Ice Nucleation and Ice Crystal Growth Questions and Answers

Answer: C

Both are true and the reason gives the principle, and silver iodide was chosen for seeding on exactly this basis. Surface defects and chemistry matter as well, so the match alone does not settle it. Empirical measurement remains necessary for any candidate material.

Answer: D

Sea salt is an excellent condensation nucleus but a poor ice nucleus, since it dissolves rather than presenting a lattice. The other three are all established sources of atmospheric ice nuclei. Solubility helps a condensation nucleus but hinders an ice nucleus.

Answer: A

Certain strains carry surface proteins that mimic the ice lattice remarkably well. They are the most efficient known ice nucleators and are used commercially in snow making. Their efficiency far exceeds that of any known mineral surface.

Answer: B

More of the aerosol population becomes capable of nucleating as supercooling deepens. Empirical relations expressing this dependence are widely used in models, though they carry large uncertainties. Measurements of these particles vary widely between instruments.

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 resolution of the discrepancy. Measured ice nucleating particle concentrations can be several orders of magnitude below the crystal concentrations observed in the same cloud, and primary nucleation cannot make up the difference. Secondary production - rime splintering, drop shattering, collisional fragmentation - multiplies the existing crystals, so the population is not limited by the nuclei available.

Answer: D

Correct answer: splinters ejected during the riming of supercooled droplets

In the Hallett–Mossop process a supercooled droplet freezes onto a rimer, and the freezing shell traps liquid that expands and ejects small splinters. Each splinter is a new ice crystal, so the process is a multiplier. It operates in a narrow window between about minus 3 and minus 8 degrees and requires droplets of both large and small sizes to be present, which is why it dominates in some clouds and not others.

Answer: A

The window is narrow and requires both large and small supercooled droplets to be present. Outside that range the mechanism produces essentially no splinters at all. The narrow window makes it sensitive to small temperature errors.

Answer: B

The first two are correct and describe the process. The third is wrong by definition, since secondary production multiplies existing ice rather than creating the first crystal from nothing. A primary nucleation event is always required to start the chain.

Answer: C

Correct answer: deposition nucleation on mineral dust

Deposition nucleation is a *primary* mechanism: vapour deposits directly onto a nucleating particle to form ice where there was none. The other three all create new ice from ice that already exists - fragmentation on collision, shattering of freezing drops, and rime splintering - which is what makes them secondary.

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.