A snowflake is a single ice crystal or an aggregation of multiple crystals. This formation begins high in the atmosphere when supercooled water vapor freezes directly onto a tiny particle, such as dust or pollen, which serves as a nucleus. As the crystal grows while suspended in the cloud, it develops a structure that reflects the atmospheric conditions it encountered. Although often perceived as unique, scientists have categorized these formations into distinct types.
The Underlying Structure of Snowflakes
The fundamental six-sided symmetry observed in almost all snowflakes is a direct consequence of the water molecule’s structure. Each water molecule is naturally polar, meaning it has a slight positive charge on one end and a slight negative charge on the other. When water freezes into ice, these molecules align themselves to maximize the attractive forces, a phenomenon known as hydrogen bonding. This molecular arrangement forces the oxygen atoms into a repeating, open pattern called the hexagonal ice lattice, or Ice Ih. This underlying hexagonal architecture dictates that every snow crystal will possess six primary arms or sides.
The Main Classification of Snow Crystal Designs
The vast array of snow crystal appearances can be organized into main types, a classification system pioneered by scientists like Ukichiro Nakaya. The most recognized form is the stellar dendrite, characterized by its fern-like branches extending from six central arms. These crystals are typically large, possessing numerous side-branches that give them their classic star-like shape.
Another common form is the simple hexagonal plate, which looks like a thin, flat six-sided prism. These plates can be solid or may feature internal etchings and patterns, sometimes leading to a sectored plate appearance where the six sectors are distinctly divided. When the crystal grows primarily along one axis rather than spreading flat, it forms columns or needles, which are elongated, pencil-shaped prisms.
Columns are short, stout hexagonal prisms, while needles are much thinner and longer. Complex forms arise when a crystal transitions between growth environments, such as a capped column, where a column develops hexagonal plates on one or both ends. Finally, irregular crystals are aggregates or fragments that have been broken or clumped together during their descent.
How Temperature and Humidity Determine the Final Shape
The shape a snow crystal adopts is determined by the air temperature and the amount of water vapor present, known as supersaturation or humidity. This relationship is summarized in the Nakaya Diagram, which maps crystal morphology to these two atmospheric variables. Temperature primarily dictates whether the crystal will grow as a flat, plate-like structure or an elongated, columnar one.
Plate-like growth, which includes plates, stars, and dendrites, tends to occur in specific temperature ranges: near the freezing point (0°C to -3°C) and in a colder zone around -10°C to -22°C. Conversely, the growth of columns and needles dominates in the intermediate temperature band of approximately -3°C to -10°C.
Humidity, or supersaturation, controls the complexity of the final form within each temperature zone. When the air is only slightly saturated with water vapor, the crystal grows slowly, resulting in simpler, more solid shapes like small hexagonal plates or simple columns. Higher levels of supersaturation cause rapid growth, which leads to the formation of branches and intricate structures, yielding the stellar dendrites.