I question one piece of what you're saying. I'm not sure that after a rain, the water vapor remains higher. I'll concede that it CAN but not that it is a given. I think it depends on the specific storm and how it played out.
What this would mean is that if excess heat was received from the sun, it would always lead to higher vapor content which would always lead to higher insulation against thermal heat leaving to space which would lead to more increased heat and more insulation and so on. In short, it would mean a runaway condition with the first occurrence of higher than normal solar activity. So either the water cycle is a self regulating process, which is what I've always been told, or there is some other factor mitigating the runaway situation. Occam's Razor suggests that rain is the great balance to this potentially catastrophic imbalance.
Another (possibly small but worth mentioning?) factor may be that rain does not always occur JUST by the content surpassing the saturation point. If this were the case, cloud seeding would not work. Is it possible that CO2 could affect this process, either by offering contamination for rain to form on or by insulating it from them as in a MIG welder's gas envelope?
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