Questions: Ice Nucleation and Freezing Processes in Clouds

Question 1

A cloud at −15°C is observed to contain only liquid water droplets with no detectable ice. What is the most likely explanation?

• A. At −15°C, water cannot exist as a liquid; the observation must be an instrument error
• B. The cloud lacks effective ice nucleating particles, so supercooled liquid droplets persist despite temperatures well below 0°C
• C. The cloud is too warm for ice formation; ice only forms below −40°C in all circumstances
• D. The droplets are too large to be nucleated by available particles

Answer: (B)

Supercooling is common in clouds. Without effective ice nucleating particles (INPs), pure water droplets can remain liquid down to about −40°C. At −15°C, ice formation requires heterogeneous nucleation on mineral dust or similar INPs. In their absence, the surface-energy barrier prevents spontaneous homogeneous nucleation at that temperature. All-liquid clouds at −15°C are physically possible and well-documented in nature.

Question 2

In a mixed-phase cloud between −10°C and −40°C, why do ice crystals grow preferentially at the expense of supercooled liquid droplets?

• A. Ice crystals are larger and physically sweep up liquid droplets through collision
• B. Ice crystals are warmer than liquid droplets, creating a vapor pressure gradient
• C. The saturation vapor pressure over ice is lower than over liquid water at the same temperature, so vapor flows from droplets to ice
• D. Ice crystals produce surface tension forces that compress neighboring droplets

Answer: (C)

This is the Bergeron process. At the same sub-zero temperature, the saturation vapor pressure over a liquid surface is higher than over an ice surface. Vapor in the cloud is supersaturated with respect to ice but subsaturated with respect to liquid. Vapor therefore diffuses from liquid droplets (where it is in excess) to ice crystals (where it is deficient), depositing as ice while the droplets shrink. This vapor-pressure differential, not collision, drives rapid ice crystal growth.

Question 3

Supercooled liquid water — water that remains liquid below 0°C — can exist in clouds under natural atmospheric conditions.

• T. True
• F. False

Answer: True

Supercooling is a well-documented atmospheric phenomenon. Without ice nucleating particles, the surface-energy barrier to forming an ice crystal lattice prevents freezing at temperatures only slightly below 0°C. Pure water droplets can persist as liquid down to approximately −40°C (the homogeneous nucleation temperature). Clouds in the −10°C to −40°C range routinely contain a mixture of supercooled liquid droplets and ice crystals — the mixed-phase zone central to mid-latitude precipitation.

Question 4

Most aerosol particles in the atmosphere are approximately equally effective at nucleating ice at a given temperature.

• T. True
• F. False

Answer: False

Ice nucleating efficiency varies enormously among particle types. Certain biological particles (e.g., Pseudomonas syringae bacteria) can nucleate ice near −2°C, while mineral dust (clay minerals like kaolinite and feldspar) operates between −10°C and −20°C, and many common aerosols (sea salt, sulfate) are poor INPs requiring temperatures below −25°C. Effectiveness depends on how closely the particle's crystal surface matches ice's lattice structure. Most atmospheric aerosols are poor INPs.

Question 5

Why can't the Bergeron process operate in an all-liquid cloud, and what role does ice nucleation play in enabling it?

Model answer: The Bergeron process relies on the vapor pressure difference between supercooled liquid and ice at the same temperature — vapor flows from liquid to ice, growing ice crystals rapidly. If no ice crystals are present (all-liquid cloud), there is no vapor pressure differential to drive this transfer. Ice nucleation, triggered by INPs, creates the initial ice crystals that then exploit the vapor pressure gradient to grow at the expense of surrounding liquid droplets. Without INPs in the −10°C to −40°C range, mid-latitude precipitation via the Bergeron process cannot occur.

Ice nucleation is not merely a microphysical detail but a gate that controls whether precipitation forms at all. An all-liquid cloud at −15°C would need droplets to grow large enough to collide and coalesce (the warm-rain process), which is much slower. The Bergeron process is far more efficient precisely because the vapor pressure differential drives rapid ice crystal growth — but this only works once ice is present, which requires nucleation by suitable particles.