Editing Lightning (section)

== Formation ==
The processes involved in lightning formation fall into the following categories:
# Large-scale atmospheric phenomena in which charge separation can occur (e.g. [[storm]])
# Microscopic and macroscopic processes that result in charge separation
# Establishment of an electric field
# Discharge through a lightning channel

=== Atmospheric phenomena in which lightning occurs ===
Lightning primarily occurs when warm air is mixed with colder air masses,<ref>{{Cite book|url=https://books.google.com/books?id=g1foWWN5odwC&q=Lightning+occurs+when+warm+air+is+mixed+with+colder+air+masses&pg=PA90|title=Sprites, Elves and Intense Lightning Discharges|last1=Füllekrug|first1=Martin|last2=Mareev|first2=Eugene A.|last3=Rycroft|first3=Michael J.|date=May 1, 2006|publisher=Springer Science & Business Media|isbn=9781402046285|url-status=live|archive-url=https://web.archive.org/web/20171104190958/https://books.google.com/books?id=g1foWWN5odwC&pg=PA90&dq=Lightning+occurs+when+warm+air+is+mixed+with+colder+air+masses&hl=en&sa=X&ved=0ahUKEwiV44XT9uXUAhVJ7mMKHb3pBUoQ6AEIMDAC#v=onepage&q=Lightning%20occurs%20when%20warm%20air%20is%20mixed%20with%20colder%20air%20masses&f=false|archive-date=November 4, 2017|bibcode=2006seil.book.....F}}</ref> resulting in atmospheric disturbances necessary for polarizing the atmosphere.<ref name="Volland1995">{{cite book | editor = Hans Volland | date = 1995 | title = Handbook of Atmospheric Electrodynamics | publisher = CRC Press | page = 204 | isbn = 978-0-8493-8647-3 | url = https://books.google.com/books?id=MNPPh7B3WTIC|author1=Rinnert, K. |chapter=9: Lighting Within Planetary Atmospheres|quote=The requirements for the production of lightning within an atmosphere are the following: (1) a sufficient abundance of appropriate material for electrification, (2) the operation of a microscale electrification process to produce classes of particles with different signs of charge and (3) a mechanism to separate and to accumulate particles according to their charge.}}</ref> The disturbances result in [[storm]]s, and when those storms also result in lightning and thunder, they are called a [[thunderstorm]].

Lightning can also occur during [[dust storm]]s, [[forest fires]], [[tornado]]es, [[volcano|volcanic eruptions]], and even in the cold of winter, where the lightning is known as [[thundersnow]].<ref>[http://news.nationalgeographic.com/news/2010/02/100203-volcanoes-lightning/ New Lightning Type Found Over Volcano?] {{webarchive|url=https://web.archive.org/web/20100209015048/http://news.nationalgeographic.com/news/2010/02/100203-volcanoes-lightning/ |date=February 9, 2010 }}. News.nationalgeographic.com (February 2010). Retrieved on June 23, 2012.</ref><ref>{{cite web|url=http://hvo.wr.usgs.gov/volcanowatch/1998/98_06_11.html|title=Bench collapse sparks lightning, roiling clouds|access-date=October 7, 2012|publisher=[[United States Geological Survey]]|date=June 11, 1998|work=Volcano Watch|url-status=live|archive-url=https://web.archive.org/web/20120114172155/http://hvo.wr.usgs.gov/volcanowatch/1998/98_06_11.html|archive-date=January 14, 2012}}</ref> [[tropical cyclone|Hurricanes]] typically generate some lightning, mainly in the rainbands as much as {{convert|160|km|mi|abbr=on}} from the center.<ref>Pardo-Rodriguez, Lumari (Summer 2009) [http://nldr.library.ucar.edu/repository/assets/soars/SOARS-000-000-000-193.pdf Lightning Activity in Atlantic Tropical Cyclones: Using the Long-Range Lightning Detection Network (LLDN)] {{webarchive|url=https://web.archive.org/web/20130309085405/http://nldr.library.ucar.edu/repository/assets/soars/SOARS-000-000-000-193.pdf |date=March 9, 2013 }}. MA Climate and Society, Columbia University Significant Opportunities in Atmospheric Research and Science Program.</ref><ref>[https://science.nasa.gov/science-news/science-at-nasa/2006/09jan_electrichurricanes/ Hurricane Lightning] {{webarchive|url=https://web.archive.org/web/20170815013425/https://science.nasa.gov/science-news/science-at-nasa/2006/09jan_electrichurricanes/ |date=August 15, 2017 }}, NASA, January 9, 2006.</ref><ref>[http://www.unidata.ucar.edu/committees/polcom/2009spring/statusreports/BusingerS09.pdf The Promise of Long-Range Lightning Detection in Better Understanding, Nowcasting, and Forecasting of Maritime Storms] {{webarchive|url=https://web.archive.org/web/20130309085405/http://www.unidata.ucar.edu/committees/polcom/2009spring/statusreports/BusingerS09.pdf |date=March 9, 2013 }}. Long Range Lightning Detection Network</ref>

Intense forest fires, such as those seen in the [[2019–20 Australian bushfire season]], can create their own weather systems that can produce lightning (also called Fire Lightning) and other weather phenomena.<ref name="ABC / Ceranic">{{Cite news |url=https://www.abc.net.au/news/2018-11-28/bushfire-storms-can-spark-fire-tornadoes-dry-lightning-and-more/10561832 |author=Ceranic, Irena |title=Fire tornadoes and dry lightning are just the start of the nightmare when a bushfire creates its own storm |date=November 28, 2020 |work=ABC News |publisher=Australian Broadcasting Corporation}}</ref> Intense heat from a fire causes air to rapidly rise within the smoke plume, causing the formation of [[pyrocumulonimbus]] clouds. Cooler air is drawn in by this turbulent, rising air, helping to cool the plume. The rising plume is further cooled by the lower atmospheric pressure at high altitude, allowing the moisture in it to condense into cloud. Pyrocumulonimbus clouds form in an unstable atmosphere. These weather systems can produce [[Dry thunderstorm|dry lightning]], [[fire tornado]]es, intense winds, and dirty hail.<ref name="ABC / Ceranic" />

As well as the [[atmospheric thermodynamics|thermodynamic]] and [[Meteorology#Dynamic meteorology|dynamic]] conditions of the atmosphere, [[aerosol]] (e.g. dust or smoke) composition is thought to influence the frequency of lightning flashes in a storm.<ref name="wang2018">{{cite journal |last1=Wang |first1=Qianqian |last2=Li |first2=Zhanqing |last3=Guo |first3=Jianping |last4=Zhao |first4=Chuanfeng |last5=Cribb |first5=Maureen |title=The climate impact of aerosols on the lightning flash rate: is it detectable from long-term measurements? |journal=Atmospheric Chemistry and Physics |date=6 September 2018 |volume=18 |issue=17 |pages=12797–12816 |doi=10.5194/acp-18-12797-2018|doi-access=free |bibcode=2018ACP....1812797W }}</ref> A specific example of this is that relatively high lightning frequency is seen along ship tracks.<ref name="thornton2017">{{cite journal |last1=Thornton |first1=Joel A. |last2=Virts |first2=Katrina S. |last3=Holzworth |first3=Robert H. |last4=Mitchell |first4=Todd P. |title=Lightning enhancement over major oceanic shipping lanes |journal=Geophysical Research Letters |date=16 September 2017 |volume=44 |issue=17 |pages=9102–9111 |doi=10.1002/2017GL074982|doi-access=free |bibcode=2017GeoRL..44.9102T }}</ref>

Airplane contrails have also been observed to influence lightning to a small degree. The water vapor-dense contrails of airplanes may provide a lower resistance pathway through the atmosphere having some influence upon the establishment of an ionic pathway for a lightning flash to follow.<ref>[[#Uman|Uman (1986)]] Ch. 4, pp. 26–34.</ref> Rocket exhaust plumes provided a pathway for lightning when it was witnessed striking the [[Apollo 12|Apollo 12 rocket]] shortly after takeoff.

[[Thermonuclear weapon|Thermonuclear explosions]], by providing extra material for electrical conduction and a very turbulent localized atmosphere, have been seen triggering lightning flashes within the mushroom cloud. In addition, intense gamma radiation from large nuclear explosions may develop intensely charged regions in the surrounding air through [[Compton scattering]]. The intensely charged space charge regions create multiple clear-air lightning discharges shortly after the device detonates.<ref name="Nuclear Lightning">{{Cite journal|date= 1987 | title= An empirical study of the nuclear explosion-induced lightning seen on IVY-MIKE | journal= Journal of Geophysical Research | volume= 92 | issue= D5 | pages= 5696–5712| bibcode=1987JGR....92.5696C | doi= 10.1029/JD092iD05p05696| author= Colvin, J. D. | last2= Mitchell | first2= C. K. | last3= Greig | first3= J. R. | last4= Murphy | first4= D. P. | last5= Pechacek | first5= R. E. | last6= Raleigh | first6= M.}}</ref>

Some high energy cosmic rays produced by supernovas as well as solar particles from the solar wind, enter the atmosphere and electrify the air, which may create pathways for lightning channels.<ref>{{cite web|url=http://www.iop.org/news/14/may/page_63245.html |title=High-speed solar winds increase lightning strikes on Earth |publisher=Iop.org |date=May 15, 2014 |access-date=May 19, 2014}}</ref>

=== Charge separation ===
==== Charge separation in thunderstorms ====
[[File: Understanding Lightning - Figure 1 - Cloud Charging Area.gif|thumb|(Figure 1) The main charging area in a thunderstorm occurs in the central part of the storm where the air is moving upward rapidly (updraft) and temperatures range from {{convert|-15|to|-25|C|F}}.]]
[[File:Graupel animation 3a.gif|thumb|(Figure 2) When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged.]]
[[File:Charged cloud animation 4a.gif|thumb|The upper part of the thunderstorm cloud becomes positively charged while the middle to the lower part of the thunderstorm cloud becomes negatively charged.]]
The details of the charging process are still being studied by scientists, but there is general agreement on some of the basic concepts of thunderstorm charge separation, also known as electrification. Electrification can be by the [[triboelectric effect]] leading to electron or ion transfer between colliding bodies.

The main charging area in a thunderstorm occurs in the central part of the storm where air is moving upward rapidly (updraft) and temperatures range from {{convert|-15|to|-25|C|F}}; see Figure 1. In that area, the combination of temperature and rapid upward air movement produces a mixture of super-cooled cloud droplets (small water droplets below freezing), small ice crystals, and [[graupel]] (soft hail). The updraft carries the [[Supercooling|super-cooled]] cloud droplets and very small ice crystals upward. At the same time, the graupel, which is considerably larger and denser, tends to fall or be suspended in the rising air.<ref name="NOAA">{{cite web|url=http://www.lightningsafety.noaa.gov/science/science_electrication.htm |title=NWS Lightning Safety: Understanding Lightning: Thunderstorm Electrification |publisher=[[National Oceanic and Atmospheric Administration]]|access-date=November 25, 2016|url-status=dead|archive-url=https://web.archive.org/web/20161130080723/http://www.lightningsafety.noaa.gov/science/science_electrication.htm |archive-date=November 30, 2016}} {{PD-notice}}</ref>

The differences in the movement of the cloud particles cause collisions to occur. When the rising ice crystals collide with graupel, the ice crystals become positively charged and the graupel becomes negatively charged; see Figure 2. The updraft carries the positively charged ice crystals upward toward the top of the storm cloud. The larger and denser graupel is either suspended in the middle of the thunderstorm cloud or falls toward the lower part of the storm.<ref name="NOAA"/> Typically, the upper part of the thunderstorm cloud becomes positively charged while the middle to lower part of the thunderstorm cloud becomes negatively charged.<ref name="NOAA"/><ref name="arizonaLecture11">{{cite web |title=Lecture 11 – Thunderstorm electrification |url=http://www.atmo.arizona.edu/students/courselinks/spring13/atmo589/ATMO489_online/lecture_11/lect11_cloud_electrification.html |website=www.atmo.arizona.edu |access-date=31 January 2025}}</ref> The above process of charge separation as a result of cloud particle collisions is normally referred to as the ''non-inductive'' charging mechanism.<ref name="yair2008">{{cite journal |last1=Yair |first1=Y. |title=Charge Generation and Separation Processes |journal=Space Science Reviews |date=June 2008 |volume=137 |issue=1–4 |pages=119–131 |doi=10.1007/s11214-008-9348-x|bibcode=2008SSRv..137..119Y }}</ref>

The upward motions within the storm and winds at higher levels in the atmosphere tend to cause the small ice crystals (and positive charge) in the upper part of the thunderstorm cloud to spread out horizontally some distance from the thunderstorm cloud base. This part of the thunderstorm cloud is called the anvil. While this is the main charging process for the thunderstorm cloud, some of these charges can be redistributed by air movements within the storm (updrafts and downdrafts). In addition, there is a small but important positive charge buildup near the bottom of the thunderstorm cloud due to the precipitation and warmer temperatures.<ref name="NOAA"/> The positive-negative-positive charge regions commonly occur in mature thunderstorms, and referred to as the '' tripolar'' charge structure.<ref name="yair2008"/>

There are also other charging processes that may play a role in thunderstorms, but are generally thought to be less important. An ''inductive'' charging mechanism has been studied, and would arise from the polarisation of cloud droplets in the presence of the [[Global atmospheric electrical circuit#Fair weather|''fair-weather'' electric field]].<ref name="yair2008"/>  It has also been stated that uncharged, colliding water-drops can become charged because of charge transfer between them (as aqueous ions) in an electric field as would exist in a [[thunderstorm]].<ref>{{cite journal | last1=Jennings | first1=S. G. | last2=Latham | first2=J. | title=The charging of water drops falling and colliding in an electric field | journal=Archiv für Meteorologie, Geophysik und Bioklimatologie, Serie A | publisher=Springer Science and Business Media LLC | volume=21 | issue=2–3 | year=1972 | doi=10.1007/bf02247978 | pages=299–306| bibcode=1972AMGBA..21..299J | s2cid=118661076 }}</ref>

The induced separation of charge in pure liquid water has been known since the 1840s as has the electrification of pure liquid water by the triboelectric effect.<ref>Francis, G. W., "Electrostatic Experiments" Oleg D. Jefimenko, Editor, Electret Scientific Company, Star City, 2005</ref> [[William Thomson, 1st Baron Kelvin|William Thomson]] (Lord Kelvin) demonstrated that charge separation in water occurs in the usual electric fields at the Earth's surface and developed a continuous electric field measuring device using that knowledge.<ref>{{cite journal |last1=Aplin |first1=K. L. |last2=Harrison |first2=R. G. |title=Lord Kelvin's atmospheric electricity measurements |journal=History of Geo- and Space Sciences |date=September 3, 2013 |volume=4 |issue=2 |pages=83–95 |doi=10.5194/hgss-4-83-2013|arxiv=1305.5347 |bibcode=2013HGSS....4...83A |s2cid=9783512 |doi-access=free }}</ref> The physical separation of charge into different regions using liquid water was demonstrated by Kelvin with the [[Kelvin water dropper]]. The most likely charge-carrying species were considered to be the aqueous hydrogen ion and the aqueous hydroxide ion.<ref>{{cite journal |last1=Desmet |first1=S |last2=Orban |first2=F |last3=Grandjean |first3=F |title=On the Kelvin electrostatic generator |journal=European Journal of Physics |date=April 1, 1989 |volume=10 |issue=2 |pages=118–122 |doi=10.1088/0143-0807/10/2/008|bibcode=1989EJPh...10..118D |s2cid=121840275 }}</ref> An electron is not stable in liquid water concerning a hydroxide ion plus dissolved hydrogen for the time scales involved in thunderstorms.<ref>Buxton, G. V., Greenstock, C. L., Helman, W. P. and Ross, A. B. "Critical Review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (OH/O in aqueous solution." J. Phys. Chem. Ref. Data 17, 513–886 (1988).</ref> The electrical charging of solid water ice has also been considered. The charged species were again considered to be the hydrogen ion and the hydroxide ion.<ref>{{cite journal |last1=Dash |first1=J G |last2=Wettlaufer |first2=J S |title=The surface physics of ice in thunderstorms |journal=Canadian Journal of Physics |date=January 1, 2003 |volume=81 |issue=1–2 |pages=201–207 |doi=10.1139/P03-011|bibcode=2003CaJPh..81..201D }}</ref><ref>{{cite journal |last1=Dash |first1=J. G. |last2=Mason |first2=B. L. |last3=Wettlaufer |first3=J. S. |title=Theory of charge and mass transfer in ice-ice collisions |journal=Journal of Geophysical Research: Atmospheres |date=September 16, 2001 |volume=106 |issue=D17 |pages=20395–20402 |doi=10.1029/2001JD900109|bibcode=2001JGR...10620395D |doi-access=free }}</ref>

=== Establishing an electric field ===
{{main|Thunderstorm}}
In order for an [[electrostatic discharge]] to occur, two preconditions are necessary: first, a sufficiently high [[potential difference]] between two regions of space must exist, and second, a high-resistance medium must obstruct the free, unimpeded equalization of the opposite charges. The atmosphere provides the electrical insulation, or barrier, that prevents free equalization between charged regions of opposite polarity. Meanwhile, a thunderstorm can provide the charge separation and aggregation in certain regions of the cloud.<ref>{{cite journal|doi=10.1175/1520-0450(1993)032<0642:AROTEP>2.0.CO;2|volume=32|title=A Review of Thunderstorm Electrification Processes |last1=Saunders|first1=C. P. R.|journal=Journal of Applied Meteorology |issue=4 |pages=642–55|bibcode=1993JApMe..32..642S |year=1993|doi-access=free}}</ref>

When the local electric field exceeds the [[dielectric strength]] of damp air (about 3 MV/m), electrical discharge results in a ''strike'', often followed by commensurate discharges branching from the same path. Mechanisms that cause the charges to build up to lightning are still a matter of scientific investigation.<ref name="how">{{cite web|url=https://www.pbs.org/wnet/savageplanet/03deadlyskies/01lforms/indexmid.html|title=How Lightning Forms|access-date=September 21, 2007|publisher=Public Broadcasting System|author=Fink, Micah|work=PBS.org|url-status=live|archive-url=https://web.archive.org/web/20070929174806/http://www.pbs.org/wnet/savageplanet/03deadlyskies/01lforms/indexmid.html|archive-date=September 29, 2007}}</ref><ref name="noaa">{{cite web|url=http://www.lightningsafety.noaa.gov/science.htm|title=Lightning Safety|access-date=September 21, 2007|publisher=National Weather Service|date=2007|author=National Weather Service|url-status=dead|archive-url=https://web.archive.org/web/20071007110300/http://www.lightningsafety.noaa.gov/science.htm|archive-date=October 7, 2007}}</ref> A 2016 study confirmed dielectric breakdown is involved.<ref>{{Cite journal|last1=Rison|first1=William|last2=Krehbiel|first2=Paul R.|last3=Stock|first3=Michael G.|last4=Edens|first4=Harald E.|last5=Shao|first5=Xuan-Min|last6=Thomas|first6=Ronald J.|last7=Stanley|first7=Mark A.|last8=Zhang|first8=Yang|date=February 15, 2016|title=Observations of narrow bipolar events reveal how lightning is initiated in thunderstorms|journal=Nature Communications|volume=7|issue=1|pages=10721|doi=10.1038/ncomms10721|pmid=26876654|pmc=4756383|bibcode=2016NatCo...710721R|doi-access=free}}</ref> Lightning may be caused by the circulation of warm moisture-filled air through [[electric field]]s.<ref>[[#Uman|Uman (1986)]] p. 61.</ref> Ice or water particles then accumulate charge as in a [[Van de Graaff generator]].<ref>[[#Rakov|Rakov and Uman]], p. 84.</ref>

As a [[Cumulonimbus cloud|thundercloud]] moves over the surface of the Earth, an equal [[electric charge]], but of opposite polarity, is [[Electrostatic induction|induced]] on the Earth's surface underneath the cloud. The induced positive surface charge, when measured against a fixed point, will be small as the thundercloud approaches, increasing as the center of the storm arrives and dropping as the thundercloud passes. The referential value of the induced surface charge could be roughly represented as a bell curve.

The oppositely charged regions create an [[electric field]] within the air between them. This electric field varies in relation to the strength of the surface charge on the base of the thundercloud – the greater the accumulated charge, the higher the electrical field.

=== 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.