Snowflakes are individual ice crystals that form in Earth’s atmosphere, known for their intricate and often six-sided structures. This article explores the atmospheric conditions and physical processes that contribute to the formation of larger-than-average snowflakes.
The Formation of Individual Snowflakes
The birth of any snowflake begins high in the atmosphere when water vapor condenses directly onto a tiny airborne particle, such as dust, pollen, or even bacteria. These microscopic particles act as “ice nuclei,” providing a surface for water molecules to arrange themselves into a hexagonal ice crystal structure. In clouds where temperatures are below freezing but water remains liquid (supercooled water droplets), these ice nuclei become sites for rapid freezing when encountered by the droplets. The ice crystal then grows as additional water vapor freezes directly onto its surface, a process called deposition.
As the nascent ice crystal grows, surrounding supercooled water droplets may evaporate, providing more water vapor for the crystal to absorb. This process, known as the Wegener-Bergeron-Findeisen process, allows ice crystals to grow at the expense of liquid droplets. A single large snow crystal can require the evaporation of roughly one million cloud droplets to reach its full size, developing from a simple hexagonal prism into a more complex form.
Key Conditions for Large Snowflake Growth
The size and intricate shape of an individual ice crystal are largely determined by the temperature and humidity of the air where it forms and grows. Higher humidity levels, meaning more water vapor is available in the air, promote faster and more elaborate crystal growth. This abundance of water molecules allows the crystal to develop complex branching patterns.
Temperature dictates the specific type of ice crystal that forms. For instance, complex, fern-like structures known as stellar dendrites tend to grow best in a narrow temperature range around -15°C (5°F). At temperatures near 0°C (32°F), ice crystals may form as hexagonal plates, while colder temperatures can produce columns or needles. Dynamic changes in temperature and humidity as a crystal falls through different cloud layers sculpt its final, unique form.
The Aggregation Phenomenon
While individual ice crystals can grow quite large, the truly “big” snowflakes commonly observed during snowfalls are often the result of a process called aggregation. This occurs when multiple individual ice crystals collide and stick together to form a single, much larger flake. This clumping creates the flimsy, puffball-like snow that can sometimes measure several inches across.
Conditions that favor aggregation include temperatures slightly above freezing, typically around -2°C to 0°C (28°F to 32°F). At these temperatures, a thin layer of liquid water can form on the surface of the ice crystals, making them stickier upon contact. The shape of the crystals also influences their ability to aggregate; branched or spiky crystals like dendrites are more likely to interlock and stick together than simpler plate-like crystals. Gentle air currents, such as updrafts and downdrafts within the cloud, facilitate these collisions without breaking the delicate aggregated structures apart.