When a cold, sunny day follows a fresh snowfall, the snowpack often shrinks even if the air temperature remains below the freezing point of 32°F (0°C). This phenomenon is not a contradiction but a direct result of energy transfer and the varying temperatures of different surfaces. The snow loses mass through processes that bypass the cold air temperature, primarily involving direct energy from the sun.
Air Temperature Versus Surface Temperature
The temperature reported in weather forecasts is typically the air temperature, measured by a thermometer shielded from direct sunlight about five feet above the ground. This ambient temperature represents the surrounding environment but does not reflect the temperature of objects exposed to the sun. Melting, the solid-to-liquid phase change, is governed by the temperature of the material itself, not the air around it. For snow to melt, the ice crystals must reach 32°F (0°C) at their surface.
Snow is an effective insulator because it is approximately 90% trapped air, which slows the transfer of heat from the warmer ground below. Consequently, the snow’s surface temperature can be dramatically different from the air temperature, especially when exposed to a direct energy source. This distinction explains why snow can begin to melt even when the official air temperature is well below freezing.
Solar Radiation: The Direct Melting Mechanism
The sun’s energy provides the primary heat source necessary to raise the snow’s surface temperature to the melting point. This energy travels as shortwave radiation and is absorbed directly by the snow or dark objects underneath it, largely bypassing the surrounding cold air. When absorbed, this energy transforms into thermal energy, directly warming the snow crystals.
The reflectivity of the surface, known as albedo, plays a substantial role in this process. Fresh, clean snow has a high albedo, reflecting up to 90% of the incoming solar radiation, which helps keep it cold. As the snow ages or becomes contaminated with dust or soot, its albedo decreases, causing it to absorb significantly more energy. This increased absorption can quickly raise the surface temperature above freezing, initiating melt even if the air remains frigid.
Solar radiation can also penetrate a thin layer of snow and warm a darker surface beneath, such as pavement or dirt. Since these dark surfaces have a low albedo, they absorb a high percentage of the sun’s energy and conduct that heat upward to the overlying snow. This process often causes the snow to melt from the bottom or edges first, creating pools of liquid water observed on cold, sunny days.
Sublimation: Snow Loss Below Freezing
Not all snow loss on a cold day is due to melting; a significant amount can disappear through sublimation. Sublimation is the physical process where a solid transitions directly into a gas, skipping the liquid phase entirely. This mechanism occurs at temperatures below freezing, meaning no liquid water is produced. The energy required to drive sublimation can be supplied by solar radiation, even if it is insufficient to raise the snow’s temperature to 32°F.
Sublimation is favored by specific atmospheric conditions, including low relative humidity and wind. Dry air has a higher capacity to hold water vapor, pulling water molecules directly from the solid ice structure. Wind accelerates this phase change by continually moving the air, preventing a localized layer of moist air from building up above the snow surface. Therefore, a cold, sunny, dry, and windy day is highly effective at reducing the snowpack through sublimation without creating visible meltwater puddles.