Snowflake

Each flake begins by forming around a tiny particle, called its nucleus, accumulating water droplets, which freeze and slowly form a crystal. Complex shapes emerge as the flake moves through differing temperature and humidity zones in the atmosphere, and possibly combines with other snowflakes. Because of this, snowflakes tend to look very different from one another. However, they 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 the base or "plate", "needle", column, and rime.

Formation

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, 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. Experiments show that "homogeneous" nucleation of cloud droplets only occurs at temperatures lower than −35 °C (−31 °F).

Growth

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.

Guinness World Records lists the world's largest aggregated snowflakes as those of January 1887 at Fort Keogh, Montana, which were claimed to be 38 cm (15 inches) wide—well outside the normally documented range of aggregated flakes of 7.5 or 10 cm (3 or 4 inches) in width. Single crystals the size of a dime (18 mm or 0.705 inches in diameter) have been observed. Snowflakes encapsulated in rime form balls known as graupel.

Appearance

Color

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 composed.

Shape

The shape of the snowflake is determined broadly by the temperature and humidity at which it is formed. Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes. 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, 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. Snowflakes that look identical, but may vary at the molecular level, have been grown under controlled conditions.

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. 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.

Empirical studies suggest less than 0.1% of snowflakes exhibit the ideal six-fold symmetric shape. Very occasionally twelve branched snowflakes are observed; they maintain the six-fold symmetry.