A snowflake is an individual ice crystal that forms high in the atmosphere, beginning when water vapor freezes onto a tiny particle of dust or pollen. The resulting ice crystal starts to grow as it collects more water vapor from the surrounding air. While the shapes they take are famously varied and complex, the foundational structure of virtually every snowflake is the same: six sides, arms, or points. This six-fold symmetry is a direct consequence of the molecular physics of water as it transitions into a solid state.
The Universal Rule of Six
The reason all snowflakes share a six-sided base structure is found in the way water molecules arrange themselves when they freeze. In its solid form, known as hexagonal ice, water creates a crystal lattice where each water molecule is bonded to four others through a network of hydrogen bonds. This arrangement naturally forms a repeating, six-sided ring structure.
The oxygen atoms in the water molecules sit at the corners of a hexagonal prism, which dictates the six-fold symmetry of the ice crystal. As the crystal grows, new water molecules are strongly favored to attach at the six points of the hexagon. The angles between the water molecules in this lattice provide the geometric basis for the hexagonal pattern.
The growth of the crystal occurs primarily along the six corners of this hexagonal base, leading to the familiar shape with six arms or points. Since the internal molecular arrangement is consistent for all ordinary ice, the six-sided structure is a constant rule of snowflake formation.
How Atmospheric Conditions Shape the Crystal
While internal physics determines the six-sided base, external atmospheric conditions dictate the final, visible shape of the snowflake, known as its morphology. The precise temperature and humidity the crystal encounters as it falls determine how and where water vapor molecules attach to the ice. These variables can change dynamically, meaning a single snowflake may grow differently at its center than at the tips of its arms.
Different temperature ranges favor the growth of distinct crystal forms. For example, temperatures around -2 degrees Celsius (28 degrees Fahrenheit) and near -15 degrees Celsius (5 degrees Fahrenheit) tend to produce flat, plate-like crystals. Conversely, air around -5 degrees Celsius (23 degrees Fahrenheit) often leads to the growth of long, slender needle-like or column-shaped crystals.
Humidity levels are also significant, determining the complexity of the shape. When the air is highly saturated with water vapor, the crystal grows rapidly, encouraging the formation of elaborate branches and intricate stellar dendrites. Lower humidity causes slower growth, resulting in simpler forms like solid hexagonal plates or columns. Because a single snowflake may pass through multiple temperature and humidity zones, its final shape is a composite record of its entire journey.
Perfect Versus Imperfect Snowflakes
Despite the scientific rule of six-fold symmetry, most snowflakes observed are not perfectly symmetrical stellar crystals. The pristine, textbook snowflake is quite rare, as the crystal’s journey from cloud to ground is turbulent and often damaging. Many flakes are irregular because they break, collide, or clump together with others during their fall.
A process called “riming” occurs when the ice crystal collides with supercooled water droplets, which freeze instantly onto the flake, giving it a bumpy, irregular, and chaotic appearance. The individual arms of a snowflake may also experience slightly different conditions, leading to asymmetrical growth even on the same crystal. This variation in micro-environments makes the formation of two identical snowflakes nearly impossible.
The common saying that no two snowflakes are alike holds true because the path each one takes is unique. Though the underlying molecular structure is the same, the sheer number of environmental variables—including changes in temperature, humidity, and collisions—means that every flake follows a unique growth history. Scientists estimate the number of possible snowflake forms is astronomically high.