A snowflake is a single ice crystal that has grown large enough within a cloud to fall through the atmosphere and reach the ground. This journey of descent sculpts the crystal into its final form. While an immense variety of shapes exist, the forms most commonly observed and classified are the delicate, complex structures known as stellar dendrites and hexagonal plates. These intricate designs embody the six-sided symmetry dictated by the physics of water molecules. The final morphology of any individual crystal tells a silent story of the temperature and humidity conditions it encountered.
The Science of Snowflake Classification
Scientists rely on classification systems to bring order to the vast diversity of snow crystal shapes that exist. Japanese physicist Ukichiro Nakaya pioneered this work, cataloging natural snow crystals into 41 distinct types in the 1930s. The most visually striking and widely recognized shapes fall under the general categories of Stellar Dendrites and Hexagonal Plates.
Stellar dendrites are the classic, star-shaped crystals featuring six main branches, or arms, that often display feathery side-branches. Their fern-like complexity makes them among the largest crystals, sometimes exceeding four millimeters in diameter. These large, flat forms fall slowly, making them the most noticeable and frequently studied crystals.
Hexagonal plates are simpler, flat, six-sided discs that can range from simple hexagons to more decorated shapes known as sectored plates. These plates and the elaborate dendrites are the most celebrated because they represent the maximum growth potential of the ice crystal structure. They form under conditions of high humidity, which leads to rapid crystal growth and the development of complex branching patterns.
The Environmental Drivers of Crystal Morphology
The precise shape a snow crystal takes is determined by the atmospheric conditions it experiences, particularly the temperature and the level of moisture, or supersaturation, in the air. These two factors are mapped out in a scientific tool called the Nakaya Diagram, which illustrates the correlation between environment and crystal shape. The diagram shows that extremely small changes in temperature can cause a dramatic switch in the crystal’s growth habit.
Thin, flat plate-like crystals grow best in two distinct temperature ranges: near \(-2^{\circ}\text{C}\) and again near \(-15^{\circ}\text{C}\). In contrast, growth shifts to the vertical axis, producing long, slender needles or hollow columns, in the intermediate temperature range around \(-5^{\circ}\text{C}\). Higher supersaturation in the air supplies more water vapor for molecules to attach, leading to faster growth and the formation of the complex, branched stellar shapes.
As the crystal falls through the cloud, it passes through layers with different temperatures and humidity levels, constantly altering its growth pattern. This dynamic journey means the final shape is a frozen record of the atmospheric conditions encountered along its path. The six arms of a single stellar crystal grow symmetrically because they travel together and experience the same microenvironment simultaneously.
The Universal Rule of Six Sides
Despite the immense variety in size and complexity, all natural snow crystals share the fundamental characteristic of having six sides, or six-fold symmetry. This universal rule is a direct consequence of the molecular structure of water (\(\text{H}_2\text{O}\)). When water freezes into ice under normal atmospheric pressure, the water molecules arrange themselves to maximize the stability of their hydrogen bonds.
This arrangement forces the molecules into a repeating, open pattern known as a hexagonal crystal lattice. The most stable configuration for this lattice is a six-sided ring structure, which forms the building block of the ice crystal. As the crystal grows, new water molecules attach to the existing structure, maintaining and extending this inherent six-sided geometry outward. The six sides are the physical manifestation of the most energetically favorable way for water molecules to stack.
The Simpler and Rarer Forms
While the ornate stellar dendrites are the most celebrated, many other crystal forms are part of the full classification spectrum. Simpler forms like columns and needles are hexagonal prisms that have grown primarily along their vertical axis, resembling tiny pencils. These are typically small and form in colder, drier conditions where growth is slower.
Another distinct form is the capped column, which begins as a column in one temperature zone and then falls into a warmer layer, causing plates to sprout from its ends. The final shape is a column with a hexagonal plate “cap” on each end, demonstrating a change in growth habit mid-fall.
The most common type of snow particle, however, is the irregular crystal. This is often a fragment, a damaged piece, or an asymmetrical clump of crystals. These irregular forms are less visually impressive than the textbook dendrites, but they make up a significant portion of natural snowfall.