Why Are Snowflakes Six-Sided? Molecules, Temperature and Humidity

A water molecule has two hydrogen atoms arranged at a characteristic angle from the oxygen, and when water freezes those molecules link through hydrogen bonds into a lattice whose arrangement has sixfold symmetry, meaning the structure repeats when rotated by sixty degrees. A crystal growing from vapour adds molecules preferentially at the six corners of that arrangement, where the geometry offers more neighbours to bond with, which is why arms develop in six directions rather than in any other number. The symmetry is therefore a direct consequence of molecular geometry rather than an accident, and it is the one feature every snow crystal shares. Everything else about the shape depends on conditions.

Form depends on temperature and on how much water vapour is available:

• •Near freezing, thin plates form
• •Around minus five degrees, needles and columns form instead
• •Around minus fifteen degrees, the classic branched stars grow, and they grow largest there
• •Below about minus twenty five, columns and plates return
• •Higher humidity at any temperature produces more elaborate branching
• •Lower humidity produces simpler, blockier forms

The six arms of a single crystal frequently resemble one another closely, which invites the assumption that something coordinates them, and the explanation is simpler. A crystal is small, so all six arms experience essentially the same temperature and humidity at every moment, and since growth depends on those conditions, all six respond identically to the same sequence of changes as the crystal falls and rises through the cloud. The complex path a crystal takes through varying conditions is what produces the complex shape, and the arms match because they shared the journey. The matching is far from perfect in most real crystals, which is visible in any careful photograph and which the popular image of a perfectly symmetric flake obscures.

Not everything falling as frozen water is a snow crystal, and distinguishing the forms matters for anyone reading a forecast. Graupel forms when a crystal falls through supercooled droplets that freeze onto it, coating it until the original shape is buried, producing soft opaque pellets. Sleet in the technical sense is rain that froze while falling through a cold layer, giving clear hard pellets. Hail forms in thunderstorms, where updraughts carry particles repeatedly through regions of supercooled water, adding layers that are visible when a stone is cut open. Freezing rain stays liquid until it touches a surface and then freezes on contact, which is the most dangerous form for infrastructure. And large snowflakes are aggregates of many crystals stuck together, which happens near freezing when surfaces are slightly wet.

The familiar claim that no two are identical deserves unpacking rather than repeating. For the large branched crystals, the number of possible arrangements of molecules is so enormous that identical ones are effectively impossible, and the claim holds comfortably. For very small simple crystals, consisting of a few hundred molecules in a plain hexagonal plate, identical ones are entirely possible and have effectively been made in laboratories. The interesting version of the question concerns what identical means, since two crystals can look alike at any given magnification while differing in the placement of individual molecules, and the differing isotopes present in natural water guarantee differences at that level. The photographer Wilson Bentley captured thousands of images from 1885 onwards and is why the popular image exists at all.

Water molecules bond into a lattice with sixfold symmetry, and growth favours the six corners, which fixes the number of arms. Temperature determines whether plates, needles, columns or branched stars form, with the classic stars growing around minus fifteen degrees. The arms match because they experienced the same conditions on the same journey rather than through any coordination.