The effectiveness of a silver iodide nucleus depends strongly on
Answer: C
Correct answer: the temperature at which it is asked to act
Silver iodide begins to nucleate at about −5 degrees and becomes effective at −8 to −10, so the same particle is useless in warmer cloud and active in colder. Knowing where in the cloud the temperature falls in that window is therefore the central targeting problem.
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State whether the following is true or false.
Releasing silver iodide into cloud warmer than about minus four degrees achieves little.
Answer: D
Correct answer: True
Above about −4 or −5 degrees the agent nucleates almost nothing, so releasing it there accomplishes nothing whatever. Seeding requires the target volume to be colder than the agent’s threshold, which is why the vertical temperature structure of the cloud must be known before an operation.
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All of the following must be true for a glaciogenic seeding operation to have any chance of success EXCEPT
Answer: A
Correct answer: the cloud is already producing heavy precipitation
A cloud already precipitating heavily has nothing to gain: its water is already being converted. Seeding is plausible only where the process is limited by a shortage of ice nuclei, so supercooled water must be present, the temperature must be below the agent’s threshold, and the agent must actually reach it.
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Weather modification research declined in many countries after the 1970s chiefly because
Answer: B
Early optimism was not sustained by the results of carefully designed experiments. Interest has revived somewhat with better instrumentation and with growing pressure on water supplies. Water scarcity has revived interest independently of the science.
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Assertion (A): Interest in seeding has revived in recent decades despite the earlier disappointments.
Reason (R): Better instrumentation and growing water scarcity have both changed the calculation.
Answer: C
Both are true and the reason gives the two drivers, and modern radar and tracer techniques address some earlier weaknesses. The fundamental evaluation problem nonetheless remains. Better instruments do not supply a counterfactual.
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Consider the following statements about modern seeding research.
1. Improved radar and tracer methods address old weaknesses.
2. The fundamental problem of the missing counterfactual persists.
3. Modern methods have removed the need for randomisation.
Answer: D
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: radar, tracers and radiometers have addressed the old complaint that nobody knew whether the agent reached the cloud, while the fundamental difficulty remains - nobody can observe what the same cloud would have done unseeded. Statement 3 inverts the consequence, since it is precisely the missing counterfactual that makes randomisation indispensable.
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Physical rather than statistical evidence for seeding effects comes from
Answer: A
Tracking the ice produced in a seeded plume demonstrates the chain of causation directly. Such studies have provided the strongest evidence available for orographic seeding. Each link in the chain was observed separately.
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Assertion (A): Direct observation of a seeded plume is more persuasive than a statistical result alone.
Reason (R): It demonstrates each link in the causal chain rather than merely a correlation.
Answer: B
Both are true and the reason gives the advantage, and recent orographic experiments have done precisely this. Statistics alone can establish association but not mechanism. Physical observation closes the gap statistics leaves.
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Three of the following provide physical evidence of a seeding effect. Which is the odd one out?
Answer: C
A seasonal total confounds seeding with every other influence on the weather. The other three each observe a specific link in the causal chain. A seasonal total confounds every influence at once. Only a plume observation isolates the seeding itself.
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The main practical reason orographic programmes dominate operational seeding is that
Answer: D
Correct answer: the cloud is anchored to fixed terrain and is easy to target
An orographic cloud forms where the air is lifted over the ridge and stays there while the wind blows, so ground generators in fixed positions can seed it reliably and the target area does not move. A convective cell must be found, intercepted and treated within minutes, which is far harder and far more expensive.
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