Editing Snow (section)

===Measurement and classification===
{{See also|Classifications of snow}}
In the field, snow scientists often excavate a snow pit within which to make basic measurements and observations. Observations can describe features caused by wind, water percolation, or snow unloading from trees. Water percolation into a snowpack can create flow fingers and ponding or flow along capillary barriers, which can refreeze into horizontal and vertical solid ice formations within the snowpack. Among the measurements of the properties of snowpacks that the ''International Classification for Seasonal Snow on the Ground'' includes are: snow height, snow water equivalent, snow strength, and extent of snow cover. Each has a designation with a code and detailed description. The classification extends the prior classifications of Nakaya and his successors to related types of precipitation and are quoted in the following table:<ref name = Classificationonground/>
[[File:Taku glacier firn ice sampling.png|thumb|right|Snow pit on the surface of a glacier, profiling snow properties where the snow becomes increasingly dense with depth as it turns to ice]]
{| class="wikitable"
|+ Frozen precipitation particles, related to snow crystals
|-
! Subclass
! Shape
! Physical process
|-
| [[Graupel]]
| Heavily rimed particles, spherical, conical, hexagonal or irregular in shape
| Heavy riming of particles by accretion of supercooled water droplets
|-
| [[Hail]]
| Laminar internal structure, translucent or milky glazed surface
| Growth by accretion of supercooled water, size: >5&nbsp;mm
|-
| [[Ice pellets]]
| Transparent, mostly small spheroids
| Freezing of raindrops or refreezing of largely melted snow crystals or snowflakes (sleet). Graupel or snow pellets encased in thin ice layer (small hail). Size: both 5&nbsp;mm
|-
| [[Hard rime|Rime]]
| Irregular deposits or longer cones and needles pointing into the wind
| Accretion of small, supercooled fog droplets frozen in place. Thin breakable crust forms on snow surface if process continues long enough.
|}
''All are formed in cloud, except for rime, which forms on objects exposed to supercooled moisture.''

It also has a more extensive classification of deposited snow than those that pertain to airborne snow. The categories include both natural and man-made snow types, descriptions of snow crystals as they metamorphose and melt, the development of hoar frost in the snow pack and the formation of ice therein. Each such layer of a snowpack differs from the adjacent layers by one or more characteristics that describe its microstructure or density, which together define the snow type, and other physical properties. Thus, at any one time, the type and state of the snow forming a layer have to be defined because its physical and mechanical properties depend on them. Physical properties include microstructure, grain size and shape, snow density, liquid water content, and temperature.<ref name=Classificationonground>
{{Citation
 |last1        = Fierz
 |first1       = C.
 |last2       = Armstrong
 |first2      = R.L.
 |last3       = Durand
 |first3      = Y.
 |last4       = Etchevers
 |first4      = P.
 |last5       = Greene
 |first5      = E.
 |display-authors=5
 |author6=McClung, D.M.
 |author7=Nishimura, K.
 |author8=Satyawali, P.K.
 |author9=Sokratov, S.A.
 |title       = The International Classification for Seasonal Snow on the Ground
 |place       = Paris
 |publisher   = UNESCO
 |series      = IHP-VII Technical Documents in Hydrology
 |volume      = 83
 |year        = 2009
 |pages       = 80
 |url         = http://unesdoc.unesco.org/images/0018/001864/186462e.pdf
 |access-date = November 25, 2016
 |url-status=live
 |archive-url  = https://web.archive.org/web/20160929205922/http://unesdoc.unesco.org/images/0018/001864/186462e.pdf
 |archive-date = September 29, 2016
 |df          = mdy-all
}}</ref>

When it comes to measuring snow cover on the ground, typically three variables are measured: the snow cover extent (SCE) — the land area covered by snow, snow cover duration (SD) — how long a particular area is covered by snow, and the snow accumulation, often expressed as snow water equivalent (SWE), which expresses how much water the snow would be if it were all melted: this last one is a measurement of the volume of the snowpack.<ref name=":AR6">{{Cite journal |last1=Fox-Kemper |first1=B. |last2=Hewitt |first2=H.T.|author2-link=Helene Hewitt |last3=Xiao |first3=C. |last4=Aðalgeirsdóttir |first4=G. |last5=Drijfhout |first5=S.S. |last6=Edwards |first6=T.L. |last7=Golledge |first7=N.R. |last8=Hemer |first8=M. |last9=Kopp |first9=R.E. |last10=Krinner |first10=G. |last11=Mix |first11=A. |date=2021 |editor-last=Masson-Delmotte |editor-first=V. |editor2-last=Zhai |editor2-first=P. |editor3-last=Pirani |editor3-first=A. |editor4-last=Connors |editor4-first=S.L. |editor5-last=Péan |editor5-first=C. |editor6-last=Berger |editor6-first=S. |editor7-last=Caud |editor7-first=N. |editor8-last=Chen |editor8-first=Y. |editor9-last=Goldfarb |editor9-first=L. |title=Ocean, Cryosphere and Sea Level Change |journal=Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change |volume=2021 |publisher=Cambridge University Press, Cambridge, UK and New York, NY, US |pages=1283–1285 |doi=10.1017/9781009157896.011|bibcode=<!-- not 2021AGUFM.U13B..09F --> |isbn=9781009157896 |url=https://www.vliz.be/imisdocs/publications/84/371584.pdf }}</ref> To measure these variables a variety of techniques are used: surface observations, [[remote sensing]], [[Land surface models (climate)|land surface models]] and [[Atmospheric reanalysis|reanalysis products]]. These techniques are often combined to form the most complete datasets.<ref name=":AR6" />