Snow is a familiar form of precipitation, yet its physical characteristics vary widely depending on the environment in which it forms and falls. The experience of walking through a blizzard or shoveling a driveway can differ dramatically. The primary difference lies in the moisture content, which determines whether the snow is categorized as “wet” or “dry.” Understanding the properties and formation of dry snow provides insight into the diverse nature of winter weather.
Understanding Snow Moisture Content
The term “dry snow” refers to snow that contains virtually no liquid water on the surface of its ice crystals. The “wetness” is determined by the presence of a thin film of liquid water between individual flakes. This characteristic is measured by the snow-to-liquid ratio (SLR). A higher SLR indicates drier snow, meaning a greater volume of snow is required to equal one inch of melted water. Dry snow typically exhibits an SLR greater than 10:1, sometimes reaching 20:1 or 30:1 in extremely dry conditions.
The low percentage of liquid water means the ice crystals remain sharp and distinct, preventing them from clumping together. This results in a very low-density snowpack because the crystals settle loosely, trapping a large amount of air. Freshly fallen dry snow can have a density as low as 50 to 70 kilograms per cubic meter (kg/m³), contrasting sharply with the average snow density, which is closer to a 12:1 ratio.
Atmospheric Conditions for Dry Snow
The formation of dry snow depends entirely on sustained, very cold air temperatures throughout the entire atmospheric column. Temperatures at the ground and aloft must remain well below the freezing point of \(32^{\circ}\)F (\(0^{\circ}\)C) to maintain the snow’s dryness. Temperatures often need to be below \(20^{\circ}\)F (approximately \(-7^{\circ}\)C) to produce the fluffiest, driest snow.
Low relative humidity also prevents the formation of liquid water on the crystal surface. When the air is frigid and dry, the snow crystals are less likely to undergo partial melting or refreezing as they descend. This preservation of the crystal structure ensures the snow reaches the ground as light, individual flakes. Furthermore, the highest snow-to-liquid ratios occur when the temperature is between \(0^{\circ}\)F and \(10^{\circ}\)F, which favors the growth of intricate snow crystals known as dendrites in the cloud layer.
Key Differences Between Dry and Wet Snow
Dry snow and wet snow, sometimes called “cement snow” or “heavy snow,” possess different physical properties based on their liquid water content. Dry snow is exceptionally light and less dense due to the high volume of trapped air between the unbonded crystals. This low density means a large accumulation of dry snow melts down to a surprisingly small amount of liquid water.
Wet snow has a higher liquid water content that acts like an adhesive, causing the snowflakes to stick together as they fall. This results in larger, heavier, and much denser snow, with fresh damp snow having a density between 100 to 200 kg/m³. Dry snow is powdery and granular, easily scattering when disturbed, while wet snow is sticky and clumps readily. The lack of liquid water in dry snow makes it difficult to pack, making it poor for making snowballs or building snowmen.
How Dry Snow Affects Recreation and Safety
The light, powdery nature of dry snow makes it highly prized for winter recreation, especially skiing and snowboarding, where it is known as “powder.” This snow allows skis and boards to “float” over the surface rather than cutting through dense material, providing a sensation of effortless gliding. The low friction and deep texture contribute to the enjoyable experience of a perfect powder day.
The properties of dry snow also introduce significant safety concerns, particularly regarding avalanche danger. Extremely dry, light snow forms loose, unconsolidated layers that are easily transported by wind. This wind transport creates dense, slab-like layers on leeward slopes. When a heavy, cohesive slab rests upon a weak layer of dry, loose crystals, the snowpack becomes highly unstable and prone to triggering a destructive slab avalanche.