRevE.81.011102|pmid=20365318|title=Biased percolation on scale-free networks|date=2010|last1=Hooyberghs|first1=Hans|last2=Van Schaeybroeck|first2=Bert|last3=Moreira|first3=André A.|last4=Andrade|first4=José S.|last5=Herrmann|first5=Hans J.|last6=Indekeu|first6=Joseph O.|journal=Physical Review E|volume=81|issue=1|page=011102|bibcode = 2010PhRvE..81a1102H |arxiv = 0908.3786 |s2cid=7872437}}</ref> {{clarify| what does 'biased percolation' mean?|date= July 2013}} describes random connectivity phenomena, which produce an evolution of connected structures similar to that of lightning strikes. A streamer avalanche model<ref>{{Cite journal|last1=Griffiths|first1=R. F.|last2=Phelps|first2=C. T.|date=1976|title=A model for lightning initiation arising from positive corona streamer development|journal=Journal of Geophysical Research|volume=81|issue=21|pages=3671–3676|doi=10.1029/JC081i021p03671|bibcode=1976JGR....81.3671G}}</ref> has recently been favored by observational data taken by LOFAR during storms.<ref>{{Cite journal|last1=Sterpka|first1=Christopher|last2=Dwyer|first2=J|last3=Liu|first3=N|last4=Hare|first4=B M|last5=Scholten|first5=O|last6=Buitink|first6=S|last7=Ter Veen|first7=S|last8=Nelles|first8=A|date=November 24, 2021|title=The Spontaneous Nature of Lightning Initiation Revealed|journal=Ess Open Archive ePrints |volume=105 |issue=23 |pages=GL095511 |doi=10.1002/essoar.10508882.1|bibcode=2021GeoRL..4895511S |s2cid=244646368|url=https://bib-pubdb1.desy.de/record/474239 |hdl=2066/242824|hdl-access=free}}</ref><ref>{{Cite web|last=Lewton|first=Thomas|date=December 20, 2021|title=Detailed Footage Finally Reveals What Triggers Lightning|url=https://www.quantamagazine.org/radio-telescope-reveals-how-lightning-begins-20211220/|access-date=December 21, 2021|website=Quanta Magazine}}</ref>

==== Upward streamers ====
[[File:Upwards streamer from pool cover.jpg| thumb|220x124px | right | Upwards streamer emanating from the top of a pool cover]]
When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the strength of the [[electric field]]. The electric field is strongest on grounded objects whose tops are closest to the base of the thundercloud, such as trees and tall buildings. If the electric field is strong enough, a positively charged ionic channel, called a positive or upward [[Streamer discharge|streamer]], can develop from these points. This was first theorized by Heinz Kasemir.<ref>Kasemir, H. W. (1950) "Qualitative Übersicht über Potential-, Feld- und Ladungsverhaltnisse Bei einer Blitzentladung in der Gewitterwolke" (Qualitative survey of the potential, field and charge conditions during a lightning discharge in the thunderstorm cloud) in ''Das Gewitter'' (The Thunderstorm), H. Israel, ed., Leipzig, Germany: [[Akademische Verlagsgesellschaft]].</ref><ref>Ruhnke, Lothar H. (June 7, 2007) "[https://archive.today/20110611231459/http://www.physicstoday.org/obits/notice_157.shtml Death notice: Heinz Wolfram Kasemir]". Physics Today.</ref><ref name="SA-Stephan">{{cite web |last1=Stephan |first1=Karl |title=The Man Who Understood Lightning |url=https://blogs.scientificamerican.com/guest-blog/the-man-who-understood-lightning/ |publisher=Scientific American |access-date=June 26, 2020 |date=March 3, 2016}}</ref>

As negatively charged leaders approach, increasing the localized electric field strength, grounded objects already experiencing [[corona discharge]] will [[Corona breakdown|exceed a threshold]] and form upward streamers.

==== Attachment ====
Once a downward leader connects to an available upward leader, a process referred to as attachment, a low-resistance path is formed and discharge may occur. Photographs have been taken in which unattached streamers are clearly visible. The unattached downward leaders are also visible in branched lightning, none of which are connected to the earth, although it may appear they are. High-speed videos can show the attachment process in progress.<ref>{{Cite journal |doi = 10.1002/2017GL072796|title = Lightning attachment process to common buildings|journal = Geophysical Research Letters|volume = 44|issue = 9|pages = 4368–4375|year = 2017|last1 = Saba|first1 = M. M. F.|last2 = Paiva|first2 = A. R.|last3 = Schumann|first3 = C.|last4 = Ferro|first4 = M. A. S.|last5 = Naccarato|first5 = K. P.|last6 = Silva|first6 = J. C. O.|last7 = Siqueira|first7 = F. V. C.|last8 = Custódio|first8 = D. M.|bibcode = 2017GeoRL..44.4368S|doi-access = free}}</ref>

==== Discharge – Return stroke ====
{{redirect|Return stroke}}
[[File:Lightnings sequence 2 animation-wcag.gif|thumb|High-speed photography showing different parts of a lightning flash during the discharge process as seen in [[Toulouse]], France.]]

Once a conductive channel bridges the air gap between the negative charge excess in the cloud and the positive surface charge excess below, there is a large drop in resistance across the lightning channel. Electrons accelerate rapidly as a result in a zone beginning at the point of attachment, which expands across the entire leader network at up to one third of the speed of light.<ref name =Ulman2001>{{cite book|url=https://books.google.com/books?id=DgHCAgAAQBAJ|title=The lightning discharge|access-date=September 1, 2020|publisher=Courier Corporation|author=Uman, M. A.| date=2001|isbn=9780486151984}}</ref> This is the "return stroke" and it is the most [[Luminous intensity|luminous]] and noticeable part of the lightning discharge.

A large electric charge flows along the plasma channel, from the cloud to the ground, neutralising the positive ground charge as electrons flow away from the strike point to the surrounding area. This huge surge of current creates large radial voltage differences along the surface of the ground. Called step potentials,{{citation needed|date=September 2020}} they are responsible for more injuries and deaths in groups of people or of other animals than the strike itself.<ref>Deamer, Kacey (August 30, 2016) [https://www.livescience.com/55916-why-reindeer-killed-by-lightning.html More Than 300 Reindeer Killed By Lightning: Here's Why]. ''Live Science''</ref> Electricity takes every path available to it.<ref>{{cite web|title=The Path of Least Resistance|url=http://ecmweb.com/content/path-least-resistance|url-status=dead|archive-url=https://web.archive.org/web/20160104215214/http://ecmweb.com/content/path-least-resistance|archive-date=January 4, 2016|date=July 2001|access-date=January 9, 2016}}</ref>
Such step potentials will often cause current to flow through one leg and out another, electrocuting an unlucky human or animal standing near the point where the lightning strikes.

The electric current of the return stroke averages 30 kiloamperes for a typical negative CG flash, often referred to as "negative CG" lightning. In some cases, a ground-to-cloud (GC) lightning flash may originate from a positively charged region on the ground below a storm. These discharges normally originate from the tops of very tall structures, such as communications antennas. The rate at which the return stroke current travels has been found to be around 100,000&nbsp;km/s (one-third of the speed of light).<ref>{{Cite journal | last1 = Idone | first1 = V. P. | last2 = Orville | first2 = R. E. | last3 = Mach | first3 = D. M. | last4 = Rust | first4 = W. D. | title = The propagation speed of a positive lightning return stroke | doi = 10.1029/GL014i011p01150 | journal = Geophysical Research Letters | volume = 14 | issue = 11 | page = 1150 | year = 1987 |bibcode = 1987GeoRL..14.1150I | url = https://zenodo.org/record/1231386 }}</ref> A typical cloud-to-ground lightning flash culminates in the formation of an electrically conducting [[plasma (physics)|plasma]] channel through the air in excess of {{convert|5|km|mi|abbr=on}} tall, from within the cloud to the ground's surface.<ref>[[#Uman|Uman (1986)]] p. 81.</ref>

The massive flow of electric current occurring during the return stroke combined with the rate at which it occurs (measured in microseconds) rapidly [[superheating|superheats]] the completed leader channel, forming a highly electrically conductive plasma channel. The core temperature of the plasma during the return stroke may exceed {{convert|50,000|F|C|order=flip}},<ref>{{Cite web |last=US Department of Commerce |first=NOAA |title=Understanding Lightning: Thunder |url=https://www.weather.gov/safety/lightning-science-thunder#:~:text=The%20lightning%20discharge%20heats%20the,the%20surface%20of%20the%20sun |access-date=December 15, 2023 |website=www.weather.gov |language=EN-US}}</ref> causing it to radiate with a brilliant, blue-white color. Once the electric current stops flowing, the channel cools and dissipates over tens or hundreds of milliseconds, often disappearing as fragmented patches of glowing gas. The nearly instantaneous heating during the return stroke causes the air to expand explosively, producing a powerful [[shock wave]] which is heard as [[#Thunder|thunder]].

==== Discharge – Re-strike ====
High-speed videos (examined frame-by-frame) show that most negative CG lightning flashes are made up of 3 or 4 individual strokes, though there may be as many as 30.<ref>[[#Uman|Uman (1986)]] Ch. 5, p. 41.</ref>

Each re-strike is separated by a relatively large amount of time, typically 40 to 50 milliseconds, as other charged regions in the cloud are discharged in subsequent strokes. Re-strikes often cause a noticeable "[[strobe light]]" effect.<ref name="uman">[[#Uman|Uman (1986)]] pp. 103–110.</ref>

To understand why multiple return strokes utilize the same lightning channel, one needs to understand the behavior of positive leaders, which a typical ground flash effectively becomes following the negative leader's connection with the ground. Positive leaders decay more rapidly than negative leaders do. For reasons not well understood, bidirectional leaders tend to initiate on the tips of the decayed positive leaders in which the negative end attempts to re-ionize the leader network. These leaders, also called ''recoil leaders'', usually decay shortly after their formation. When they do manage to make contact with a conductive portion of the main leader network, a return stroke-like process occurs and a ''dart leader'' travels across all or a portion of the length of the original leader. The dart leaders making connections with the ground are what cause a majority of subsequent return strokes.<ref name="Warner">{{cite web |url=https://ztresearch.blog/education/ground-flashes/ |title=Ground Flashes |last=Warner |first=Tom |website=ZT Research |access-date=November 9, 2017|date=May 6, 2017 }}</ref>

Each successive stroke is preceded by intermediate dart leader strokes that have a faster rise time but lower amplitude than the initial return stroke. Each subsequent stroke usually re-uses the discharge channel taken by the previous one, but the channel may be offset from its previous position as wind displaces the hot channel.<ref>[[#Uman|Uman (1986)]] Ch. 9, p. 78.</ref>

Since recoil and dart leader processes do not occur on negative leaders, subsequent return strokes very seldom utilize the same channel on positive ground flashes which are explained later in the article.<ref name="Warner"/>

==== Discharge – Transient currents during flash ====
The electric current within a typical negative CG lightning discharge rises very quickly to its peak value in 1–10 microseconds, then decays more slowly over 50–200 microseconds. The transient nature of the current within a lightning flash results in several phenomena that need to be addressed in the effective protection of ground-based structures. Rapidly changing (alternating) currents tend to travel on the surface of a conductor, in what is called the [[skin effect]], unlike direct currents, which "flow-through" the entire conductor like water through a hose. Hence, conductors used in the protection of facilities tend to be multi-stranded, with small wires woven together. This increases the total bundle [[surface area]] in inverse proportion to the individual strand radius, for a fixed total [[Cross section (geometry)|cross-sectional area]].

The rapidly changing currents also create [[Electromagnetic pulse|electromagnetic pulses (EMPs)]] that radiate outward from the ionic channel. This is a characteristic of all electrical discharges. The radiated pulses rapidly weaken as their distance from the origin increases. However, if they pass over conductive elements such as power lines, communication lines, or metallic pipes, they may induce a current which travels outward to its termination. The surge current is inversely related to the surge impedance: the higher in impedance, the lower the current.<ref>{{Cite web|url=https://site.ieee.org/sas-pesias/files/2016/03/Lightning-Protection-and-Transient-Overvoltage_Rogerio-Verdolin.pdf|title=Lightning Protection and Transient Overvoltage}}</ref> This is the [[Voltage spike|surge]] that, more often than not, results in the destruction of delicate [[electronics]], [[electrical appliance]]s, or [[electric motor]]s. Devices known as [[Surge protector|surge protectors (SPD) or transient voltage surge suppressors (TVSS)]] attached in parallel with these lines can detect the lightning flash's transient irregular current, and, through alteration of its physical properties, route the spike to an attached [[Electrical ground|earthing ground]], thereby protecting the equipment from damage.