Mineral dust is an effective ice nucleating particle chiefly because
Answer: B
A lattice match lowers the energy barrier to forming an ice embryo on the surface. Feldspar in particular is among the most efficient natural ice nucleating minerals known. Desert dust outbreaks can therefore alter cloud phase downwind.
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Assertion (A): Lattice matching is a useful guide to ice nucleating ability.
Reason (R): A surface resembling the ice lattice lowers the barrier to forming an ice embryo.
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.
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Three of the following are recognised ice nucleating particles. Which is the odd one out?
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.
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State whether the following is true or false.
Some bacteria produce proteins that nucleate ice at temperatures as warm as minus two degrees.
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.
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The concentration of ice nucleating particles active at a given temperature generally
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.
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Assertion (A): Ice crystal concentrations in real clouds often exceed the measured ice nucleating particle concentration.
Reason (R): Secondary ice production multiplies the crystals already present.
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.
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The Hallett Mossop process produces secondary ice through
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.
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The Hallett Mossop rime splintering process operates over roughly which temperature range?
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.
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Consider the following statements about secondary ice production.
1. It can raise crystal concentrations far above the nuclei available.
2. Rime splintering is the best established of the proposed mechanisms.
3. It requires no pre-existing ice in the cloud at all.
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.
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Three of the following are proposed secondary ice production mechanisms. Which is the odd one out?
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.
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