My SO asked a science-based question that I couldn’t come up with an answer for, which frankly surprised the hell out of me! Not that I’m any science genius but I consider myself a fairly well-informed layman. Just goes to show you never know as much as you think you do, and that anyone can ask a smart question.
The question is in the title … a bit of Googling shows a bunch of contradictory and fairly silly theories, including that there is more space between the air molecules to fit the water in.
I did find one sensible site that tries to offer an explanation but it seems to lose clarify in trying to be brief. At core it’s saying that the air doesn’t “hold” water vapour at all.
I think what it’s trying to say is that it’s not the air or air temperature per se that matter but the temperature of the water itself (whether vapour or not). That is, liquid water which is above freezing point will always evaporate (ie move from the liquid state to the gaseous state) at some rate. This obviously happens faster the closer it gets to 100c, and at some stage this happens faster than the opposite (condensation) occurs. So a volume of air gets warmer for whatever reason and the water in that same volume (whether existing as liquid on the ground or suspended in the air) starts to evaporate into the air at the same time because it too is getting warmer (possibly by radiation, conduction, or convection from that same volume of air, but also possibly from other sources like the ground nearby or sunlight, ie not specifically because the air it’s in or next to is warming).
When the air containing that vapour cools, it’s not that the air itself loses capacity to hold water, it’s that the water vapour which is a component (or perhaps co-habitant) of the air also cools, and thus more water vapour transitions to the liquid state than the other way around.
So clouds are liquid water that has condensed out of the water vapour in the air - not because the N[sub]2[/sub], O[sub]2[/sub] etc can hold less water, but because the water that is co-existing in that air is also cooling, and so is condensing from vapour to liquid state faster than vice versa. When that happens enough the liquid water drops are too heavy to remain suspended and fall as rain.
At core, I think he’s saying that the source of any heat that causes water to evaporate is irrelevant, all that matters are the competing rates of evaporation and condensation, which are driven by the temperature of the water itself, regardless of source.
So: do I have that right?