Ice Nucleation and Ice Crystal Growth Questions and Answers

Q: Consider the following statements about mixed phase clouds.1. They contain ice and supercooled liquid at the same time.2. They are thermodynamically unstable and tend to glaciate.3. They persist indefinitely once formed without change.

Correct answer: 1 and 2 onlyStatements 1 and 2 are correct: a mixed phase cloud contains both, and it is thermodynamically unstable for exactly the reason given by the Wegener–Bergeron–Findeisen process - the ice grows at the liquid’s expense. Statement 3 contradicts the second. Such clouds persist only where a continuing supply of liquid, usually from an updraught, replaces what the ice consumes, and once that supply fails the cloud glaciates.

Q: Match each crystal habit with the temperature at which it forms.List IA. PlatesB. NeedlesC. DendritesD. ColumnsList II1. near minus 15 degrees2. near minus 2 degrees3. near minus 30 degrees4. near minus 5 degrees

Correct answer: A-2, B-4, C-1, D-3The habit changes with temperature in a sequence that reverses twice. Plates form near minus 2, needles near minus 5, dendrites near minus 15, and columns again below about minus 22, near minus 30. The alternation between plate-like and column-like growth arises from which crystal face grows faster at each temperature, and it means the shape of a snow crystal records the temperature it grew at.

Q: Dendritic crystals form most readily at around

Correct answer: minus 15 degrees CelsiusAround minus 15 the difference between the saturation vapour pressures over water and ice is near its maximum, so a crystal in a water-saturated cloud grows fastest there and does so along the branching, dendritic habit. That combination - rapid growth and a large capacitance from the branches - makes the dendritic growth zone the layer where most snow mass is produced.

21.

The Wegener Bergeron Findeisen process depends on the fact that

A.saturation vapour pressure over ice is higher than over water
B.ice and water have identical saturation vapour pressures
C.ice crystals fall more slowly than water droplets
D.saturation vapour pressure over ice is lower than over water

Answer: D

Correct answer: saturation vapour pressure over ice is lower than over water

Ice holds its molecules more tightly than liquid water does, so at the same subzero temperature its equilibrium vapour pressure is lower. Air saturated with respect to water is therefore supersaturated with respect to ice, and the crystals grow by deposition while the droplets evaporate to feed them. The difference peaks near minus 12 degrees, which is why that region of a cloud glaciates fastest.

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22.

Assertion (A): In a mixed phase cloud, ice crystals grow at the expense of the surrounding droplets.
Reason (R): The air is supersaturated with respect to ice while being subsaturated with respect to water.

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

Answer: A

Both are true and the reason gives the vapour pressure difference driving the transfer. The droplets supply the vapour that the crystals consume, so the cloud glaciates over time. The transfer runs one way only, from liquid towards ice.

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23.

The difference between saturation vapour pressure over water and over ice is greatest at approximately

A.plus 10 degrees Celsius
B.minus 12 degrees Celsius
C.0 degrees Celsius
D.minus 40 degrees Celsius

Answer: B

The two curves coincide at zero and converge again at very low temperatures. The maximum difference near minus twelve is where the Bergeron process transfers water most rapidly. This is also where dendritic crystals grow most readily.

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24.

State whether the following is true or false.
The Wegener Bergeron Findeisen process requires both ice and supercooled liquid to be present.

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

Answer: C

Without liquid droplets there is no reservoir of vapour to transfer, and without ice there is nothing to grow. A fully glaciated cloud can no longer operate the process at all. Glaciation therefore extinguishes the very process that caused it.

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25.

Consider the following statements about mixed phase clouds.
1. They contain ice and supercooled liquid at the same time.
2. They are thermodynamically unstable and tend to glaciate.
3. They persist indefinitely once formed without change.

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

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: a mixed phase cloud contains both, and it is thermodynamically unstable for exactly the reason given by the Wegener–Bergeron–Findeisen process - the ice grows at the liquid’s expense. Statement 3 contradicts the second. Such clouds persist only where a continuing supply of liquid, usually from an updraught, replaces what the ice consumes, and once that supply fails the cloud glaciates.

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26.

Assertion (A): Mixed phase clouds can persist for hours despite being thermodynamically unstable.
Reason (R): Continuing updraught keeps forming new droplets as fast as the ice consumes them.

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

Answer: A

Both are true and the reason gives the sustaining mechanism, and Arctic stratocumulus are the classic long lived example. Without that supply the cloud would glaciate within minutes. Persistent mixed phase cloud requires continual droplet formation.

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27.

Diffusional growth of an ice crystal is faster than that of a droplet of the same mass because

A.latent heat of sublimation is smaller than that of condensation
B.the crystal is not spherical and has a larger capacitance
C.ice has a higher saturation vapour pressure than water
D.the crystal falls faster and ventilates more strongly

Answer: B

Growth depends on the electrostatic capacitance of the shape, and a plate or dendrite exceeds a sphere considerably. The larger vapour excess over ice adds to the advantage. Both geometry and thermodynamics favour the crystal over the drop.

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28.

Assertion (A): The growth equation for an ice crystal uses a capacitance in place of a radius.
Reason (R): The vapour diffusion problem is mathematically identical to an electrostatic one.

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

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the justification. Steady-state vapour diffusion to a crystal obeys Laplace’s equation with a fixed value on the surface, which is exactly the electrostatic problem of a charged conductor, so the geometry enters through the electrical capacitance of that shape. It is why a branched dendrite grows faster than a compact crystal of the same mass: capacitance depends on shape, not merely on volume.

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29.

Match each crystal habit with the temperature at which it forms.
List I
A. Plates
B. Needles
C. Dendrites
D. Columns
List II
1. near minus 15 degrees
2. near minus 2 degrees
3. near minus 30 degrees
4. near minus 5 degrees

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

Answer: D

Correct answer: A-2, B-4, C-1, D-3

The habit changes with temperature in a sequence that reverses twice. Plates form near minus 2, needles near minus 5, dendrites near minus 15, and columns again below about minus 22, near minus 30. The alternation between plate-like and column-like growth arises from which crystal face grows faster at each temperature, and it means the shape of a snow crystal records the temperature it grew at.

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30.

Dendritic crystals form most readily at around

A.minus 15 degrees Celsius
B.minus 2 degrees Celsius
C.minus 40 degrees Celsius
D.plus 2 degrees Celsius

Answer: A

Correct answer: minus 15 degrees Celsius

Around minus 15 the difference between the saturation vapour pressures over water and ice is near its maximum, so a crystal in a water-saturated cloud grows fastest there and does so along the branching, dendritic habit. That combination - rapid growth and a large capacitance from the branches - makes the dendritic growth zone the layer where most snow mass is produced.

Discuss in Forum

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Ice Nucleation and Ice Crystal Growth Questions and Answers

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