The common image of a snowflake, the intricate, six-pointed star, is often the subject of both art and winter folklore. This idealized shape, known scientifically as a stellar dendrite, is real, but it represents only a small fraction of the forms that frozen precipitation can take. The true diversity of snow crystals extends far beyond the familiar star, encompassing a vast gallery of geometric shapes.
The Foundation of Six Sides
Every single snow crystal adheres to a fundamental six-sided symmetry. This consistent hexagonal shape is dictated by the precise molecular geometry of water. A water molecule (H2O) consists of two hydrogen atoms and one oxygen atom arranged in a bent configuration.
When water vapor freezes into ice, hydrogen bonds pull the molecules together. These bonds force the molecules to align into the most stable and efficient arrangement possible: a hexagonal lattice structure. This lattice forms the blueprint for the entire crystal, meaning the six-fold symmetry persists throughout the growth process, even as the crystal becomes a complex, multi-branched structure.
The True Diversity of Snow Crystal Shapes
While the stellar dendrite is the most recognizable, it is merely one type within a vast classification system that includes at least 80 individual variants. Snow crystals are broadly categorized by their appearance, ranging from simple geometric forms to highly complex structures.
Simple forms include hexagonal plates, which are thin, flat crystals, and columns, which are pencil-like or barrel-shaped prisms. Needles are another common simple form, appearing as thin, elongated rods. Complex crystals include:
- Stellar dendrites, characterized by six large, fern-like arms branching from a central point.
- Capped columns, which are column-shaped crystals with flat, plate-like structures growing on each end.
- Spatial dendrites, which are three-dimensional clusters of crystals grown together in random orientations.
Irregular crystals are the most common type, consisting of fragments or aggregates of other shapes that have collided or partially melted during their descent.
How a Snowflake’s Journey Determines Its Final Form
The specific shape a snow crystal adopts is determined by the atmospheric conditions it encounters as it grows, acting like a detailed record of its journey. The two most influential factors are the surrounding air temperature and the level of supersaturation, which is the amount of water vapor available for crystal growth.
Crystal morphology shifts dramatically with temperature. Thin plates tend to form at temperatures near -2°C (28°F), while slender columns and needles develop between -5°C and -10°C (23°F and 14°F). Stellar dendrites grow in a specific temperature range around -15°C (5°F), provided the air has high supersaturation.
High supersaturation means abundant water vapor is available, leading to rapid growth that favors the development of complex branches and arms. Since a single crystal might fall through several layers of air, each with a different temperature and humidity, its final shape is a composite of its entire “growth history.” Because no two crystals ever follow the exact same path through the atmosphere, the sequence of environmental changes they experience is unique, leading to the infinite variation in their final forms.