Ice Nucleation and Ice Crystal Growth Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.