Growth by Collision and Coalescence Questions and Answers

Answer: A

Both are true and the reason gives the cause, and the cloud must grow deep enough to glaciate before it can rain. This is part of why continental convection tends to be deeper before precipitating at all. Maritime convection rains from much shallower clouds.

Answer: B

Correct answer: 2 - 4 - 1 - 3

The history runs from a nucleus to a raindrop and out again. A droplet activates on a condensation nucleus, grows by diffusion of vapour until that becomes too slow, then grows by collecting other droplets, and finally breaks up on reaching the aerodynamic size limit.

Answer: C

Correct answer: a collector radius below which collision efficiency vanishes

Below about eighteen to twenty micrometres a collector has too little inertia relative to the flow around it, and droplets are carried past rather than striking it. Collection therefore cannot begin until some droplets have reached that size by condensation, which is why the two growth mechanisms operate in sequence rather than together.

Answer: D

The first two are correct and define the difficulty. The third is exactly what does not hold, since neither mechanism works well in the range between them and something else must bridge it. Turbulence, giant nuclei and mixing are the leading candidates.

Answer: A

Salt particles of several micrometres produce droplets already past the collection threshold at cloud base. Concentrations of only a few per litre can measurably shorten the time to first rain. Their rarity is offset by how quickly they begin collecting.

Answer: B

Because collection accelerates so strongly with size, a handful of early collectors can seed the whole process. Their scarcity is more than compensated by the speed at which they grow. Collection accelerates as the fourth power of collector radius.

Answer: C

Spectral breadth concerns how the water is divided, not how much there is. The other three all follow directly from droplets of differing sizes coexisting in the same volume. Water content is a separate quantity from spectral shape.

Answer: D

The kernel depends on the difference between the two fall speeds, not on their sum or their mean. The large disparity here is what makes such a pair an efficient collection partnership. Relative motion, not absolute fall speed, drives the kernel.

Answer: A

Averaging over the population hides the fortunate drops that collect several times early on. Those outliers are precisely the ones that reach precipitation size first and set the timing of rain. The tail of the distribution governs when rain appears.

Answer: B

Both are true and the reason gives why the mean misleads, and rain onset is governed by the tail of the distribution. This was the central insight that made stochastic collection theory necessary. The mean is the wrong statistic for a threshold process.