Snow

Snow
CargoNet Di 12 Euro 4000 Lønsdal - Bolna.jpg
Norwegian train plowing through drifted snow
Physical properties
Density (ρ)0.1 – 0.8 g/cm3
Mechanical properties
Tensile strengtht)1.5 – 3.5 kPa[1]
Compressive strength (σc)3 – 7 MPa[1]
Thermal properties
Melting temperature (Tm)0 °C
Thermal conductivity (k) For densities 0.1 to 0.5 g/cm30.05 – 0.7 W K−1 m−1
Electrical properties
Dielectric constant (εr) For dry snow density 0.1 to 0.9 g/cm31 – 3.2
The physical properties of snow vary considerably from event to event, sample to sample, and over time.

Snow refers to forms of ice crystals that precipitate from the atmosphere (usually from clouds) and undergo changes on the Earth's surface.[2] It pertains to frozen crystalline water throughout its life cycle, starting when, under suitable conditions, the ice crystals form in the atmosphere, increase to millimeter size, precipitate and accumulate on surfaces, then metamorphose in place, and ultimately melt, slide or sublimate away. Snowstorms organize and develop by feeding on sources of atmospheric moisture and cold air. Snowflakes nucleate around particles in the atmosphere by attracting supercooled water droplets, which freeze in hexagonal-shaped crystals. Snowflakes take on a variety of shapes, basic among these are platelets, needles, columns and rime. As snow accumulates into a snowpack, it may blow into drifts. Over time, accumulated snow metamorphoses, by sintering, sublimation and freeze-thaw. Where the climate is cold enough for year-to-year accumulation, a glacier may form. Otherwise, snow typically melts seasonally, causing runoff into streams and rivers and recharging groundwater.

Major snow-prone areas include the polar regions, the upper half of the Northern Hemisphere and mountainous regions worldwide with sufficient moisture and cold temperatures. In the Southern Hemisphere, snow is confined primarily to mountainous areas, apart from Antarctica.[3]

Snow affects such human activities as transportation: creating the need for keeping roadways, wings, and windows clear; agriculture: providing water to crops and safeguarding livestock; sports such as skiing, snowboarding, and snowmachine travel; and warfare. Snow affects ecosystems, as well, by providing an insulating layer during winter under which plants and animals are able to survive the cold.[1]

Precipitation

Worldwide occurrence of snowfall. Snow at reference above sea level (meters):
  Below 500: annually.
  Below 500: annually, but not in all of its territory.
  500: above annually, below occasionally.
  Above 500: annually.
  Above 2,000: annually.
  Any elevation: none.

Snow develops in clouds 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.[4]

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 incorporate warm and cold fronts as part of their 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

Extratropical cyclonic snowstorm, February 24, 2007—(Click for animation.)

Mid-latitude cyclones are low pressure areas which are capable of producing anything from cloudiness and mild snow storms to heavy blizzards.[5] During a hemisphere's 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.[6] For the southern mid-latitudes, the side of a cyclone that produces the most snow is the southern side.

Fronts

Frontal snowsquall moving toward Boston, Massachusetts

A cold front, the leading edge of a cooler mass of air, can produce 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 line passes over as the wind causes intense blowing snow.[7] 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 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 only separated by 25 miles (40 kilometers) with each passing the same point in 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.[7]

Lake and ocean effects

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.[8][9]

The same effect also occurs over bodies of salt water, when it is termed ocean-effect or bay-effect snow. The effect is enhanced when the moving air mass is uplifted by the orographic influence of higher elevations on the downwind shores. This uplifting can produce narrow but very intense bands of precipitation, which deposit at a rate of many inches of snow each hour, often resulting in a large amount of total snowfall.[10]

The areas affected by lake-effect snow are called snowbelts. 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.[11]

Mountain effects

Orographic or relief snowfall is caused when masses of air pushed by wind are forced up the side of elevated land formations, such as large mountains. The lifting of air up the side of a mountain or range results in adiabatic cooling, and ultimately condensation and precipitation. Moisture is removed by orographic lift, leaving drier, warmer air on the descending, leeward side.[12] The resulting enhanced productivity of snow fall[13] and the decrease in temperature with elevation[14] means that snow depth and seasonal persistence of snowpack increases with elevation in snow-prone areas.[1][15]

Cloud physics

Freshly fallen snowflakes

A snowflake consists of roughly 1019 water molecules, 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 vary among themselves, while following similar patterns.[16][17][18]

Snow crystals form when tiny supercooled cloud droplets (about 10 μm in diameter) freeze. These droplets are able to remain liquid at temperatures lower than −18 °C (0 °F), 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. Then 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 that cloud condensation nuclei on which liquid droplets form. Clays, desert dust and biological particles can be nuclei.[19] Artificial nuclei include particles of silver iodide and dry ice, and these are used to stimulate precipitation in cloud seeding.[20]

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 molecules 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 due to their sheer abundance, 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. The corresponding depletion of water vapor causes the ice crystals to grow at the droplets' expense. 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 snowflakes, and are usually the type of ice particle that falls to the ground.[21] 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.[22]

Classification of snowflakes

An early classification of snowflakes by Israel Perkins Warren[23]

Micrography of thousands of snowflakes from 1885 onward, starting with Wilson Alwyn Bentley, revealed the wide diversity of snowflakes within a classifiable set of patterns.[24] Closely matching snow crystals have been observed.[25]

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.[1]

Crystal structure morphology as a function of temperature and water saturation
Temperature range Saturation range Types of snow crystal
°C °F g/m3 oz/cu yd below saturation above saturation
0 to −3.5 32 to 26 0.0 to 0.5 0.000 to 0.013 Solid plates Thin plates

Dendrites

−3.5 to −10 26 to 14 0.5 to 1.2 0.013 to 0.032 Solid prisms

Hollow prisms

Hollow prisms

Needles

−10 to −22 14 to −8 1.2 to 1.4 0.032 to 0.038 Thin plates

Solid plates

Sectored plates

Dendrites

−22 to −40 −8 to −40 1.2 to 0.1 0.0324 to 0.0027 Thin plates

Solid plates

Columns

Prisms

As Nakaya discovered, shape is also a function of whether the prevalent moisture is above or below saturation. Forms below the saturation line trend more towards solid and compact. Crystals formed in supersaturated air trend more towards lacy, delicate and ornate. Many more complex growth patterns also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei.[26][27][28] If a crystal has started forming in a column growth regime, at around −5 °C (23 °F), and then falls into the warmer plate-like regime, then plate or dendritic crystals sprout at the end of the column, producing so called "capped columns".[21]

Magono and Lee devised a classification of freshly formed snow crystals that includes 80 distinct shapes. They documented each with micrographs.[29]

Other Languages
Afrikaans: Sneeu
Alemannisch: Schnee
አማርኛ: አመዳይ
Ænglisc: Snāw
العربية: ثلج
aragonés: Nieu
armãneashti: Neauâ
asturianu: Ñeve
Avañe'ẽ: Ro'yrypy'a
Aymar aru: Khunu
azərbaycanca: Qar
تۆرکجه: قار
বাংলা: তুষার
Bân-lâm-gú: Seh
башҡортса: Ҡар
беларуская: Снег
беларуская (тарашкевіца)‎: Сьнег
भोजपुरी: बर्फबारी
български: Сняг
Boarisch: Schnää
bosanski: Snijeg
brezhoneg: Erc'h
буряад: Саһан
català: Neu
Чӑвашла: Юр
čeština: Sníh
chiShona: Sinowo
Cymraeg: Eira
dansk: Sne
Deitsch: Schnee
Deutsch: Schnee
Diné bizaad: Yas
eesti: Lumi
Ελληνικά: Χιόνι
emiliàn e rumagnòl: Naiv
эрзянь: Лов
español: Nieve
Esperanto: Neĝo
estremeñu: Ñevi
euskara: Elur
فارسی: برف
føroyskt: Kavi
français: Neige
Frysk: Snie
Gaeilge: Sneachta
Gaelg: Sniaghtey
Gàidhlig: Sneachd
galego: Neve
贛語:
ગુજરાતી: હિમવર્ષા
客家語/Hak-kâ-ngî: Siet
한국어: 눈 (날씨)
հայերեն: Ձյուն
हिन्दी: हिम
hrvatski: Snijeg
Ido: Nivo
Bahasa Indonesia: Salju
interlingua: Nive
Interlingue: Nive
ᐃᓄᒃᑎᑐᑦ/inuktitut: ᐊᐳᑦ
íslenska: Snjór
italiano: Neve
עברית: שלג
Basa Jawa: Salju
ქართული: თოვლი
कॉशुर / کٲشُر: شیٖن
қазақша: Қар
Kiswahili: Theluji
Kreyòl ayisyen: Lanèj
kurdî: Berf
Кыргызча: Кар
Ladino: Inyeve
лезги: Жив
Latina: Nix
latviešu: Sniegs
Lëtzebuergesch: Schnéi
lietuvių: Sniegas
Limburgs: Snieë
lingála: Neje
lumbaart: Neu
magyar:
македонски: Снег
Malagasy: Oram-panala
മലയാളം: ഹിമം
मराठी: हिमवर्षा
მარგალური: თირი
Bahasa Melayu: Salji
Mìng-dĕ̤ng-ngṳ̄: Suók
мокшень: Лов
монгол: Цас
မြန်မာဘာသာ: နှင်း
Nāhuatl: Cepayahuitl
Nederlands: Sneeuw
Nedersaksies: Snee
नेपाली: हिउँ
नेपाल भाषा: च्वापु गायु
日本語:
Napulitano: Neva
нохчийн: Ло
norsk: Snø
norsk nynorsk: Snø
occitan: Nèu
олык марий: Лум
oʻzbekcha/ўзбекча: Qor
ਪੰਜਾਬੀ: ਬਰਫ਼ (ਵਰਖਾ)
پنجابی: برف پیناں
پښتو: واوره
Перем Коми: Лым
Piemontèis: Fiòca
polski: Śnieg
português: Neve
română: Zăpadă
Runa Simi: Rit'i
русиньскый: Снїг
русский: Снег
саха тыла: Хаар
sardu: Nie
Scots: Snaw
Sesotho sa Leboa: Lehlwa
shqip: Dëbora
sicilianu: Nivi
සිංහල: හිම
Simple English: Snow
سنڌي: برف باري
slovenčina: Sneh
slovenščina: Sneg
ślůnski: Śńyg
српски / srpski: Снијег
srpskohrvatski / српскохрватски: Snijeg
Basa Sunda: Salju
suomi: Lumi
svenska: Snö
Tagalog: Niyebe
татарча/tatarça: Кар
తెలుగు: మంచుగళ్లు
ไทย: หิมะ
тоҷикӣ: Барф
ᏣᎳᎩ: ᎤᎾᏥ
Tsetsêhestâhese: Hésta'se
Türkçe: Kar
українська: Сніг
اردو: برف باری
Vahcuengh: Nae
vèneto: Neve
vepsän kel’: Lumi
Tiếng Việt: Tuyết
Volapük: Nif
Võro: Lumi
walon: Nive
文言:
Winaray: Nyebe
吴语:
ייִדיש: שניי
粵語:
Zazaki: Vewre
žemaitėška: Snėigs
中文:
डोटेली: हिउँ
ГӀалгӀай: Лоа
Kabɩyɛ: Ɖomba mʋlʋm
Lingua Franca Nova: Neva