The process where snow or ice turns directly into a gas without first becoming a liquid is called sublimation. This transformation is a significant part of the Earth’s water cycle, allowing frozen water to bypass the melting phase entirely. Sublimation involves water molecules escaping from the solid surface directly into the atmosphere as water vapor. This physical change is responsible for the gradual disappearance of snowpacks and ice layers even when the air temperature remains below freezing.
Defining Sublimation
Sublimation is a phase transition where a substance moves from the solid state directly into the gaseous state, bypassing the intermediate liquid phase common in processes like melting and boiling. For water, this means the change from ice or snow directly to water vapor. The process is endothermic, meaning it requires the absorption of energy, often from solar radiation or surrounding air.
This transition is distinct from melting (solid to liquid) and evaporation (liquid to gas). A non-water example is dry ice, which is solid carbon dioxide that turns into a gas at standard atmospheric pressure. The occurrence of sublimation is controlled by the thermodynamic concept of the triple point, a single temperature and pressure where all three phases of a substance can exist together in equilibrium.
For water, the triple point is at \(0.01^\circ\text{C}\) and a pressure of \(611.657\) pascals. Sublimation can only happen at temperatures and pressures below this triple point on a phase diagram. If the pressure is higher, the substance will melt into a liquid before turning into a gas.
The Atmospheric Factors Required
Sublimation in the natural environment is driven by specific atmospheric conditions. The most important factor is a difference in vapor pressure between the ice surface and the surrounding air. The air must have a lower partial pressure of water vapor than the ice surface, causing water molecules to move outward. This pressure gradient acts as the driving force for the solid-to-gas transition.
Low atmospheric humidity is an accelerating factor, as dry air readily accepts more moisture, increasing the vapor pressure difference and speeding up sublimation. Strong winds also enhance the process by continually removing the water vapor that has escaped the snowpack, preventing the air immediately above the snow from becoming saturated. This constant replacement of moist air with drier air, known as advection, significantly enhances the loss of snow mass.
The ice does not need to be above the freezing point for sublimation to occur. Even at sub-freezing temperatures, the ice molecules possess enough thermal energy to escape into the vapor phase. Sunlight plays a major role by providing the necessary energy for this phase change. High altitudes also favor the process because lower atmospheric pressure shifts conditions closer to the triple point boundary that promotes sublimation.
Sublimation’s Role in the Hydrological Cycle
Sublimation represents a significant loss mechanism for frozen water in the hydrological cycle, particularly in mountainous and polar regions. Instead of melting into liquid water that contributes to streamflow and groundwater recharge, a portion of the snowpack returns directly to the atmosphere as vapor. This means less water is available for ecosystems and human consumption downstream.
The magnitude of this water loss can be substantial, sometimes accounting for up to more than half of the total seasonal snowfall depending on local conditions. In the western United States, where snowmelt provides the primary source of water for agriculture and municipal use, this effect has major implications for water resource management. Hydrologists must accurately measure and forecast sublimation rates to predict the amount of meltwater contributing to reservoirs and rivers.
This is relevant in the context of climate change, as altered temperature and precipitation patterns influence the frequency of the dry, windy conditions that promote sublimation. An increased rate of sublimation contributes to the observed gap between the snow expected on a mountain and the actual runoff received. Understanding this atmospheric loss is necessary for accurate snow prediction models and effective water budgeting in regions reliant on snowpack.
The Opposite Phase Change
The reverse process of sublimation, where water vapor transforms directly into a solid without an intermediate liquid phase, is called deposition, also known as desublimation. Deposition occurs when gaseous water molecules lose energy and bond together directly to form an ice structure.
A common example of deposition is the formation of frost on cold surfaces, such as grass or car windshields, on clear winter nights. The water vapor in the air contacts a surface below the freezing point and converts immediately into delicate ice crystals. This change is a direct gas-to-solid transition, bypassing the formation of liquid dew.