Questions: Bergeron Process and Ice Crystal Precipitation

Question 1

A mixed-phase cloud at −15°C contains both supercooled liquid droplets and a small number of ice crystals. The air is at exactly the saturation vapor pressure for liquid water. What happens to the droplets and crystals over the next 15–20 minutes?

• A. Nothing changes — the cloud is in thermodynamic equilibrium since the air is at saturation
• B. The supercooled droplets gradually freeze as small temperature fluctuations push them below the homogeneous nucleation threshold
• C. The ice crystals grow by vapor deposition while the liquid droplets evaporate, because air saturated with respect to liquid is supersaturated with respect to ice
• D. The ice crystals melt and the droplets grow, because temperatures above −20°C favor the liquid phase

Answer: (C)

This is the Bergeron process in action. The key physical fact is that the saturation vapor pressure over ice is lower than over liquid water at the same subfreezing temperature. Air at liquid saturation is therefore supersaturated with respect to ice — vapor deposits onto ice crystals faster than it leaves them. As ice crystals consume vapor, the vapor pressure drops below liquid saturation, forcing droplets to evaporate to restore equilibrium. Mass transfers from liquid to ice via the vapor phase. The droplets do not freeze; they evaporate. This transfer is remarkably fast — a single ice crystal can grow to precipitation size in 15–20 minutes.

Question 2

Warm maritime clouds with bases near sea level and tops that barely reach 0°C produce heavy rainfall in tropical regions. Which precipitation mechanism dominates in these clouds, and why?

• A. The Bergeron process, because ice crystals always nucleate at the very top of any cloud that reaches freezing temperature
• B. Collision-coalescence of liquid droplets, because the cloud lacks the extensive subfreezing layer of coexisting liquid and ice needed for the Bergeron process to operate efficiently
• C. Both mechanisms operate with equal contribution — the Bergeron process above the freezing level and collision-coalescence below
• D. Neither mechanism — tropical rain requires strong updrafts to loft droplets high enough to encounter ice

Answer: (B)

The Bergeron process requires a mixed-phase zone where supercooled liquid droplets and ice crystals coexist. Clouds with tops barely at 0°C have little or no such zone. Warm maritime tropical clouds instead produce 'warm rain' entirely through collision-coalescence: large cloud droplets collide with smaller ones, grow, and eventually become heavy enough to fall. This is the dominant precipitation mechanism in the tropics where cloud tops rarely reach deep subfreezing temperatures. Bergeron dominates in mid-latitude and polar clouds where extensive cold layers allow mixed-phase conditions to persist.

Question 3

In the Bergeron process, supercooled liquid droplets freeze directly to form ice crystals, which then grow by aggregating with other crystals.

• T. True
• F. False

Answer: False

This is the most common misconception about the Bergeron process. Supercooled liquid droplets do not freeze — they evaporate. The water vapor produced by evaporation is then deposited directly onto existing ice crystals. The entire mass transfer happens through the vapor phase. The ice crystals remain solid throughout; only their size changes as vapor deposits on them. This vapor-phase transfer is what makes the process so efficient: ice crystals grow rapidly without needing to collide with other droplets or crystals.

Question 4

Cloud seeding with silver iodide can enhance precipitation from supercooled clouds by introducing artificial ice nuclei that trigger the Bergeron process.

• T. True
• F. False

Answer: True

Silver iodide has a crystal structure similar to ice and is an effective ice nucleus even at temperatures as warm as −4°C. Introducing it into a supercooled cloud creates many ice crystals where few existed before, triggering the vapor pressure imbalance that drives the Bergeron process — ice crystals grow rapidly at the expense of liquid droplets. However, this only works if the cloud contains sufficient supercooled liquid water to begin with. Seeding a cloud that is already glaciated (all ice) or has too little liquid water produces no enhancement.

Question 5

Explain why introducing a small number of ice crystals into a supercooled liquid cloud triggers rapid ice crystal growth, without any change in the cloud's temperature.

Model answer: The saturation vapor pressure over ice is lower than over liquid water at the same subfreezing temperature. A cloud in equilibrium with its liquid droplets (air at liquid saturation) is therefore supersaturated with respect to ice. When ice crystals are introduced, vapor deposits onto them faster than it leaves, so the crystals grow rapidly. As they consume water vapor, the air pressure drops below liquid saturation, forcing some droplets to evaporate to restore equilibrium. This self-sustaining cycle transfers mass from droplets to ice crystals entirely through the vapor phase — no temperature change is required. The driving force is purely the thermodynamic difference in vapor pressure between the two phases.

The magnitude of the vapor pressure difference between liquid and ice peaks at around −10 to −20°C, which is why the Bergeron process is most efficient in clouds with tops in that temperature range. Below about −40°C, spontaneous homogeneous nucleation freezes all remaining liquid, removing the mixed-phase condition necessary for the process. Above 0°C, there is no ice phase. The Bergeron process therefore operates in a specific temperature window where mixed-phase conditions can be sustained.