Editing Lightning (section)

==== 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>