Explain why do snowflakes have such a variety of shapes?

Initial formation of snow crystals

A snow crystal begins to form from a tiny speck of dust or a particle present in the clouds. This tiny impurity acts as a nucleus around which the water vapor present will gradually condense to form a tiny ice grain, also known as an ice nucleus. This nucleus then serves as an anchor for nearby water molecules, which attach themselves to its surface by transforming directly into ice without passing through the liquid water stage: this is solid condensation, or more simply, reverse sublimation. And that’s the discreet yet essential beginning of what will later become a beautiful snowflake with unique shapes.

Combined effects of humidity and ambient temperature

Temperature and humidity are somewhat like the conductors of snow crystal growth. When it is very cold, around -15°C, combined with sufficient humidity, we get beautiful star-shaped flakes, fine and very detailed. If the air is milder, close to -5°C, and quite humid, the crystals will be rather large and flat, like small hexagonal plates. Conversely, in very cold but dry air, it’s often simple small flakes in the shape of sticks or columns that appear, without extravagant details. In short, each precise combination of humidity and temperature gives rise to a specific shape of flakes—a sort of unique signature in the winter sky.

Impact of atmospheric impurities on morphology

When particles such as dust, sea salts, or industrial pollutants float in the atmosphere, they often serve as nuclei where snow crystals begin their growth. As a result, the final shape largely depends on these small impurities, as they locally alter the way water molecules freeze. Some impurities promote very regular shapes, hexagonal or star-like, while others push the crystals to adopt more complex or irregular—sometimes strange—appearances. The more numerous or varied the impurities present, the greater the diversity of snowflake shapes observed.

Physical mechanisms governing the varied growth of crystals

The shape and diversity of snow crystals directly result from how water molecules come together to form ice. These molecules group into hexagonal structures, giving the characteristic symmetrical appearance to snowflakes. But why so much variety? Simply because as they fall, the crystals pass through different physical conditions (temperature, humidity, pressure) that constantly change the way they grow. For example, when it's relatively warm (close to 0°C) but very humid, we get wide, branched crystals with complex star-shaped patterns. In contrast, in very cold and dry weather, the crystals remain small and flat, resembling plates or columns. As the crystals fall, they can also rotate or tilt, exposing their different faces to variable conditions; their morphology then evolves even further. In short, each snowflake is shaped in real-time by a lovely cocktail of physical and environmental parameters, resulting in this incredible diversity of shapes.

Environmental variations and diversity of final forms

The great diversity of the final shapes of snow crystals mainly depends on subtle changes in their environment during the fall. Even a small change in temperature or a slight breeze is enough to influence the growth of the crystal, altering its speed or axis of formation. For example, a sudden increase in temperature can transform a star pattern into a crystal with thicker, rounded branches. Conversely, a sudden and rapid drop in temperature generally favors more complex shapes with fine, delicate branches. Thus, it is a whole meteorological journey, filled with small climatic twists and turns, that gives rise to the incredible beauty that is both diverse and unique to each snowflake.