Phase change at subzero conditions: The disappearance of snow in freezing (subzero) weather is an example of which physical process?

Introduction / Context:
Snow and ice can vanish even when ambient temperatures remain below 0°C. Understanding the underlying phase change explains everyday observations, informs meteorology, and clarifies drying and preservation technologies (e.g., freeze-drying) that exploit similar physics.

Given Data / Assumptions:

• Ambient temperature is below 0°C (subzero), so bulk melting is not sustained.
• Air is unsaturated with respect to ice, allowing net mass transfer to the vapor phase.
• Wind and low humidity enhance mass transfer from the solid surface.

Concept / Approach:
Sublimation is the direct transition of a substance from solid to vapor without passing through the liquid phase. For ice/snow, the vapor pressure of ice at subzero temperatures, though low, is nonzero. If the surrounding air’s partial pressure of water vapor is below the equilibrium vapor pressure over ice at that temperature, molecules leave the solid and diffuse away—net sublimation occurs. Evaporation and vaporisation refer to liquid → vapor transitions, which are not dominant here because liquid water is not present or stable at sustained subzero conditions. Melting requires temperatures at or above the melting point (or at least localized heating), which is not the scenario described.

Step-by-Step Solution:
Identify phases: solid (snow/ice) and gas (water vapor).Check temperature regime: subzero → liquid phase is not stable overall.Equilibrium criterion: if p_H2O,air < p_sat,ice(T), mass transfer occurs from solid to gas.Therefore the correct mechanism is sublimation.

Verification / Alternative check:
Psychrometric charts and ice vapor pressure correlations show positive sublimation driving forces at low relative humidity and cold temperatures, consistent with snow loss without melting.

Why Other Options Are Wrong:
Evaporation/vaporisation involve a liquid phase.Melting requires temperatures near/above 0°C or strong localized heating.

Common Pitfalls:
Assuming all disappearance of snow must involve melting; overlooking the role of humidity and wind in sublimation rates.

Final Answer:
Sublimation