The question of whether snow simply evaporates is common, but the answer involves a specific phase change that bypasses the liquid state entirely. Snow disappears into the atmosphere without melting first, a process distinct from the evaporation of liquid water. This phenomenon is responsible for a significant, often unseen, loss of water stored in the snowpack. Understanding this mechanism is important because it directly affects how much water ultimately flows into rivers and reservoirs during the spring melt.
Understanding Sublimation
The process by which snow disappears directly into the air is called sublimation. This is the transition of a substance from a solid to a gas without ever becoming a liquid. For snow, water molecules move directly from the solid ice crystal into the gaseous water vapor state, fundamentally differing from melting (solid-to-liquid) and standard evaporation (liquid-to-gas).
This phase change requires a substantial amount of energy, known as the latent heat of sublimation. To complete the full transition from ice to vapor, the process must absorb the energy equivalent of both melting and boiling. For instance, a cubic centimeter of ice needs approximately 720 calories to convert completely into water vapor, bypassing the liquid phase. This energy is typically supplied by the surrounding air or, more powerfully, by direct solar radiation.
Key Conditions Driving Snow Loss
Several environmental factors must align to drive a high rate of snow sublimation. The primary factor is the presence of a strong vapor pressure gradient, meaning the air above the snow must be much drier than the air directly at the snow’s surface. Low relative humidity allows the atmosphere to absorb more moisture, continuously pulling water molecules away from the ice crystals.
Wind plays a substantial role by sweeping away the thin layer of moist, saturated air that forms immediately above the snowpack. This action constantly refreshes the air with drier molecules, maintaining the steep moisture gradient necessary for efficient sublimation. High winds can also lift snow off the ground, turning it into “blowing snow,” which increases the surface area exposed to the dry air and accelerates the loss.
Intense solar radiation provides the direct energy needed to break the bonds holding the water molecules in the ice crystal structure. This energy input causes sublimation even when the air temperature remains below the freezing point. While cold air can sublimate snow, warmer, drier air further accelerates the process by increasing the atmosphere’s capacity to hold the resulting water vapor.
Impact on Water Resources
Sublimation represents a permanent loss of potential runoff, as water moving into the atmosphere via this process is unavailable for human or ecological use downstream. In mountain environments, where snowpack serves as a natural reservoir, the amount of water lost to sublimation can be substantial. Studies in the intermountain Western United States suggest that between 15% and 20% of the annual snowfall can be lost this way.
The loss can be much higher under specific conditions; some alpine ridge sites report that sublimation accounts for up to 90% of the snow mass lost during certain periods. In the Colorado River Basin, which is heavily reliant on snowmelt, this loss is a major concern for water resource managers. Accurately predicting spring and summer streamflow depends heavily on correctly quantifying how much water is lost to the atmosphere through this process.
The mismatch between a healthy snowpack and surprisingly low resulting river runoff has highlighted the importance of sublimation in the overall water balance. For example, some regions have experienced snowpacks reaching 80% of average, yet the resulting streamflow was only 30% of average. This discrepancy indicates that a significant portion of the snow is being lost directly to the atmosphere rather than melting into the ground.