Snow and ice are the solid form of water. For pure water ice, the melting point at standard atmospheric pressure is \(0^\circ C\) or \(32^\circ F\). While this is the textbook answer for when snow melts, the actual process is more complicated than air temperature alone suggests. Snow often lingers when temperatures are above freezing or disappears when the air is still cold. The disappearance of a snowpack is determined by how energy is transferred to the snow and the presence of other substances, not just the ambient air temperature.
The Baseline Temperature of Phase Change
The melting point of pure water ice is \(0^\circ C\) (\(32^\circ F\)), where the solid and liquid phases coexist in equilibrium. Melting requires energy to break the ice’s crystalline structure, known as the latent heat of fusion. This is the thermal energy a substance must absorb to change state from solid to liquid without increasing its temperature. For water, this value is approximately 334 kilojoules per kilogram. Even when snow reaches \(0^\circ C\), it must absorb this large amount of energy before fully converting to liquid water.
During the entire melting process, the mixture of snow and water remains locked at \(0^\circ C\) until the last ice crystal is gone. The absorbed heat overcomes the intermolecular forces holding the water molecules in their fixed solid arrangement. This explains why a large snowdrift can stay at the freezing point for many hours, even if the surrounding air temperature is slightly above freezing.
External Factors That Drive Melting
Many variables contribute to the speed and efficiency of melting, often overriding the influence of air temperature. One powerful external factor is solar radiation, which transfers heat directly to the snow’s surface. The sun’s radiant energy is absorbed by the snow, providing the necessary latent heat of fusion more directly than warm air molecules.
The reflectivity of the snow, known as albedo, determines how much solar energy is absorbed. Fresh, clean snow has a high albedo and reflects up to \(90\%\) of incoming solar radiation. This value decreases significantly as the snow ages or becomes contaminated. When fine particles like dust or dirt settle on the snow, the surface albedo drops, allowing the snow to absorb more heat and melt faster.
Another major factor is freezing point depression caused by impurities, most commonly road salt. When substances like sodium chloride dissolve in water, the salt ions interfere with the ability of water molecules to form a solid crystal lattice. This disruption forces the melting point to be lower than \(0^\circ C\).
Depending on the concentration, salt can significantly lower the temperature at which ice remains stable. Ocean water, with a salt content of about \(3.5\%\), has a freezing point of approximately \(-2^\circ C\) (\(28^\circ F\)). Highly concentrated salt brine solutions used on roads can lower the melting point to as low as \(-16^\circ C\) (\(2^\circ F\)). Melting is also accelerated by direct contact with warmer surfaces, such as dark asphalt pavement that conducts heat into the base of the snowpack.
How Snow Disappears Without Turning to Water
Snow can disappear completely without ever passing through the liquid phase in a process called sublimation. Sublimation is the transition where a substance moves directly from its solid state to its gaseous state, bypassing the liquid state entirely. This phenomenon explains how a snowpack can shrink even when the air temperature remains well below \(0^\circ C\).
The conditions most favorable for sublimation involve dry air, low humidity, and high winds, along with a source of energy like sunlight. The moisture-free air absorbs the water vapor molecules as they escape from the snow surface. Strong winds carry away this water vapor, preventing the air immediately above the snow from becoming saturated and allowing the process to continue.
Sublimation requires an immense amount of energy to free the water molecules from their solid bonds and accelerate them into the gas phase. It takes roughly seven times the energy required to boil a comparable amount of liquid water to sublimate ice. This energy is primarily supplied by solar radiation, which is why bright, sunny days with a drying wind can cause a noticeable reduction in snow depth, even if temperatures are far below freezing.