The sentence in question is not an intermediary conclusion because it is not supported by any facts in the passage.
The meteorologist concludes that heavy downpours are likely to become more frequent if Earth's atmosphere becomes significantly warmer.
Why? Because rain clouds will form more quickly and there will be larger clouds which generally cause heavier downpours.
The only other information that we learn about this is that warmer atmospheres hold more moisture, creating larger clouds. This does not support the conclusion that larger clouds are likely to cause heavier downpours. Therefore, the statement in question is a premise (or a general principle) used to support the conclusion, but is not a conclusion in and of itself.
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