Snow crystals form when tiny
supercooled cloud droplets (approx 1
mm in diameter)
freeze. These droplets are able to remain liquid at temperatures colder than 0
°C because, in order to freeze, a few
molecules in the liquid droplet need to get together by chance to form an arrangement close to that in an ice lattice; then the droplet freezes around this 'nucleus'. Experiments show that this 'homogeneous' nucleation of cloud droplets only occurs at temperatures colder than -35 °C.
[1] In warmer clouds an aerosol particle or 'ice nucleus' must be present in (or in contact with) the droplet to act as a nucleus. Our understanding of what particles make efficient ice nuclei is poor - what we do know is they are very rare compared to that cloud condensation nuclei which liquid droplets form on. Clays, desert dust and biological particles may be effective,
[2] although to what extent is unclear. Artificial nuclei include
silver iodide and
dry ice, and these form the basis of
cloud seeding.
Once a droplet has frozen, it grows in the supersaturated environment (air saturated with respect to liquid water is supersaturated with respect to ice when the temperature is below the freezing point) and grows by diffusion of water molecules in the air (vapor) onto the ice crystal surface where they are deposited. Because the droplets are so much more numerous than the ice crystals (because of the relative numbers of ice vs droplet nuclei) the crystals are able to grow to hundreds of
micrometres or millimetres in size at the expense of the water droplets (the
Wegner-Bergeron-Findeison process). The corresponding depletion of water vapour causes the droplets to evaporate, meaning that the ice crystals effectively 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 (aggregates). These aggregates are snowflakes, and are usually the type of ice particle which falls at the ground.
[3] The exact details of the sticking mechanism remains controversial (and probably there are different mechanisms active in different clouds), possibilities include mechanical interlocking, sintering, electrostatic attraction as well as the existence of a 'sticky' liquid-like layer on the crystal surface.
The individual ice crystals often have an
hexagonal symmetry. Although the ice is clear, scattering of light by the crystal facets and hollows/imperfections mean that the crystals often appear white in colour due to
diffuse reflection of all
spectrum of
light by the small ice particles.
Ice crystals formed in the appropriate conditions can often be thin and flat. These planar crystals may be simple hexagons, or if the
supersaturation is high enough, develop branches and dendritic (fern-like) features and have six approximately identical arms, as per the iconic 'snowflake' popularised by
Wilson Bentley. The 6-fold symmetry arises from the hexagonal
crystal structure of
ordinary ice, the branch formation is produced by unstable growth, with deposition occurring preferentially near the tips of branches.
[1]
The shape of the snowflake is determined broadly by the temperature, and humidity at which it forms.
[3] Rarely, at a temperature of around −2 °C (28 °F), snowflakes can form in threefold symmetry — triangular snowflakes.
[4] The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing.
Planar crystals (thin and flat) grow in air between 0 °C (32 °F) and −3 °C (27 °F). Between −3 °C (27 °F) and −8 °C (18 °F), the crystals will form needles or hollow columns or prisms (long thin pencil-like shapes). From −8 °C (18 °F) to −22 °C (−8 °F) the habit goes back to plate like, often with branched or dendritic features. Note that the maximum difference in vapour pressure between liquid and ice is at approx. −15 °C (5 °F) where crystals grow most rapidly at the expense of the liquid droplets. At temperatures below −22 °C (−8 °F), the crystal habit again becomes column-like again, although many more complex habits also form such as side-planes, bullet-rosettes and also planar types depending on the conditions and ice nuclei.
[5]
Interestingly, if a crystal has started forming in a column growth regime, say at around −5 °C (23 °F), and then falls into the warmer plate-like regime, plate or dendritic crystals sprout at the end of the column producing so called 'capped columns'.
[3]
There is a widely held belief that no two snowflakes are alike. Strictly speaking, it is extremely unlikely for any two
macroscopic objects in the universe to contain an identical molecular structure; but there are, nonetheless, no known scientific laws that prevent it. In a more pragmatic sense, it's more likely—albeit not much more—that two snowflakes are virtually identical if their environments were similar enough, either because they grew very near one another, or simply by chance. The
American Meteorological Society has reported that matching snow crystals were discovered in Wisconsin in 1988 by Nancy Knight of the
National Center for Atmospheric Research.
[6] The crystals were not flakes in the usual sense but rather hollow
hexagonal prisms.