Editing Snow (section)

===Cloud physics===
{{Main|Snowflake}}
[[File:Snow day in Adachi Tokyo - 2018 1 22.webm|thumb|Snow falling in [[Tokyo]], [[Japan]]]]
[[File:Feathery Snow Crystals (2217830221).jpg|thumb|Freshly fallen snowflakes]]
A snowflake consists of roughly 10<sup>19</sup> water [[molecule]]s which are added to its core at different rates and in different patterns depending on the changing temperature and humidity within the atmosphere that the snowflake falls through on its way to the ground. As a result, snowflakes differ from each other though they follow similar patterns.<ref>{{cite web|url=http://news.nationalgeographic.com/news/2007/02/070213-snowflake.html|title="No Two Snowflakes the Same" Likely True, Research Reveals|author=John Roach|date=February 13, 2007|access-date=July 14, 2009|publisher=[[National Geographic Society|National Geographic]] News|url-status=dead|archive-url=https://web.archive.org/web/20100109031550/http://news.nationalgeographic.com/news/2007/02/070213-snowflake.html|archive-date=January 9, 2010|df=mdy-all}}</ref><ref>{{cite journal|title=Origin of diversity in falling snow|author=Jon Nelson|journal=Atmospheric Chemistry and Physics|date=September 26, 2008|doi=10.5194/acp-8-5669-2008|volume=8|issue=18|pages=5669–5682|df=mdy-all|bibcode=2008ACP.....8.5669N|doi-access=free}}</ref><ref>{{cite journal|url=http://www.aft.org/pubs-reports/american_educator/issues/winter04-05/Snowflake.pdf |title=Snowflake Science |author=Kenneth Libbrecht |journal=American Educator |date=Winter 2004–2005 |access-date=July 14, 2009 |url-status=dead |archive-url=https://web.archive.org/web/20081128094655/http://www.aft.org/pubs-reports/american_educator/issues/winter04-05/Snowflake.pdf |archive-date=November 28, 2008 }}</ref>

Snow crystals form when tiny [[supercool]]ed cloud droplets (about 10&nbsp;[[micrometre|μm]] in diameter) [[freezing|freeze]]. These droplets are able to remain liquid at temperatures lower than {{convert|-18|°C|°F|0}}, because to freeze, a few molecules in the droplet need to get together by chance to form an arrangement similar to that in an ice lattice. The droplet freezes around this "nucleus". In warmer clouds, an aerosol particle or "ice nucleus" must be present in (or in contact with) the droplet to act as a nucleus. Ice nuclei are very rare compared to cloud condensation nuclei on which liquid droplets form. Clays, desert dust, and biological particles can be nuclei.<ref name=Christner2008>{{cite journal
|author = Brent Q Christner
|author2=Cindy E Morris |author3=Christine M Foreman |author4=Rongman Cai |author5=David C Sands
 |year = 2008
|title = Ubiquity of Biological Ice Nucleators in Snowfall
|journal = Science
|volume = 319
|issue = 5867
|page = 1214
|doi = 10.1126/science.1149757
|pmid = 18309078
|bibcode=2008Sci...319.1214C|citeseerx=10.1.1.395.4918 |s2cid=39398426 }}</ref> Artificial nuclei include particles of [[silver iodide]] and [[dry ice]], and these are used to stimulate precipitation in [[cloud seeding]].<ref>{{cite web|url=http://amsglossary.allenpress.com/glossary/search?p=1&query=cloud+seeding&submit=Search|title=Cloud seeding|author=Glossary of Meteorology|year=2009|access-date=June 28, 2009|publisher=[[American Meteorological Society]]|url-status=dead|archive-url=https://web.archive.org/web/20120315161127/http://amsglossary.allenpress.com/glossary/search?p=1&query=cloud+seeding&submit=Search|archive-date=March 15, 2012|df=mdy-all}}</ref>

Once a droplet has frozen, it grows in the supersaturated environment—one where air is saturated with respect to ice when the temperature is below the freezing point. The droplet then grows by diffusion of water [[molecule]]s in the air (vapor) onto the ice crystal surface where they are collected. Because water droplets are so much more numerous than the ice crystals, the crystals are able to grow to hundreds of micrometers or millimeters in size at the expense of the water droplets by the [[Wegener–Bergeron–Findeisen process]]. These large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their mass, and may collide and stick together in clusters, or aggregates. These aggregates are [[snowflake]]s, and are usually the type of ice particle that falls to the ground.<ref name="natgeojan07">{{cite journal|author=M. Klesius| title=The Mystery of Snowflakes| journal=National Geographic| volume=211| issue=1| year=2007| issn=0027-9358|page=20}}</ref> Although the ice is clear, scattering of light by the crystal facets and hollows/imperfections mean that the crystals often appear white in color due to [[diffuse reflection]] of the whole [[spectrum]] of [[light]] by the small ice particles.<ref>{{cite book|url=https://books.google.com/books?id=4T-aXFsMhAgC&pg=PA39|title=Hands-on Science: Light, Physical Science (matter) – Chapter 5: The Colors of Light|page=39|author=Jennifer E. Lawson|isbn=978-1-894110-63-1|year=2001|access-date=June 28, 2009|publisher=Portage & Main Press}}</ref>