Is there any combination of factors that would allow the average human to walk on a cloud without the aid of magic or technology? If so, could buildings and cities be constructed on clouds in those conditions?
By "the aid of magic or technology," I mean only that the aid in question is actively or passively causing the appearance of the user walking on the cloud. Aid in the form of protection against the environment is acceptable, but a jet pack is not.
... Not really, I'm afraid. Clouds are just water vapour, they would put up very little resistance at all if you were to step onto one. So, outside of magic or technology, no, no way on Earth.
Which is really begging the question: are we talking about Earth? The physics of other planets could be different. But if you want humans to walk about unprotected, I'm afraid it'd have to be so close to Earth that the answer is the same: without magic, or technology, clouds are quite literally as insubstantial as mist.
I've thought about it some more, and I think the answer is still no, for any reasonable definition of "cloud". The reason is, for a cloud to be substantial enough to walk on (let alone build on) there would have to be some force holding the cloud together, and that force would have to be stronger than the force of you (or your building) forcing it apart. For vapour (of whatever substance) there just isn't enough force keeping the particles together: effectively, you're just standing on air.
Now, that does raise another possibility: what if you were less dense than the air? Well, then you'd float on top of it! (Or rather, you'd float at a particular altitude where your buoyancy cancelled out the force of gravity, but that's about the same.) Ignoring the clouds entirely for a moment, that would almost get you what you're asking for, but it would be like floating on a lake, not like standing on solid ground. However, there are floating buildings in our world, so you could just build a platform that floated in the air, and stand on that!
Now, that does sound a lot like technology, but what if there was some natural material floating in the atmosphere too? That could form a natural platform that perhaps you could walk on, and that might meet your needs. It wouldn't be a cloud: if it were just vapour, you would sink into it and float, as if it wasn't there; but it might be good enough, and your characters could stand on it and even build on it if it was substantial enough.
However, there are floating buildings in our world, I would like to see these "floating buildings" of yours :P.
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Commented
Apr 23, 2015 at 22:11
Based on this question/answer: https://physics.stackexchange.com/questions/25918/
There is a place you could kind of do this, namely one of the gas giants. If you fell into a gas giant you would fall until you hit a point where the cloud density matched your own density. If you were wearing an unubtainium suit that kept you from being crushed, you wouldn't really be able to walk, but you would bob about nicely like a cork on the water.
If you think about it, water is much much denser than cloud, and it's pretty hard to walk on. You can do it with water walking shoes which are essentially feet boats.
So could you make shoes that let you walk on clouds, without using the principle of lift? On Jupiter, maybe... down where the clouds get as dense as water.
EDIT: More thoughts...
So, why can't we walk on water? The problem is that the water molecules are loose; they move about freely, this way and that, and when something pushes against them, like the bottom of your foot, they just move out of the way.
But sometimes an outside event can cause the molecules to become fixed in place, frozen if you will, and when that happens they don't move when pressure is placed against them.
Clouds are even less cohesive than water because the density is a lot lower, and because the density of ice is greater than air, there aren't really any natural ways to freeze a cloud on an Earth like planet, but there may be ways to do it if the atmosphere was very alien.
Straying into the magic technology arena, there might be a way to generate a field that holds the cloud molecules in place (magic), or the clouds are made of aerogel (technology), but without that you're going to have to have a very dense atmosphere, where some of it can freeze out and float up above the surface like a cloud.
If something like this is possible, the best candidate in our solar system is Uranus.
As on Earth, the atmosphere of Uranus is broken into layers, depending upon temperature and pressure. Like the other gas giants, the planet doesn't have a firm surface. Scientists define the surface as the region where the atmospheric pressure exceeds one bar, the pressure found on Earth at sea level.
Just above the "surface" of Uranus lies the troposphere, where the atmosphere is the densest. The temperature ranges from minus 243 degrees Fahrenheit (minus 153 degrees Celsius) to minus 370 F (minus 218 C) , with the upper regions being the coldest. This makes the atmosphere of Uranus the coldest in the solar system. Within the troposphere are layers of clouds — water clouds at the lowest pressures, with ammonium hydrosulfide clouds above them. Ammonia and hydrogen sulfide clouds come next. Finally, thin methane clouds lay on the top. The troposphere extends 30 miles (50 kilometers) from the surface of the planet.
So if you were going to have clouds that were solid enough to walk on, that would be the best place to start.
There's nothing that would let a human walk on clouds even with the aid of technology. Clouds aren't the small, compact things they look like from the ground, they're massive things sometimes spread out across miles. If you look at one up close, it doesn't look like a solid structure with clearly defined edges, it looks like a roughly defined mist. Which gives us the key problem even if we ignore the fact that they're not strong enough to hold you up. If you actually get up there, there's no ground-analogue to walk on.
Part one of your question is impossible, but part two (can you build cities and buildings in the clouds) oddly enough isn't with a slight adjustment to your thinking.
Buckminster Fuller, who invented the Geodesic dome, observed that as you double the radius of the dome, the interior volume increases 8X. After a certain point, the mass of the entrained air becomes greater than that of the material of the dome itself, so if the air were to be heated, it would then displace the cooler air outside and the dome could, in theory, lift off with as little as a 1 degree F temperature differential.
The reason sports stadiums don't take flight today is this effect only becomes pronounced when domes reach truly gigantic sizes. A dome would have to be on the order of a kilometre in diameter before it could take off with a reasonably small temperature change, and a dome that carried hundreds or thousands of people (like a flying cruise ship) would need to be much larger to have sufficient lift capability.
Going back to individuals, a person could float solo with a balloon of sufficient size or lots of small balloons (people have lifted off in lawn chairs with hundreds of helium party balloons tied off to them), so there is a [ossibility of getting close to the effect you are envisioning.
Here's a arachnophobia inducing idea:
Spiders. Spiders everywhere.
Imagine a colony of spiders that weave a web so tight that it traps air. Then one of two things could happen: (1) through some biological process the spiders/their webbing removes gases other than hydrogen, leaving the air pocket less dense than the air around it or (2) the webbing traps heat well, keeping the air inside warmer (and therefore less dense) than the air outside it. In either case, that would cause the spider colony to airlift themselves to a new location. These aren't exactly clouds, but a flying web like this would look like a cloud and could be strong enough to walk on. Also, it wouldn't be too hard to imagine a world in which spiders have evolved this mechanism to move en masse to a new location as a form of seasonal migration.
You wouldn't be able to (and I doubt you'd want to) build anything on a cloud like this, though.
Each cloud is essentially a bunch of water droplets and snowflakes. You could probably compress and freeze a part of it to have a more solid snowy/icy surface, but your result could obviously start to fall down.
