Randomised seeding trials commonly require how long to reach a statistically useful conclusion?
Answer: C
The effect sought is small relative to natural variability, so the sample must be large. Several of the best known trials ran for a decade or more. Short programmes cannot settle the question. A decade of cases is a realistic requirement.
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Three of the following are genuine obstacles to demonstrating a seeding effect. Which is the odd one out?
Answer: D
Precipitation can be measured perfectly adequately by gauge and radar. The other three are the genuine difficulties that make evaluation so hard. Measurement itself is not among the problems. Gauges and radar are entirely adequate for the task.
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Targeting failure in a seeding operation means that
Answer: A
Tracer studies have repeatedly shown agent drifting away from the intended volume. A null result may therefore reflect a delivery failure rather than a failure of the underlying physics. Verification of delivery is therefore essential.
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Assertion (A): A null result in a seeding trial does not necessarily disprove the physical hypothesis.
Reason (R): The agent may never have reached the supercooled region it was meant to treat.
Answer: B
Both are true and the reason gives the alternative explanation, and this is why tracer verification matters. Distinguishing a failed delivery from a failed hypothesis is a central difficulty. Tracer studies exist precisely to separate the two.
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Tracer studies in seeding programmes are used to
Answer: C
An inert tracer released with the agent can be detected later in the target volume. Without such verification a programme cannot tell whether its agent arrived where it was meant to. An unverified delivery leaves the result uninterpretable.
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State whether the following is true or false.
Some national assessments have concluded that operational rain enhancement remains scientifically unproven.
Answer: D
Major reviews have repeatedly found the evidence for operational programmes inconclusive. Orographic winter seeding has the strongest case among the various applications. Convective enhancement remains considerably weaker.
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Consider the following statements about the evidence for seeding.
1. Orographic winter seeding has the strongest supporting evidence.
2. Convective rain enhancement remains substantially unproven.
3. All forms of seeding are equally well established.
Answer: A
The first two are correct and reflect the balance of assessments. The third flattens a genuine distinction, since the orographic case is far better constrained than the convective one. Targeting and comparison are both easier there.
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Assertion (A): Orographic seeding is easier to evaluate than convective seeding.
Reason (R): The cloud is anchored to fixed terrain, so targeting and comparison are far more tractable.
Answer: B
Both are true and the reason gives the advantage, and the seeded volume can be identified reliably. Convective clouds move and evolve too quickly for comparable control. A stationary target is a large practical advantage.
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Claimed precipitation increases from orographic seeding are typically of the order of
Answer: C
Correct answer: five to fifteen per cent
This is the range claimed by orographic programmes, and it is worth noticing what it implies for evaluation: an effect of that size is far smaller than the natural variability from one season to the next, so many seasons of randomised operation are needed before it can be distinguished from chance.
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Weather modification raises legal questions chiefly because
Answer: D
Downwind communities may claim that their rainfall has been diverted. The difficulty of attribution cuts both ways, making both claims and defences hard to establish. Attribution defeats litigants on both sides equally.
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