**Ask a monkey a physics question thread**

[MadMojoMonkey]

Hmm. I assumed (I know you love that word) that the presence of water vapour in the air would make it more dense, not less.

I don't know of anyone who got this right the first time they guessed. Myself included.

I don't hate assumptions. I use them all the time.
(The ideal gas law is an assumption. There is no gas that acts exactly like an "ideal" gas.)

I just love it when the assumptions are explicitly stated, if not implied by the question.

Well cooler air is certainly denser than warm air, which is why hot air rises.

Bless you for the wording. I have a pet peeve with the phrase, "hot air rises." There is no mystical anti-gravity power that makes heat rise. It's the cooler, denser stuff that is pulled down by gravity, displacing the warmer stuff.

In fairness, even other physicists think I'm going overboard with nit-picking on this.

Ok, the increase in air density above the river caused by the cooling effect of evaporation will need to be more than sufficient to account for the reduction in density caused by the increase in humdity. Do your not-assuming thing. Numbers and whatnot.

lol

Surface water temp / ambient air temp and humidity / wind
I think it's tractable with some assumptions.

I'm actually curious how much cooler it is. Anecdotally... Riding a motorcycle across a river valley is a noticeable shift in temp and humidity while I'm in the lowlands between the river bluffs.

Ok so I've thought this through for a minute or two while smoking.

Water evaporates from the river, cooling the air. The cooling makes the air denser, but the humidity makes the air less dense, so there is a battle. I propose that the humidity will tend to rise perpendicular to the ground, as it is less dense, while cool less humid air moves outward, parallel to the ground, sucked into a lower pressure area away from the river. Warm dry air will be sucked in at a 45 degree angle to continue the cycle.

IDK about the 45 degree angle. It could just come in from the side, above the cool air moving out. Wind is going to alter the angle, at any rate. The atmosphere is fairly thin sheet in cross-section.

Other than that... bravo, sir.

The splitting of the humidity and the cooled air seems like a good model to work with. I can try to make some very loose approximations of the effect, but fluid dynamics is intense. Most fluid dynamics is done computationally. The people who work on this generally have a dozen or so computers working on a single problem for days or weeks just to get a result. The result is sensitive to the software settings, so it can take a while to get the result you want.

I'm saying... I can give a "back of an envelope" idea of this, but it's probably going to be so vague as to be common sense.
That's my hedge.
We'll see.