Classifications of snow
Classifications of snow describe and categorize the attributes of snow-generating weather events, including the individual crystals both in the air and on the ground, and the deposited snow pack as it changes over time. Snow can be classified by describing the weather event that is producing it, the shape of its ice crystals or flakes, how it collects on the ground, and thereafter how it changes form and composition. Depending on the status of the snow in the air or on the ground, a different classification applies. Snowfall arises from a variety of events that vary  in intensity and cause, subject to classification by weather bureaus. Some snowstorms are part of a larger weather pattern. Other snowfall occurs from lake effects or atmospheric instability near mountains. Falling snow takes many different forms, depending on atmospheric conditions, especially vapor content and temperature, as it falls to the ground. Once on the ground, snow crystals metamorphose into different shapes, influenced by wind, freeze-thaw and sublimation. Snow on the ground forms a variety of shapes, formed by wind and thermal processes, all subject to "formal" classifications both by scientists and by ski resorts. Those who work and play  in snowy landscapes have "informal" classifications, as well. There is a long history of northern and alpine cultures describing snow in their different languages, including Inupiat, Russian and Finnish. However, the lore about the multiplicity of Eskimo words for snow originates from controversial scholarship on a topic that's difficult to define, because of the structures of the languages involved. Classification of snow events. Snow events reflect the type of storm that generates them and the type of precipitation that results. Classification systems use rates of deposition, types of precipitation, visibility, duration and wind speed to characterize such events. Snow -producing events. The following terms are consistent with the classifications of United States National Weather Service and the Meteorological Service of Canada: Precipitation. Precipitation may be characterized by type and intensity. Type. Frozen precipitation includes snow, snow pellets, snow grains, ice crystals, ice pellets, and hail. Falling snow comprises ice crystals, growing in a hexagonal pattern and combining as snowflakes. Ice crystals may be "any one of a number of macroscopic, crystalline forms in which ice appears, including hexagonal columns, hexagonal platelets, dendritic crystals, ice needles, and combinations of these forms". Terms that refer to falling snow particles include: Intensity . In the US, the intensity of snowfall is characterized by visibility through the falling precipitation, as follows: Snow crystal classification. Ice approximates hexagonal symmetry in most of its atmospheric manifestations of a crystal lattice as snow. Temperature and vapor pressure determine the growth of the hexagonal crystal lattice in different forms that include columnar growth in the axis perpendicular to the hexagonal plane to form snow crystals. Ukichiro Nakaya developed a crystal morphology diagram, relating crystal shape to the temperature and moisture conditions under which they formed. Magono and Lee devised a classification of freshly formed snow crystals that includes  80 distinct shapes. They are summarized in the following principal snow crystal categories (with symbol): Classifications of snow on the ground. Classification of snow on the ground comes from two sources: the science community and the community of those who encounter it in their daily lives. Snow on the ground exists both as a material with varying properties and as a variety of structures, shaped by wind, sun, temperature, and precipitation. Classification of snowpack material properties. The "International Classification for Seasonal Snow on the Ground" describes snow crystal classification, once it is deposited on the ground, that include grain shape  and grain size. The system also characterizes the snowpack, as the individual crystals metamorphize and coalesce. It uses the following characteristics (with units) to describe deposited snow: microstructure, grain shape, grain size (mm), snow density (kg/m), snow hardness, liquid water content, snow temperature (°C), impurities (mass fraction), and layer thickness (cm). The grain shape is further characterized, using the following categories (with code): precipitation particles (PP), machine-made snow (MM), decomposing and fragmented precipitation particles (DF), rounded grains (RG), faceted crystals (FC), depth hoar (DH), surface hoar (SH), melt forms (MF), and ice formations (IF). Other measurements and  characteristics are used as well, including a snow profile of a vertical section of the snowpack. Some snowpack features include: Classifications of snowpack surface and structure. In addition to having material properties, snowpacks have structure which can be characterized. These properties are primarily determined through the actions of wind, sun, and temperature. Such structures have been described by mountaineers and others encountering frozen landscapes, as follows: Ski resort classification. Ski resorts use standardized terminology to describe their snow conditions. In North America terms include: Informal classification. Skiers and others living with snow provide informal terms for snow conditions that they  encounter. Other cultures. Not surprisingly, in languages and cultures where snow is common, having different words for distinct weather conditions and types of snowfall is desirable for efficient communication. Finnish, Icelandic, Norwegian, Russian, and Swedish have multiple words and phrases relating to snow and snowfall, in some cases dozens or even hundreds, depending upon how one counts. Studies of the Sámi languages of Norway, Sweden and Finland, conclude that the languages have anywhere from 180 snow- and ice-related words and as many as 300 different words for types of snow, tracks in snow, and conditions of the use of  snow. The claim that Eskimo–Aleut languages (specifically, Yupik and Inuit) have an unusually large number of words for "snow", has been attributed to the work of anthropologist Franz Boas. Boas, who lived among Baffin islanders and learnt their language, reportedly included "only words representing meaningful distinctions" in his account. A 2010 study follows the sometimes questionable scholarship regarding the question whether these languages have many more root words for "snow" than the English language.