Editing Snow (section)

==Precipitation==
[[File:Countries receiving snowfall.svg|right|thumb|Occurrence of snowfall:{{Legend|#54E4E4|All elevations}}
{{Legend |#57AFFD|All elevations, not in all areas}}
{{Legend|#0000FF|Higher elevations (mainly above 500 meters), below rarely}}
{{Legend|#EC74E4|Higher elevations (above 500 meters) only}}
{{Legend|#D300CA|Very high elevations (such as above 2,000 meters) only}}
{{Legend|#C0C0C0|None at any elevation}}]]
Snow develops in [[cloud]]s that themselves are part of a larger weather system. The physics of snow crystal development in clouds results from a complex set of variables that include [[moisture]] content and temperatures. The resulting shapes of the falling and fallen crystals can be classified into a number of basic shapes and combinations thereof. Occasionally, some plate-like, dendritic and stellar-shaped snowflakes can form under clear sky with a very cold temperature inversion present.<ref name = Classificationonground/>

===Cloud formation===
Snow clouds usually occur in the context of larger weather systems, the most important of which is the low-pressure area, which typically incorporates warm and cold fronts as part of its circulation. Two additional and locally productive sources of snow are lake-effect (also sea-effect) storms and elevation effects, especially in mountains.

====Low-pressure areas====
{{Main|Extratropical cyclone}}
[[File:Feb242007 blizzard.gif|thumb|right|Extratropical cyclonic snowstorm, February 24, 2007—(Click for animation.)]]

[[Extratropical cyclone|Mid-latitude cyclones]] are [[low-pressure area]]s which are capable of producing anything from cloudiness and mild [[Winter storm#Snow|snow storms]] to heavy [[blizzard]]s.<ref name="ExtraLessonMillUni">{{cite web| title = ESCI 241&nbsp;– Meteorology; Lesson 16&nbsp;– Extratropical Cyclones | author = DeCaria| publisher = Department of Earth Sciences, [[Millersville University]]| date = December 7, 2005| url = http://www.atmos.millersville.edu/~adecaria/ESCI241/esci241_lesson16_cyclones.html | access-date = June 21, 2009 |archive-url = https://web.archive.org/web/20080208224320/http://www.atmos.millersville.edu/~adecaria/ESCI241/esci241_lesson16_cyclones.html |archive-date = February 8, 2008}}</ref>  During a hemisphere's [[autumn|fall]], winter, and spring, the atmosphere over continents can be cold enough through the depth of the [[troposphere]] to cause snowfall. In the Northern Hemisphere, the northern side of the low-pressure area produces the most snow.<ref>
{{cite journal
 |last        = Tolme
 |first       = Paul
 |title       = Weather 101: How to track and bag the big storms
 |journal     = Ski Magazine
 |volume      = 69
 |issue       = 4
 |page       = 126
 |date        = December 2004
 |url         = https://books.google.com/books?id=t1DaXO7wF20C&pg=PA126
 |issn        = 0037-6159
 }}</ref> For the southern [[mid-latitudes]], the side of a [[cyclone]] that produces the most snow is the southern side.

====Fronts====
{{Main|Weather front}}
[[File:Snowsquall line-Bourrasque neige frontal NOAA.png|thumb|right|Frontal snowsquall moving toward [[Boston]], [[Massachusetts]]]]
A [[cold front]], the leading edge of a cooler mass of air, can produce [[Snowsquall#Frontal snowsquall|frontal snowsqualls]]—an intense frontal [[convective]] line (similar to a [[rainband]]), when [[temperature]] is near freezing at the surface. The strong convection that develops has enough moisture to produce whiteout conditions at places which the line passes over as the wind causes intense blowing snow.<ref name=EC-2>{{Cite web
 |url         = http://www.ec.gc.ca/meteo-weather/default.asp?lang=En&n=46FBA88B-1#Snow
 |title       = Snow
 |work        = Winter Hazards
 |author      = Meteorological Service of Canada
 |author-link = Meteorological Service of Canada
 |publisher   = [[Environment Canada]]
 |date        = September 8, 2010
 |access-date  = October 4, 2010
 |url-status=live
 |archive-url  = https://web.archive.org/web/20110611163137/http://www.ec.gc.ca/meteo-weather/default.asp?lang=En&n=46FBA88B-1#Snow
 |archive-date = June 11, 2011
 |df          = mdy-all
}}</ref> This type of snowsquall generally lasts less than 30 minutes at any point along its path, but the motion of the line can cover large distances. Frontal squalls may form a short distance ahead of the surface cold front or behind the cold front where there may be a deepening low-pressure system or a series of [[Trough (meteorology)|trough]] lines which act similar to a traditional cold frontal passage. In situations where squalls develop post-frontally, it is not unusual to have two or three linear squall bands pass in rapid succession separated only by {{convert|25|mi|km|abbr=off|sp=us}}, with each passing the same point roughly 30 minutes apart. In cases where there is a large amount of vertical growth and mixing, the squall may develop embedded cumulonimbus clouds resulting in lightning and thunder which is dubbed [[thundersnow]].

A [[warm front]] can produce snow for a period as warm, moist air overrides below-freezing air and creates precipitation at the boundary. Often, snow transitions to rain in the warm sector behind the front.<ref name=EC-2/>

====Lake and ocean effects====
{{Main|Lake-effect snow}}
[[File:Lake Effect Snow on Earth.jpg|thumb|Cold northwesterly wind over [[Lake Superior]] and [[Lake Michigan]] creating lake-effect snowfall]]

Lake-effect snow is produced during cooler atmospheric conditions when a cold air mass moves across long expanses of warmer [[lake]] water, warming the lower layer of air which picks up [[water vapor]] from the lake, rises up through the colder air above, freezes, and is deposited on the [[leeward]] (downwind) shores.<ref>{{cite web|url=http://www.noaa.gov/features/02_monitoring/lakesnow.html|title=NOAA - National Oceanic and Atmospheric Administration - Monitoring & Understanding Our Changing Planet|url-status=live|archive-url=https://web.archive.org/web/20150102173430/http://www.noaa.gov/features/02_monitoring/lakesnow.html|archive-date=January 2, 2015|df=mdy-all}}</ref><ref>{{cite web|url=http://www.comet.ucar.edu/class/smfaculty/byrd/sld012.htm |title=Fetch |url-status=dead |archive-url=https://web.archive.org/web/20080515101954/http://www.comet.ucar.edu/class/smfaculty/byrd/sld012.htm |archive-date=May 15, 2008 }}</ref>

The same effect occurring over bodies of salt water is termed ''ocean-effect'' or ''bay-effect snow''. The effect is enhanced when the moving air mass is uplifted by the [[orographic lift|orographic]] influence of higher elevations on the downwind shores. This uplifting can produce narrow but very intense bands of precipitation which may deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall.<ref name=mass>{{cite book |last= Mass |first= Cliff |title= The Weather of the Pacific Northwest |year= 2008 |publisher= [[University of Washington Press]] |isbn= 978-0-295-98847-4 |page= 60}}</ref>

The areas affected by lake-effect snow are called [[snowbelt]]s. These include areas east of the [[Great Lakes]], the west coasts of northern Japan, the [[Kamchatka Peninsula]] in Russia, and areas near the [[Great Salt Lake]], [[Black Sea]], [[Caspian Sea]], [[Baltic Sea]], and parts of the northern Atlantic Ocean.<ref name="SCHMID">Thomas W. Schmidlin. [https://kb.osu.edu/dspace/bitstream/1811/23329/1/V089N4_101.pdf Climatic Summary of Snowfall and Snow Depth in the Ohio Snowbelt at Chardon.] {{webarchive|url=https://web.archive.org/web/20080408225438/https://kb.osu.edu/dspace/bitstream/1811/23329/1/V089N4_101.pdf |date=April 8, 2008 }} Retrieved on March 1, 2008.</ref>

====Mountain effects====
{{Main|Precipitation types#Orographic}}
[[Orography|Orographic]] or [[relief]] snowfall is created when moist air is forced up the [[windward]] side of [[mountain]] ranges by a large-scale [[wind]] flow. The lifting of moist air up the side of a mountain range results in [[Adiabatic lapse rate|adiabatic]] cooling, and ultimately [[condensation]] and precipitation. Moisture is gradually removed from the air by this process, leaving [[Foehn wind|drier and warmer air]] on the descending, or [[leeward]], side.<ref name="MT">Physical Geography. [http://www.physicalgeography.net/fundamentals/8e.html CHAPTER 8: Introduction to the Hydrosphere (e). Cloud Formation Processes.] {{webarchive|url=https://web.archive.org/web/20081220230524/http://www.physicalgeography.net/fundamentals/8e.html |date=December 20, 2008 }} Retrieved on January 1, 2009.</ref> The resulting enhanced snowfall,<ref>{{Citation
 |first1        = Mark T.
 |last1         = Stoelinga
 |first2       = Ronald E.
 |last2        = Stewart
 |first3       = Gregory
 |last3        = Thompson
 |first4       = Julie M.
 |last4        = Theriault
 |editor-last  = Chow
 |editor-first = Fotini K.
 |display-editors=1
 |editor2=Stephan F.J. De Wekker
 |editor3=Bradley J. Snyder
 |title        = Mountain Weather Research and Forecasting: Recent Progress and Current Challenges
 |contribution = Micrographic processes within winter orographic cloud and precipitation systems
 |series       = Springer Atmospheric Sciences
 |year         = 2012
 |page         = 3
 |publisher    = Springer Science & Business Media
 |bibcode = 2013mwrf.book.....C
 |url          = https://books.google.com/books?id=ihjFd5Q8oPMC&pg=PA3
 |isbn         = 978-94-007-4098-3
 }}</ref> along with the [[Lapse rate|decrease in temperature]] with elevation,<ref>{{cite book|author=Mark Zachary Jacobson|title=Fundamentals of Atmospheric Modeling|publisher=Cambridge University Press|edition=2nd|year=2005|isbn=978-0-521-83970-9}}</ref> combine to increase snow depth and seasonal persistence of snowpack in snow-prone areas.<ref name = Snowenclyclopedia/><ref name=Singh>
{{cite book
 |last        = P.
 |first       = Singh
 |title       = Snow and Glacier Hydrology
 |publisher   = Springer Science & Business Media
 |series      = Water Science and Technology Library
 |volume      = 37
 |date        = 2001
 |page        = 75
 |url         = https://books.google.com/books?id=0VW6Tv0LVWkC&pg=PA75
 |isbn        = 978-0-7923-6767-3
 }}</ref>

[[Mountain waves]] have also been found to help enhance precipitation amounts downwind of mountain ranges by enhancing the lift needed for condensation and precipitation.<ref>{{cite journal|last1=Gaffin|first1=David M.|last2=Parker|first2=Stephen S.|last3=Kirkwood|first3=Paul D.|date=2003|title=An Unexpectedly Heavy and Complex Snowfall Event across the Southern Appalachian Region|journal=Weather and Forecasting|volume=18|issue=2|pages=224–235|bibcode=2003WtFor..18..224G|doi=10.1175/1520-0434(2003)018<0224:AUHACS>2.0.CO;2|doi-access=free}}</ref>

===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>

===Classification of snowflakes===
{{Main||Snowflake#Classification}}
[[File:Snowflakeschapte00warriala-p11-p21-p29-p39.jpg|thumb|right|An early classification of snowflakes by [[Israel Perkins Warren]]<ref>
{{cite book
 |last        = Warren
 |first       = Israel Perkins
 |author-link = Israel Perkins Warren
 |title       = Snowflakes: a chapter from the book of nature
 |publisher   = American Tract Society
 |date        = 1863
 |location    = Boston
 |pages       = 164
 |url         = https://archive.org/details/snowflakeschapte00warriala
 |access-date = November 25, 2016
 |url-status=live
 |archive-url  = https://web.archive.org/web/20160909180658/https://archive.org/details/snowflakeschapte00warriala
 |archive-date = September 9, 2016
 |df          = mdy-all
}}</ref>]]

[[Micrography]] of thousands of snowflakes from 1885 onward, starting with [[Wilson Bentley|Wilson Alwyn Bentley]], revealed the wide diversity of snowflakes within a classifiable set of patterns.<ref>{{cite news|url=http://www.digitaljournal.com/article/263168|title=No two snowflakes are alike|date=December 7, 2008|author=Chris V. Thangham|access-date=July 14, 2009|work=Digital Journal|url-status=live|archive-url=https://web.archive.org/web/20091228011231/http://www.digitaljournal.com/article/263168|archive-date=December 28, 2009|df=mdy-all}}</ref> Closely matching snow crystals have been observed.<ref name="identical_crystals">{{cite news
 |author      = Randolph E. Schmid
 |title       = Identical snowflakes cause flurry
 |agency      = Associated Press
 |newspaper   = The Boston Globe
 |date        = June 15, 1988
 |access-date  = November 27, 2008
 |quote       = But there the two crystals were, side by side, on a glass slide exposed in a cloud on a research flight over Wausau, Wis.
 |url         = http://www.highbeam.com/doc/1P2-8066647.html
 |url-status=live
 |archive-url  = https://web.archive.org/web/20110624033612/http://www.highbeam.com/doc/1P2-8066647.html
 |archive-date = June 24, 2011
 |df          = mdy-all
}}</ref>

[[Ukichiro Nakaya]] developed a crystal morphology diagram, relating crystal shapes to the temperature and moisture conditions under which they formed, which is summarized in the following table.<ref name = Snowenclyclopedia/>
{| class="wikitable"
|+ Crystal structure morphology as a function of temperature and water saturation
|-
! colspan=2| Temperature range
! colspan=2| Saturation range
! colspan=2| Types of snow crystal
|-
!°C
!°F
!g/m<sup>3</sup>
!oz/cu yd
! ''below'' saturation
! ''above'' saturation
|-
| {{convert|0|to|-3.5|C|F|0|disp=table}}
| {{convert|0.0|to|0.5|g/m3|oz/cuyd|disp=table}}
| style="text-align:center;" | Solid plates
| style="text-align:center;" | Thin plates
Dendrites
|-
| {{convert|-3.5|to|-10|C|F|0|disp=table}}
| {{convert|0.5|to|1.2|g/m3|oz/cuyd|disp=table}}
| style="text-align:center;" | Solid prisms
Hollow prisms
| style="text-align:center;" | Hollow prisms
Needles
|-
| {{convert|-10|to|-22|C|F|0|disp=table}}
| {{convert|1.2|to|1.4|g/m3|oz/cuyd|disp=table}}
| style="text-align:center;" | Thin plates
Solid plates
| style="text-align:center;" | Sectored plates
Dendrites
|-
| {{convert|-22|to|-40|C|F|0|disp=table}}
| {{convert|1.2|to|0.1|g/m3|oz/cuyd|disp=table}}
| style="text-align:center;" | Thin plates
Solid plates
| style="text-align:center;" | Columns
Prisms
|}
Nakaya discovered that the shape is also a function of whether the prevalent moisture is above or below saturation. Forms below the saturation line tend more toward solid and compact while crystals formed in supersaturated air tend more toward lacy, delicate, and ornate. Many more complex growth patterns also form, which include side-planes, bullet-rosettes, and planar types, depending on the conditions and ice nuclei.<ref name=BaileyHallett>{{cite journal
|author = Matthew Bailey
|author2=John Hallett
 |year = 2004
|title = Growth rates and habits of ice crystals between −20 and −70C
|journal = Journal of the Atmospheric Sciences
|volume = 61
|issue = 5
|pages = 514–544
|doi = 10.1175/1520-0469(2004)061<0514:GRAHOI>2.0.CO;2
|bibcode = 2004JAtS...61..514B|doi-access = free
}}</ref><ref>{{cite web|author=Kenneth G. Libbrecht|url=http://www.its.caltech.edu/~atomic/snowcrystals/primer/primer.htm|title=A Snowflake Primer|date=October 23, 2006|publisher=[[California Institute of Technology]]|access-date=June 28, 2009|url-status=live|archive-url=https://web.archive.org/web/20090710022028/http://www.its.caltech.edu/~atomic/snowcrystals/primer/primer.htm|archive-date=July 10, 2009|df=mdy-all}}</ref><ref>{{cite journal|author=Kenneth G. Libbrecht|title=The Formation of Snow Crystals|journal=American Scientist|volume=95|issue=1|pages=52–59|date=January–February 2007|doi=10.1511/2007.63.52}}</ref> If a crystal has started forming in a column growth regime at around {{convert|-5|C|F|0}} and then falls into the warmer plate-like regime, plate or dendritic crystals sprout at the end of the column, producing so called "capped columns".<ref name="natgeojan07" />

Magono and Lee devised a classification of freshly formed snow crystals that includes 80 distinct shapes. They documented each with micrographs.<ref name = magono-lee>
{{Citation
 | last1 = Magono
 | first1 = Choji
 | last2 = Lee
 | first2 = Chung Woo
 | title = Meteorological Classification of Natural Snow Crystals
 | place = Hokkaido
 | journal = Journal of the Faculty of Science
 | volume = 3
 | issue = 4
 | date = 1966
 | edition = Geophysics
 | series = 7
 | pages = 321–335
 | language = en
 | hdl = 2115/8672
 }}</ref>