Ice Nucleation and Ice Crystal Growth Questions and Answers

Q: When ice cystals collide with supercooled liquid droplets, upon contact, the liquid droplets freeze into ice and stick to the ice crystal. A process is called :

The process is accretion (riming).When an ice crystal falls through a cloud of supercooled droplets, the droplets it hits freeze on contact and stick to it. This growth by accretion produces rime and, carried far enough, graupel and hail.The Bergeron process is growth of ice crystals by vapour diffusion at the expense of droplets, without collisions. Coalescence is the merging of liquid drops, not freezing on ice, and nucleation is the initial formation of a droplet or crystal.

When ice cystals collide with supercooled liquid droplets, upon contact, the liquid droplets freeze into ice and stick to the ice crystal. A process is called :

A.Bergeron process
B.Accretion
C.Coalescence
D.Nucleation

Exam Year(s)

MH SET - 2021, SET Exam - 2021,

Answer: B

The process is accretion (riming).

When an ice crystal falls through a cloud of supercooled droplets, the droplets it hits freeze on contact and stick to it. This growth by accretion produces rime and, carried far enough, graupel and hail.

The Bergeron process is growth of ice crystals by vapour diffusion at the expense of droplets, without collisions. Coalescence is the merging of liquid drops, not freezing on ice, and nucleation is the initial formation of a droplet or crystal.

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Supercooled water in a cloud is water that is

A.still liquid at a temperature below zero degrees
B.frozen at a temperature above zero degrees
C.at exactly zero degrees and partly frozen
D.warmer than the air immediately around it

Answer: A

Correct answer: still liquid at a temperature below zero degrees

Pure water does not freeze at zero - it needs a surface to nucleate on, and in the absence of one it can remain liquid to about minus 38 degrees, where homogeneous freezing takes over. Cloud droplets are small and often clean, so supercooled liquid is normal rather than exotic, and it is what makes mixed phase clouds, aircraft icing and freezing rain possible.

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Assertion (A): Cloud droplets can remain liquid at temperatures far below zero.
Reason (R): Freezing requires a nucleus, and a small pure droplet may contain none.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

Answer: B

Both are true and the reason gives the requirement, and the smaller the droplet the less likely it is to contain a suitable particle. This is why supercooled liquid persists in clouds where bulk water would have frozen long before.

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Homogeneous freezing of pure water droplets occurs at approximately

A.minus 10 degrees Celsius
B.minus 100 degrees Celsius
C.minus 40 degrees Celsius
D.0 degrees Celsius

Answer: C

Below about minus forty the ice embryo forms spontaneously without any foreign particle. Above that temperature freezing in the atmosphere requires an ice nucleating particle of some kind. Such particles are far too scarce to freeze every droplet.

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State whether the following is true or false.
Above minus forty degrees, freezing in a cloud requires an ice nucleating particle.

A.False
B.True only at large scales
C.False only at small scales
D.True

Answer: D

Homogeneous nucleation is negligible at those temperatures, so heterogeneous nucleation is the only available route. The scarcity of such particles is why supercooled clouds are so common. Most cloud droplets never encounter a suitable particle at all.

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Ice nucleating particles differ from cloud condensation nuclei chiefly in that they are

A.far scarcer and effective only at low temperatures
B.far more numerous and effective at any temperature
C.always soluble in water rather than insoluble
D.unable to influence cloud microphysics at all

Answer: A

Condensation nuclei number in the hundreds per cubic centimetre while ice nucleating particles are often fewer than one per litre. Their scarcity is the reason ice forms in only a small fraction of the available droplets.

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Assertion (A): Ice forms on only a tiny fraction of the droplets in a supercooled cloud.
Reason (R): Ice nucleating particles are orders of magnitude scarcer than condensation nuclei.

A.A is false but R is true
B.Both A and R are true, and R is the correct explanation of A
C.Both A and R are true, but R is not the correct explanation of A
D.A is true but R is false

Answer: B

Both are true and the reason gives the cause, and this scarcity is exactly what makes the ice phase so effective at producing precipitation. A few crystals grow rapidly among a great many droplets. Concentrating water into few particles is what produces precipitation.

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Match each heterogeneous ice nucleation mode with its description.
List I
A. Deposition
B. Condensation freezing
C. Immersion freezing
D. Contact freezing
List II
1. a particle already inside a droplet initiates freezing
2. vapour deposits directly as ice on a particle
3. a particle collides with a supercooled droplet
4. a droplet forms then freezes on the same particle

A.A-2, B-4, C-3, D-1
B.A-3, B-1, C-4, D-2
C.A-2, B-4, C-1, D-3
D.A-4, B-2, C-1, D-3

Answer: C

The four modes differ in whether the particle is inside, outside or acting on vapour directly. Immersion freezing is generally regarded as the most important of them in mixed phase clouds. The four modes differ markedly in their temperature dependence.

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Contact freezing occurs when

A.a particle already suspended within a droplet triggers freezing
B.vapour deposits directly onto an aerosol particle as ice
C.two ice crystals collide and stick together
D.a particle collides with the surface of a supercooled droplet

Answer: D

The collision itself initiates freezing at a warmer temperature than immersion would. Contact nucleation is efficient per event but is limited by how often such collisions actually occur. Collision rates in a cloud are low, so the mode contributes modestly.

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10.

Consider the following statements about immersion freezing.
1. The nucleating particle is already inside the droplet.
2. It is widely regarded as the dominant mode in mixed phase clouds.
3. It requires the particle to strike the droplet from outside.

A.1 and 2 only
B.3 only
C.1 only
D.All of them

Answer: A

The first two are correct and describe the mode. The third describes contact freezing instead, which is a different mechanism with a different temperature dependence. Distinguishing the modes matters for parameterising ice formation.

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