Cloud microphysics: Under what cloud temperature conditions does snow formation occur?

Introduction / Context:
Snow formation in clouds depends on the thermodynamic state and microphysical processes, primarily ice nucleation and growth. Knowing the temperature regime for snow is crucial for weather forecasting and interpreting satellite/radar observations of winter precipitation.

Given Data / Assumptions:

• Cloud parcels can be supercooled but still liquid above 0°C depending on nuclei availability.
• Ice crystal formation generally requires temperatures below 0°C in real clouds.
• Surface precipitation type can change below the cloud due to melting, but the question addresses formation within the cloud.

Concept / Approach:
In-cloud snow forms as ice crystals via deposition (vapor to ice), aggregation, and riming. Ice nucleation typically initiates at subfreezing temperatures; heterogeneous nucleation becomes more likely as temperature falls (e.g., below −10°C), while homogeneous freezing of droplets occurs near −38°C. Therefore, snow formation is associated with cloud temperatures below 0°C, even though flakes may encounter warmer layers later.

Step-by-Step Solution:
Relate snow to ice phase processes → requires T below 0°C for nucleation/growth.Recognize supercooled water can exist slightly below 0°C but does not imply snow formation above 0°C.Conclude that “below the freezing point” best characterizes the temperature regime for snow formation aloft.Reject “at” or “just above” 0°C because liquid phase tends to dominate near the melting level.

Verification / Alternative check:
Soundings showing dendritic growth zones (around −12 to −18°C) align with prolific snow crystal growth, reinforcing the subfreezing requirement.

Why Other Options Are Wrong:
Just above/at 0°C: Marginal for ice nucleation; melting dominates.None/Far above 0°C: Contradict the need for ice phase within the cloud.

Common Pitfalls:
Confusing formation temperature with precipitation type at the ground; melting layers can transform snow to rain before reaching the surface.

Final Answer:
Below the freezing point.