Snow is a form of precipitation consisting of ice crystals that form when water vapor in the atmosphere freezes directly onto a tiny particle, such as dust or pollen, in a process called deposition. This initial ice crystal begins with a hexagonal structure due to the molecular arrangement of water. The journey of these crystals from the cloud to the ground, governed by changing atmospheric conditions, results in a vast diversity of types. Analyzing these forms requires looking at both the individual structure created in the sky and the collective texture created once the snow lands.
The Six Primary Snow Crystal Structures
The final shape of an individual snow crystal is primarily determined by the temperature and humidity it encounters as it falls through the cloud layer. Temperature dictates the crystal’s basic form, either a flat plate or a long prism, while humidity drives the complexity of branching.
At temperatures between 0 and -3 degrees Celsius, crystals generally form as thin hexagonal plates or stars. As the temperature drops further, to approximately -3 to -10 degrees Celsius, the preferred shape shifts to elongated, hollow columns and slender needles.
The most intricate forms, the stellar dendrites, develop in a narrow band around -15 degrees Celsius where rapid growth and high humidity allow for complex, fern-like branching. Colder temperatures, below -22 degrees Celsius, typically yield simpler structures, such as small plates or solid columns. High humidity causes faster growth at the corners, enhancing the development of arms. Conversely, low humidity results in slower growth, favoring simple, compact forms like solid hexagonal plates and short prisms.
Snowpack Characteristics and Texture
Once snow lands, it immediately begins to change through a process called metamorphism, which alters the texture and density of the snowpack. The most sought-after texture is powder snow, which forms in cold, dry conditions with air temperatures typically below -7 degrees Celsius. This snow is light and fluffy because the flakes remain small and unbonded, resulting in a low-density layer with high air content.
In contrast, wet snow forms when air temperatures are near or slightly above the freezing point, causing the crystals to partially melt. This introduces liquid water, which acts as a glue, making the flakes stick together into larger, denser aggregates. Wet snow is significantly heavier and packs easily, which is why it is often used for building snowmen. However, its high water content can cause stress on structures.
Crusted snow results from surface metamorphism where the top layer melts slightly and then refreezes into a hard, ice-like shell. This melt-freeze cycle often occurs due to direct sunlight or warm air exposure, creating a surface layer stronger than the powder beneath it. A specialized form is firn, the intermediate stage between snow and glacial ice, characterized by dense, granular ice that has survived at least one melt season. Firn forms as the weight of overlying snow compresses the crystals, causing them to recrystallize into coarse, rounded grains.
Related Forms of Frozen Precipitation
Other common precipitation types are distinct from snow because their formation involves liquid water. Sleet consists of translucent ice pellets that form when snow melts into a raindrop and then refreezes in a thick layer of below-freezing air near the surface. These small pellets bounce upon impact because they have already solidified in the atmosphere.
Freezing rain occurs when the layer of sub-freezing air near the ground is too shallow for raindrops to fully refreeze before impact. The rain remains in a supercooled liquid state until it strikes a surface, such as a road or power line, that is below freezing. This causes it to instantly freeze into a clear glaze of ice, creating the hazardous conditions associated with ice storms.
Hail is fundamentally different as it forms within the strong updrafts of powerful thunderstorms, often during warmer months. It begins as an ice particle repeatedly carried upward into the cloud’s cold upper regions, collecting layers of supercooled water that freeze upon contact. The layered ice ball eventually becomes too heavy for the updraft to support and falls to the ground as a hailstone.