Snowflake

Last updated
Macro photography of a natural snowflake Snowflake (lumehelves).jpg
Macro photography of a natural snowflake

A snowflake is a single ice crystal that has achieved a sufficient size, and may have amalgamated with others, which falls through the Earth's atmosphere as snow. [1] [2] [3] Each flake nucleates around a tiny particle in supersaturated air masses by attracting supercooled cloud water droplets, which freeze and accrete in crystal form. Complex shapes emerge as the flake moves through differing temperature and humidity zones in the atmosphere, such that individual snowflakes differ in detail from one another, but may be categorized in eight broad classifications and at least 80 individual variants. The main constituent shapes for ice crystals, from which combinations may occur, are needle, column, plate, and rime. Snow appears white in color despite being made of clear ice. This is due to diffuse reflection of the whole spectrum of light by the small crystal facets of the snowflakes. [4]

Contents

Formation

Freshly fallen snowflakes ComputerHotline - Snow crystals (by).jpg
Freshly fallen snowflakes

Snowflakes nucleate around mineral or organic particles in moisture-saturated, subfreezing air masses. They grow by net accretion to the incipient crystals in hexagonal formations. The cohesive forces are primarily electrostatic.

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. The particles that make ice nuclei are very rare compared to nuclei upon which liquid cloud droplets form; however, it is not understood what makes them efficient. Clays, desert dust, and biological particles may be effective, [5] although to what extent is unclear. Artificial nuclei include particles of silver iodide and dry ice, and these are used to stimulate precipitation in cloud seeding. [6] Experiments show that "homogeneous" nucleation of cloud droplets only occurs at temperatures lower than −35 °C (−31 °F). [7]

Growth

Scanning electron microscope image of rime frost on both ends of a "capped column" snowflake. Snowflake 300um LTSEM, 13368.jpg
Scanning electron microscope image of rime frost on both ends of a "capped column" snowflake.

Once a water droplet has frozen as an ice nucleus, it grows in a supersaturated environment—wherein liquid moisture coexists with ice beyond its equilibrium point at temperatures below freezing. The droplet then grows by deposition 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. This process is known as the Wegener–Bergeron–Findeisen process. The corresponding depletion of water vapor causes the droplets to evaporate, meaning that the ice crystals 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 usually the type of ice particle that falls to the ground. [8] Guinness World Records lists the world's largest aggregated snowflakes as those of January 1887 at Fort Keogh, Montana, which were claimed to be 15 inches (38 cm) wide—well outside the normally documented range of aggregated flakes of three or four inches in width. Single crystals the size of a dime (17.91 mm in diameter) have been observed. [3] Snowflakes encapsulated in rime form balls known as graupel.

Appearance

Color

Snow crystals in strong direct sunlight act like small prisms Snow crystals glittering in strong direct sunlight 45 - tight crop - high contrast.jpg
Snow crystals in strong direct sunlight act like small prisms

Although ice by itself is clear, snow usually appears white in color due to diffuse reflection of the whole spectrum of light by the scattering of light by the small crystal facets of the snowflakes of which it is comprised. [4]

Shape

The shape of the snowflake is determined broadly by the temperature and humidity at which it is formed. [8] Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes. [9] Most snow particles are irregular in form, despite their common depiction as symmetrical. It is unlikely that any two snowflakes are alike due to the estimated 1019 (10 quintillion) water molecules which make up a typical snowflake, [10] which grow 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. [11] Snowflakes that look identical, but may vary at the molecular level, have been grown under controlled conditions. [12]

Although snowflakes are never perfectly symmetrical, the growth of a non-aggregated snowflake often approximates six-fold radial symmetry, arising from the hexagonal crystalline structure of ice. [13] At that stage, the snowflake has the shape of a minute hexagon. The six "arms" of the snowflake, or dendrites, then grow independently from each of the corners of the hexagon, while either side of each arm grows independently. The microenvironment in which the snowflake grows changes dynamically as the snowflake falls through the cloud and tiny changes in temperature and humidity affect the way in which water molecules attach to the snowflake. Since the micro-environment (and its changes) are very nearly identical around the snowflake, each arm tends to grow in nearly the same way. However, being in the same micro-environment does not guarantee that each arm grows the same; indeed, for some crystal forms it does not because the underlying crystal growth mechanism also affects how fast each surface region of a crystal grows. [14] Empirical studies suggest less than 0.1% of snowflakes exhibit the ideal six-fold symmetric shape. [15] Very occasionally twelve branched snowflakes are observed; they maintain the six-fold symmetry. [16]

Classification

An early classification of snowflakes by Israel Perkins Warren. Snowflakeschapte00warriala-p11-p21-p29-p39.jpg
An early classification of snowflakes by Israel Perkins Warren.

Snowflakes form in a wide variety of intricate shapes, leading to the notion that "no two are alike". Although nearly-identical snowflakes have been made in laboratory, they are very unlikely to be found in nature. [18] [10] [19] [20] Initial attempts to find identical snowflakes by photographing thousands of them with a microscope from 1885 onward by Wilson Alwyn Bentley found the wide variety of snowflakes we know about today.

Ukichiro Nakaya developed a crystal morphology diagram, relating crystal shape to the temperature and moisture conditions under which they formed, which is summarized in the following table: [21]

Crystal structure morphology as a function of temperature and water saturation
Temperature rangeSaturation range (g/m3)Types of snow crystal

below saturation

Types of snow crystal

above saturation

0 °C (32 °F) to −3.5 °C (26 °F)0.0 to 0.5Solid platesThin plates

Dendrites

−3.5 °C (26 °F) to −10 °C (14 °F)0.5 to 1.2Solid prisms

Hollow prisms

Hollow prisms

Needles

−10 °C (14 °F) to −22 °C (−8 °F)1.2 to 1.2Thin plates

Solid plates

Sectored plates

Dendrites

−22 °C (−8 °F) to −40 °C (−40 °F)0.0 to 0.4Thin plates

Solid plates

Columns

Prisms

Wilson Bentley micrograph showing two classes of snowflake, plate and column. Missing is an example of a needle. Bentley snowflake micrograph no. 777.jpg
Wilson Bentley micrograph showing two classes of snowflake, plate and column. Missing is an example of a needle.

The shape of a snowflake is determined primarily by the temperature and humidity at which it is formed. [8] Freezing air down to −3 °C (27 °F) promotes planar crystals (thin and flat). In colder air down to −8 °C (18 °F), the crystals form as hollow columns, prisms or needles. In air as cold as −22 °C (−8 °F), shapes become plate-like again, often with branched or dendritic features. At temperatures below −22 °C (−8 °F), the crystals become plate-like or columnar, depending on the degree of saturation. 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. [22] [23] [24] 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". [8]

Magono and Lee devised a classification of freshly formed snow crystals that includes 80 distinct shapes. They are listed in the following main categories (with symbol): [25]

They documented each with micrographs. [26]

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. [27]

Use as a symbol

Snowflake in the coat of arms of Lumijoki Lumijoki.vaakuna.svg
Snowflake in the coat of arms of Lumijoki

The snowflake is often a traditional seasonal image or motif used around the Christmas season, especially in Europe and North America. As a Christian celebration, Christmas celebrates the incarnation of Jesus, who according to Christian belief atones for the sins of humanity; so, in European and North American Christmas traditions, snowflakes symbolize purity. [28] [29] Snowflakes are also traditionally associated with the "White Christmas" weather that often occurs during Christmastide. [29] During this period, it is quite popular to make paper snowflakes by folding a piece of paper several times, cutting out a pattern with scissors and then unfolding it. [30] [31] The Book of Isaiah refers to the atonement of sins causing them to appear "white as snow" before God (cf. Isaiah 1:18); [29]

Snowflakes are also often used as symbols representing winter or cold conditions. For example, snow tires which enhance traction during harsh winter driving conditions are labelled with a snowflake on the mountain symbol. [32] A stylized snowflake has been part of the emblem of the 1968 Winter Olympics, 1972 Winter Olympics, 1984 Winter Olympics, 1988 Winter Olympics, 1998 Winter Olympics and 2002 Winter Olympics. [33] [34]

The three grades in the Order of Canada (Companion, Officer and Member, respectively) . 3 Order of Canada grades.JPG
The three grades in the Order of Canada (Companion, Officer and Member, respectively) .

A six pointed stylized hexagonal snowflake used for the Order of Canada (a national honor system) has come to symbolize Canadians northern heritage and diversity. [35]

In heraldry, the snowflake is a stylized charge. Three different snowflake symbols are encoded in Unicode: "snowflake" at U+2744 (❄); "tight trifoliate snowflake" at U+2745 (❅); and "heavy chevron snowflake" at U+2746 (❆).

In the Tang Dynasty, snowflakes in poetry sometimes served as a symbol of the cosmic energy of the Tao and the Milky Way galaxy. [36]

A selection of photographs taken by Wilson Bentley (1865–1931):

Comprehensive photographic studies of fresh snowflakes show the simple symmetry represented in Bentley's photographs to be rare. [37]

See also

Related Research Articles

<span class="mw-page-title-main">Crystal</span> Solid material with highly ordered microscopic structure

A crystal or crystalline solid is a solid material whose constituents are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macroscopic single crystals are usually identifiable by their geometrical shape, consisting of flat faces with specific, characteristic orientations. The scientific study of crystals and crystal formation is known as crystallography. The process of crystal formation via mechanisms of crystal growth is called crystallization or solidification.

<span class="mw-page-title-main">Frost</span> Coating or deposit of ice

Frost is a thin layer of ice on a solid surface, which forms from water vapor that deposits onto a freezing surface. Frost forms when the air contains more water vapor than it can normally hold at a specific temperature. The process is similar to the formation of dew, except it occurs below the freezing point of water typically without crossing through a liquid state.

<span class="mw-page-title-main">Ice</span> Frozen water: the solid state of water

Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 °C, 32 °F, or 273.15 K. As a naturally occurring crystalline inorganic solid with an ordered structure, ice is considered to be a mineral. Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaque bluish-white color.

<span class="mw-page-title-main">Snow</span> Precipitation in the form of ice crystal flakes

Snow comprises individual ice crystals that grow while suspended in the atmosphere—usually within clouds—and then fall, accumulating on the ground where they undergo further changes. It consists of 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.

<span class="mw-page-title-main">Cirrus cloud</span> Genus of atmospheric cloud

Cirrus is a genus of high cloud made of ice crystals. Cirrus clouds typically appear delicate and wispy with white strands. Cirrus are usually formed when warm, dry air rises, causing water vapor deposition onto rocky or metallic dust particles at high altitudes. Globally, they form anywhere between 4,000 and 20,000 meters above sea level, with the higher elevations usually in the tropics and the lower elevations in more polar regions.

Diamond dust is a ground-level cloud composed of tiny ice crystals. This meteorological phenomenon is also referred to simply as ice crystals and is reported in the METAR code as IC. Diamond dust generally forms under otherwise clear or nearly clear skies, so it is sometimes referred to as clear-sky precipitation. Diamond dust is most commonly observed in Antarctica and the Arctic, but can occur anywhere with a temperature well below freezing. In the polar regions of Earth, diamond dust may persist for several days without interruption.

<span class="mw-page-title-main">Altostratus cloud</span> A type of middle-altitude cloud

Altostratus is a middle-altitude cloud genus made up of water droplets, ice crystals, or a mixture of the two. Altostratus clouds are formed when large masses of warm, moist air rise, causing water vapor to condense. Altostratus clouds are usually gray or blueish featureless sheets, although some variants have wavy or banded bases. The sun can be seen through thinner altostratus clouds, but thicker layers can be quite opaque.

<span class="mw-page-title-main">Ice crystal</span> Water ice in symmetrical shapes

Ice crystals are solid ice in symmetrical shapes including hexagonal columns, hexagonal plates, and dendritic crystals. Ice crystals are responsible for various atmospheric optic displays and cloud formations.

<span class="mw-page-title-main">Precipitation</span> Product of the condensation of atmospheric water vapor that falls under gravity

In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, sleet, snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor, so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation but colloids, because the water vapor does not condense sufficiently to precipitate. Two processes, possibly acting together, can lead to air becoming saturated: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called showers.

<span class="mw-page-title-main">Cloud physics</span> Study of the physical processes in atmospheric clouds

Cloud physics is the study of the physical processes that lead to the formation, growth and precipitation of atmospheric clouds. These aerosols are found in the troposphere, stratosphere, and mesosphere, which collectively make up the greatest part of the homosphere. Clouds consist of microscopic droplets of liquid water, tiny crystals of ice, or both, along with microscopic particles of dust, smoke, or other matter, known as condensation nuclei. Cloud droplets initially form by the condensation of water vapor onto condensation nuclei when the supersaturation of air exceeds a critical value according to Köhler theory. Cloud condensation nuclei are necessary for cloud droplets formation because of the Kelvin effect, which describes the change in saturation vapor pressure due to a curved surface. At small radii, the amount of supersaturation needed for condensation to occur is so large, that it does not happen naturally. Raoult's law describes how the vapor pressure is dependent on the amount of solute in a solution. At high concentrations, when the cloud droplets are small, the supersaturation required is smaller than without the presence of a nucleus.

<span class="mw-page-title-main">Rime ice</span> Granular whitish deposit of ice formed by freezing fog

Rime ice forms when supercooled water droplets freeze onto surfaces. In the atmosphere, there are three basic types of rime ice:

<span class="mw-page-title-main">Wilson Bentley</span> American photographer known for photographing snowflakes

Wilson Alwyn Bentley, also known as Snowflake Bentley, was an American meteorologist and photographer, who was the first known person to take detailed photographs of snowflakes and record their features. He perfected a process of catching flakes on black velvet in such a way that their images could be captured before they either melted or sublimated, and elaborated the theory that no two snowflakes are alike.

<span class="mw-page-title-main">Ice spike</span> Upward projection of ice from surface of frozen water body

An ice spike is an ice formation, often in the shape of an inverted icicle, that projects upwards from the surface of a body of frozen water. Ice spikes created by natural processes on the surface of small bodies of frozen water have been reported for many decades, although their occurrence is quite rare. A mechanism for their formation, now known as the Bally–Dorsey model, was proposed in the early 20th century but this was not tested in the laboratory for many years. In recent years a number of photographs of natural ice spikes have appeared on the Internet as well as methods of producing them artificially by freezing distilled water in domestic refrigerators or freezers. This has allowed a small number of scientists to test the hypothesis in a laboratory setting and, although the experiments appear to confirm the validity of the Bally–Dorsey model, they have raised further questions about how natural ice spikes form, and more work remains to be done before the phenomenon is fully understood. Natural ice spikes can grow into shapes other than a classic spike shape, and have been variously reported as ice candles, ice towers or ice vases as there is no standard nomenclature for these other forms. One particularly unusual form takes the shape of an inverted pyramid.

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<span class="mw-page-title-main">Graupel</span> Precipitation that forms when supercooled droplets of water freeze on a falling snowflake

Graupel, also called soft hail, corn snow, hominy snow, or snow pellets, is precipitation that forms when supercooled water droplets in air are collected and freeze on falling snowflakes, forming 2–5 mm (0.08–0.20 in) balls of crisp, opaque rime.

<span class="mw-page-title-main">Ukichiro Nakaya</span> Japanese physicist and science essayist (1900–1962)

Ukichiro Nakaya was a Japanese physicist and science essayist known for his work in glaciology and low-temperature sciences. He is credited with making the first artificial snowflakes.

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The hexagonal snowflake, a crystalline formation of ice, has intrigued people throughout history. This is a chronology of interest and research into snowflakes. Artists, philosophers, and scientists have wondered at their shape, recorded them by hand or in photographs, and attempted to recreate hexagonal snowflakes.

<span class="mw-page-title-main">Snow science</span> Interdisciplinary field of hydrology, mechanics and meteorology

Snow science addresses how snow forms, its distribution, and processes affecting how snowpacks change over time. Scientists improve storm forecasting, study global snow cover and its effect on climate, glaciers, and water supplies around the world. The study includes physical properties of the material as it changes, bulk properties of in-place snow packs, and the aggregate properties of regions with snow cover. In doing so, they employ on-the-ground physical measurement techniques to establish ground truth and remote sensing techniques to develop understanding of snow-related processes over large areas.

<span class="mw-page-title-main">Glossary of meteorology</span> List of definitions of terms and concepts commonly used in meteorology

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Further reading