Editing Lightning (section)

=== 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.