A seeding programme's cost effectiveness is usually argued on the basis of
Answer: A
Water supply and hydroelectric interests fund most operational programmes. The argument depends entirely on the assumed percentage increase, which is the very quantity in dispute. The economics rest on the contested scientific claim.
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Consider the following statements about operational seeding programmes.
1. Most are funded by water supply or agricultural interests.
2. Their economic case rests on an assumed percentage increase.
3. Their effectiveness has been established beyond dispute.
Answer: B
The first two are correct and describe how such programmes are justified. The third is contradicted by successive scientific assessments, which have remained cautious. Funding and evidence have moved in different directions.
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Arrange the steps of a properly designed seeding experiment in order.
1. verify by tracer that the agent reached the target
2. specify the physical hypothesis to be tested
3. analyse the accumulated cases statistically
4. randomise the decision to seed each qualifying case
Answer: C
Omitting any step undermines the conclusion, and the tracer verification is the one most often left out. Without it a null result cannot be interpreted physically. Verification is the step most often omitted in practice.
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The physical hypothesis in a seeding experiment should be specified in advance because
Answer: D
Correct answer: otherwise any result can be reinterpreted after the fact
A hypothesis stated in advance says what should happen, where and when, so the experiment can fail. Without it, whatever the data show can be described afterwards as the effect that was expected - which is why the physical hypothesis is as much a part of the design as the randomisation.
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Assertion (A): Post hoc subgroup analysis has undermined several seeding trials.
Reason (R): Selecting a favourable subset after the event guarantees an apparent effect.
Answer: A
Both are true and the reason gives the statistical failure, and reanalysis has overturned more than one claimed success. Pre-registration of the analysis is the accepted remedy. Several claimed successes have not survived reanalysis.
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Three of the following are elements of a credible seeding trial. Which is the odd one out?
Answer: B
Correct answer: selection of the most promising cases after the season
Choosing the cases afterwards is exactly what invalidates a trial: the selection can follow the result, and the effect claimed is then an artefact of the choosing. Randomisation, a hypothesis stated in advance and verification that the agent reached the target are the three elements that make a result credible.
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Seeding is most likely to be effective in clouds that are
Answer: C
There is little scope to improve a cloud that is already converting its water efficiently. Identifying the inefficient cases is a large part of operational practice. Case selection matters as much as the seeding itself.
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Assertion (A): Seedability is a property of the individual cloud rather than of the region.
Reason (R): Only clouds that precipitate inefficiently leave room for seeding to make a difference.
Answer: D
Both are true and the reason gives the criterion, and case selection therefore dominates programme design. A programme that seeds indiscriminately dilutes any effect it might have. Selectivity concentrates whatever signal exists.
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Consider the following statements about cloud selection for seeding.
1. Clouds already precipitating efficiently offer little scope.
2. Supercooled liquid water must be present for glaciogenic seeding.
3. Any cloud will respond equally well to seeding.
Answer: A
The first two are correct and are the operational criteria. The third is wrong, and indiscriminate seeding is among the reasons some programmes show no measurable effect. Treating unseedable clouds wastes both agent and evidence.
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Silver iodide flares release nuclei in numbers of the order of how many per gram of agent?
Answer: B
A single gram yields an enormous number of active nuclei at suitable temperatures. This is why the mass of agent required for a large operation remains comparatively modest. A few kilograms can treat a substantial cloud volume.
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