Editing Lightning (section)

=== Cloud to ground (CG) ===
[[File:Lightning strikes in slow motion (240fps) in Muurame, Finland.webm|thumb|Cloud to ground seen in slow motion]]

{{em|Cloud-to-ground}} (CG) lightning is a lightning discharge between a thundercloud and the ground. It is initiated by a stepped leader moving down from the cloud, which is met by a streamer moving up from the ground.

CG is the least common, but best understood of all types of lightning. It is easier to study scientifically because it terminates on a physical object, namely the ground, and lends itself to being measured by instruments on the ground. Of the three primary types of lightning, it poses the greatest threat to life and property, since it terminates on the ground or "strikes".

The overall discharge, termed a flash, is composed of a number of processes such as preliminary breakdown, stepped leaders, connecting leaders, return strokes, dart leaders, and subsequent return strokes.<ref>{{cite book |chapter=Mechanism of the Lightning Flash |title=The Lightning Flash |edition=2nd |editor-first=V.|editor-last=Cooray |publisher=[[Institution of Engineering and Technology]] |location=London |year=2014 |pages=119–229}}</ref> The conductivity of the electrical ground, be it soil, [[fresh water]], or [[saline water|salt water]], may affect the lightning discharge rate and thus visible characteristics.<ref>{{cite magazine |last=Jones|first=Nicola |title=Salty Seas Make Lightning Brighter |url=https://hakaimagazine.com/news/salty-seas-make-lightning-brighter |access-date=January 11, 2021 |magazine=[[Smithsonian (magazine)|Smithsonian]] |date=January 4, 2021}}</ref>

===={{anchor|Positive lightning}}Positive and negative lightning====
Cloud-to-ground (CG) lightning is either positive or negative, as defined by the direction of the [[conventional current flow|conventional electric current]] between cloud and ground. Most CG lightning is negative, meaning that a negative charge is transferred (electrons flow) downwards to ground along the lightning channel (the conventional current flows from the ground up to the cloud). The reverse happens in a positive CG flash, where electrons travel upward along the lightning channel, while also a positive charge is transferred downward to the ground (the conventional current travels from cloud to ground).

Positive lightning is less common than negative lightning and on average makes up less than 5% of all lightning strikes.<ref>{{cite web |title=NWS JetStream – The Positive and Negative Side of Lightning |publisher=[[National Oceanic and Atmospheric Administration]] |url=https://srh.noaa.gov/jetstream/lightning/positive.htm |access-date=September 25, 2007 |url-status=live |archive-url=https://web.archive.org/web/20070705205815/http://www.srh.noaa.gov/jetstream/lightning/positive.htm |archive-date=July 5, 2007 | df = mdy-all }}</ref>

[[File: Anvil-to-ground lightning.jpg|thumb|A ''bolt from the blue'' lightning strike which appears to initiate from the clear, but{{clarify|date=December 2024}} the turbulent sky above the [[anvil cloud]] and drive a bolt of plasma through the cloud directly to the ground. They are commonly referred to as positive flashes, despite the fact that they are usually negative in polarity.]]

There are a number of mechanisms theorized to result in the formation of positive lightning.<ref>{{cite journal |title=Positive lightning: An overview, new observations, and inferences |journal=Journal of Geophysical Research: Atmospheres |volume=117 |issue=D8 |year=2012 |last1=Nag|first1=Amitabh |last2=Rakov|first2=Vladimir A. |pages= |bibcode=2012JGRD..117.8109N |doi=10.1029/2012JD017545 |doi-access=free}}</ref> These are mainly based on movement or intensification of charge centres in the cloud. Such changes in cloud charging may come about as a result of variations in vertical wind shear or precipitation, or dissipation of the storm. Positive flashes may also result from certain behaviour of in-cloud discharges, e.g. breaking off or branching from existing flashes.

Positive lightning strikes tend to be much more intense than their negative counterparts. An average bolt of [[electrical polarity|negative]] lightning creates an electric current of 30,000 [[ampere]]s (30&nbsp;kA), transferring a total 15&nbsp;C ([[coulomb]]s) of [[electric charge]] and 1 [[joule|gigajoule]] of [[energy]]. Large bolts of positive lightning can create up to 120&nbsp;kA and transfer 350&nbsp;C.<ref>Hasbrouck, Richard. [https://llnl.gov/str/pdfs/05_96.1.pdf Mitigating Lightning Hazards] {{webarchive|url=https://web.archive.org/web/20131005005230/https://www.llnl.gov/str/pdfs/05_96.1.pdf |date=October 5, 2013 }}, Science & Technology Review May 1996. Retrieved on April 26, 2009.</ref> The average positive ground flash has roughly double the peak current of a typical negative flash, and can produce peak currents up to 400&nbsp;kA and charges of several hundred coulombs.<ref>V. A. Rakov, M. A. Uman, Positive and bipolar lightning discharges to ground, in: Light. Phys. Eff., [[Cambridge University Press]], 2003: pp. 214–240</ref><ref name="U.A.BakshiM.V.Bakshi2009">{{cite book |first1=U. A.|last1=Bakshi |first2=M. V.|last2=Bakshi |title=Power System – II |url=https://books.google.com/books?id=oOj4NjQ8xGQC&pg=SA12-PA5 |date=January 1, 2009 |publisher=Technical Publications |isbn=978-81-8431-536-3 |page=12 |url-status=live |archive-url=https://web.archive.org/web/20170312135216/https://books.google.com/books?id=oOj4NjQ8xGQC&pg=SA12-PA5 |archive-date=March 12, 2017 }}</ref> Furthermore, positive ground flashes with high peak currents are commonly followed by long continuing currents, a correlation not seen in negative ground flashes.<ref>{{cite journal |doi=10.1029/2010JD014330 |title=High-speed video observations of positive lightning flashes to ground |journal=Journal of Geophysical Research: Atmospheres |volume=115 |issue=D24 |pages=201 |year=2010 |last1=Saba|first1=Marcelo M. F. |last2=Schulz|first2=Wolfgang |last3=Warner|first3=Tom A. |last4=Campos|first4=Leandro Z. S. |last5=Schumann|first5=Carina |last6=Krider|first6=E. Philip |last7=Cummins|first7=Kenneth L. |last8=Orville|first8=Richard E. |bibcode=2010JGRD..11524201S |s2cid=129809543 |doi-access=free}}</ref>

As a result of their greater power, positive lightning strikes are considerably more dangerous than negative strikes.{{citation needed|date=January 2025}} Positive lightning produces both higher peak currents and longer continuing currents, making them capable of heating surfaces to much higher levels which increases the likelihood of a fire being ignited. The long distances positive lightning can propagate through clear air explains why they are known as "bolts from the blue", giving no warning to observers.

Positive lightning has also been shown to trigger the occurrence of upward lightning flashes from the tops of tall structures and is largely responsible for the initiation of [[Upper-atmospheric lightning#Sprites|sprites]] several tens of kilometers above ground level. Positive lightning tends to occur more frequently in [[winter storm]]s, as with [[thundersnow]], during intense [[tornado]]es<ref name="Tornadic lightning">{{cite journal|author1=Perez, Antony H. |author2=Wicker, Louis J. |author3=Richard E. Orville |title=Characteristics of Cloud-to-Ground Lightning Associated with Violent Tornadoes|journal=Weather Forecast.|volume=12|issue=3|pages=428–37|doi=10.1175/1520-0434(1997)012<0428:COCTGL>2.0.CO;2|bibcode = 1997WtFor..12..428P|year=1997 |doi-access=free}}</ref> and in the dissipation stage of a [[thunderstorm]].<ref name="GHCC primer2">{{cite web
| url = http://thunder.nsstc.nasa.gov/primer/primer2.html|title = A Lightning Primer – Characteristics of a Storm |access-date = February 8, 2009 |author1=Christian, Hugh J. |author2=McCook, Melanie A. |work =NASA | url-status = dead|archive-url=https://web.archive.org/web/20160305002214/http://thunder.nsstc.nasa.gov/primer/primer2.html |archive-date=March 5, 2016}}</ref> Huge quantities of [[extremely low frequency]] (ELF) and [[very low frequency]] (VLF) [[radio wave]]s are also generated.<ref name="boccippio">{{cite journal | last1 = Boccippio | first1 = DJ | title = Sprites, ELF Transients, and Positive Ground Strokes | journal = [[Science (journal)|Science]] | volume = 269 | pages = 1088–1091 |date=August 1995 | doi = 10.1126/science.269.5227.1088 | pmid = 17755531 | last2 = Williams | first2 = ER | last3 = Heckman | first3 = SJ | last4 = Lyons | first4 = WA | last5 = Baker | first5 = IT | last6 = Boldi | first6 = R | issue = 5227 | bibcode = 1995Sci...269.1088B | s2cid = 8840716 }}</ref>

Contrary to popular belief, positive lightning flashes do ''not'' necessarily originate from the anvil or the upper positive charge region and strike a rain-free area outside of the thunderstorm. This belief is based on the outdated idea that lightning leaders are unipolar and originate from their respective charge region.{{citation needed|date=May 2021}} Despite the popular misconception that flashes originating from the anvil are positive, because they seem to originate from the positive charge region, observations have shown that these are in fact negative flashes. They begin as IC flashes within the cloud, the negative leader then exits the cloud from the positive charge region before propagating through clear air and striking the ground some distance away.<ref name="ReferenceA">{{cite journal|doi=10.1029/2011JD016890|title=Lightning morphology and impulse charge moment change of high peak current negative strokes|journal=Journal of Geophysical Research: Atmospheres|volume=117|issue=D4|year=2012|last1=Lu|first1=Gaopeng|last2=Cummer|first2=Steven A|last3=Blakeslee|first3=Richard J|last4=Weiss|first4=Stephanie|last5=Beasley|first5=William H|pages=n/a|bibcode=2012JGRD..117.4212L|citeseerx=10.1.1.308.9842}}</ref><ref name="ReferenceB">{{cite journal|doi=10.1038/ngeo162|title=Upward electrical discharges from&nbsp;thunderstorms|journal=Nature Geoscience|volume=1|issue=4|page=233|year=2008|last1=Krehbiel|first1=Paul R|last2=Riousset|first2=Jeremy A|last3=Pasko|first3=Victor P|last4=Thomas|first4=Ronald J|last5=Rison|first5=William|last6=Stanley|first6=Mark A|last7=Edens|first7=Harald E|bibcode=2008NatGe...1..233K|s2cid=8753629}}</ref>