A big snowflake is a spectacular sight, often covering the palm of a hand, but it is rarely a single, perfectly formed ice crystal. In meteorology, the term “snowflake” refers to any form of ice precipitation, ranging from an individual crystal to a large cluster. Exceptionally large, fluffy flakes are aggregates—collections of many smaller crystals fused together during their descent. This aggregation process, rather than individual crystal growth, is the true explanation for the sheer size of the largest snowflakes.
Growth of Individual Snow Crystals
The journey of any snowflake begins high in the clouds as a single, microscopic ice crystal. This initial formation occurs through vapor deposition, where water vapor transitions directly into a solid state without first becoming liquid water. The crystal forms around a tiny nucleus, such as a dust particle, in air highly saturated with moisture relative to ice. This mechanism, part of the Bergeron-Findeisen process, allows ice crystals to grow at the expense of surrounding supercooled water droplets.
The temperature and moisture levels of the cloud layers determine the resulting crystal habit, or the shape of the individual ice particle. For instance, temperatures near \(5^{\circ}\mathrm{F}\) tend to produce flat, plate-like crystals, while warmer temperatures near \(23^{\circ}\mathrm{F}\) often result in long, needle-like structures. These single crystals, whether plates, columns, or the familiar dendrites, typically grow only to a size of a few millimeters at most before being limited by the rate of vapor diffusion. The complexity of the crystal’s initial branching structure is a prerequisite for the later sticking process.
The Primary Mechanism Aggregation
The vast size of large snowflakes is almost entirely due to aggregation, the mechanism by which multiple ice crystals collide and bond together. This clumping happens when falling crystals encounter others, creating a larger, less dense snowflake aggregate. Intricate, branching forms like stellar dendrites are the most effective at entanglement because their arms easily interlock upon impact, increasing the probability of a successful bond. The final size of these aggregates can be substantial, with most large flakes measuring up to four inches across under ideal conditions.
This bonding relies on weak adhesive forces on the crystal surfaces, which are strongly influenced by temperature. The most effective sticking mechanism involves the quasi-liquid layer (QLL), a thin, liquid-like film that forms on the surface of ice even below \(0^{\circ}\mathrm{C}\). This layer becomes thicker as the temperature of the ice surface approaches the bulk melting point. The QLL is a surface phenomenon, not true bulk melting, that makes the ice crystals remarkably sticky.
When two crystals collide, the presence of the QLL acts as a temporary micro-glue, which then quickly re-freezes at the point of contact to create a solid, permanent bond between the two particles. Without this surface effect, which is essentially pre-melting, colliding ice crystals would often simply bounce off one another, resulting only in small, dry, powdery snow. The constant collision and fusion of individual crystals and small clusters progressively builds the massive, fragile aggregates.
The resulting aggregates are often irregular and lack the perfect symmetry seen in single crystals. These large, feathery structures have a low terminal velocity, meaning they fall slowly and have more time to collect additional crystals during their descent. Because the flakes are large but contain a lot of trapped air, they contribute to the fluffy, high-volume snow often associated with near-freezing conditions. The low density of these large aggregates also causes them to float down gently.
Essential Atmospheric Conditions for Large Flakes
The physical mechanism of aggregation requires a specific meteorological environment to be successful. The most significant condition is an atmospheric temperature profile near the freezing point, typically between \(0^{\circ}\mathrm{C}\) and \(2^{\circ}\mathrm{C}\) in the lower atmosphere. This warmer range promotes the development of the quasi-liquid layer on the ice surfaces, ensuring the crystals are sticky enough to fuse upon collision. Snow falling in much colder, drier conditions remains small and powdery because the crystals lack this adhesive surface layer.
High moisture content, or high saturation, is also necessary. It allows the initial crystals to grow to a sufficient size and provides the water mass needed to create a large aggregate. Warm air holds significantly more water vapor than very cold air, which explains why the heaviest snowfalls often occur when air temperatures are relatively mild. The abundance of water molecules fuels the entire process, ensuring crystal growth is not diffusion-limited.
The movement of air within the cloud layer also plays a role in the collision rate of the crystals. Turbulent air movement increases the frequency of collisions between falling crystals, which is necessary for the aggregation process to occur. This turbulence forces the particles into contact more often, rapidly building the aggregate size. However, the wind speed must not be so strong that it breaks apart the large, delicate aggregates before they reach the ground. The ideal scenario combines near-freezing temperatures, high moisture, and moderate turbulence.