If humid air is just air plus water, then it has to be heavier than dry air, right? Sure, if it was only a matter of simple addition, but molecular physics is a lot like a bouncer at a club: Nothing gets in unless something else goes out.
Who likes getting caught in a downpour without an umbrella? Not this guy and not us. Are we ever going to achieve rainmaker status so we can dial up a few gentle showers one day and a blast of sunshine the next?