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What causes the seasons? Why are the seasons reversed in the Northern and Southern Hemispheres? And if the Earth moves in an elliptical orbit around the Sun, why don't we have two summers and two winters every year?
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.