A cloud is placed in exactly one genus because
Answer: C
The scheme is designed so that a given cloud belongs to one genus and no other, in the manner of a biological classification. Several genera may of course be present in the sky at the same time. The exclusivity applies to a single cloud, not to the sky as a whole.
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Match each genus with its étage.
List I
A. Cirrostratus
B. Altocumulus
C. Stratocumulus
D. Cumulonimbus
List II
1. low
2. high
3. spanning all three
4. middle
Answer: D
Cumulonimbus is the classic example of a cloud of great vertical extent, with a base in the low étage and a top that may reach the tropopause. Its assignment to a single étage is therefore a convention rather than a description.
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Stratiform clouds differ from cumuliform clouds principally in that stratiform clouds
Answer: A
The distinction reflects the kind of ascent that produced them, gentle and widespread against vigorous and localised. It is the most useful single division in the whole classification because it points directly at the physics.
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Assertion (A): The stratiform and cumuliform division reflects how the cloud was formed.
Reason (R): Layered cloud arises from slow widespread lifting and heaped cloud from local convection.
Answer: B
Both are true and the reason gives the underlying process, and the appearance follows from it rather than being an arbitrary label. Knowing which kind is present tells an observer a good deal about the state of the atmosphere.
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All of the following genera commonly produce precipitation reaching the ground EXCEPT
Answer: C
Cirrostratus is a thin ice cloud of the high étage and any fall from it evaporates long before reaching the surface. Stratus is included because it does yield drizzle, even if only lightly. Steady moderate rain over hours is nearly always nimbostratus.
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The halo phenomenon around the sun is most characteristically produced by
Answer: D
Correct answer: cirrostratus
A halo requires ice crystals, and cirrostratus is the thin, uniform ice cloud that veils the whole sky - often so tenuous that the halo is the only sign it is there. Altostratus is thicker and dims the sun to a watery disc, while stratocumulus and cumulus are water droplet clouds and produce a corona rather than a halo.
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Assertion (A): A halo indicates an ice crystal cloud rather than a water droplet cloud.
Reason (R): The halo arises from refraction through hexagonal ice crystals of a particular geometry.
Answer: A
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the optics. The common 22-degree halo is produced by refraction through the sixty-degree prism formed by alternate faces of hexagonal ice crystals, and water droplets cannot do it - being spherical, they produce a corona of coloured rings close around the sun instead. A halo is therefore a reliable statement about the phase of the cloud.
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State whether the following is true or false.
Nimbostratus is classified as a middle level cloud although its base is often low.
Answer: B
Correct answer: True
Nimbostratus is placed in the middle étage because that is where its greatest depth lies and where it is generated, even though the base is frequently low and ragged and the cloud may extend through all three levels. It is the deep, grey, precipitating sheet of a warm front, and the classification here follows the body of the cloud rather than its base.
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Lenticular clouds form characteristically
Answer: C
Correct answer: in the wave crests downwind of a mountain range
Stable air forced over a mountain range oscillates downwind in a train of standing waves, and cloud forms at each crest where the air is lifted and cooled. Because the wave is stationary while the air flows through it, the cloud appears motionless in a strong wind, continuously forming at its upwind edge and evaporating at the downwind one.
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Assertion (A): A lenticular cloud can remain stationary while a strong wind is blowing.
Reason (R): It marks a fixed wave crest through which the air is continuously flowing.
Answer: D
Both are true and the reason gives the apparent paradox, and the cloud is being formed at one edge and destroyed at the other continuously. The shape rather than the substance is what stays put. Pilots treat them as a reliable marker of turbulent mountain wave activity.
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