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Making a world where you live in the air, as the surface is not survivable (Venus-like) but there are mountains which serve as islands. I'm gonna leave out the story of the planet as it's not important.

So could it be possible for a ship-like structure to "float", maybe with the help of some tanks filled with lifting gas such as hydrogen (not as big as today's zeppelins in proportion to the weight they carry though), on dense clouds or rather could dense enough "clouds" like this form on an alien planet?

What environmental factors would have to exist on such a planet, things like a dense atmosphere maybe even with different layers etc. for these clouds to form and could they support a ship-like structure? (They don't have to be "clouds" as we know them on earth, I use this term loosely)

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Ultimately, the question of whether or not something will 'float' on top of something else is a matter of density. Put simply, weight does not matter...if the average density of your entire craft is lower than that of the substance you are trying to float on, you will float. If you are denser, you will sink. It's why an Aircraft carrier (lower density than water) will float while a pebble (higher density than water) will sink.

So, ultimately, if you want to actually float on a cloud, you need a world where a 'ship' is less dense than a 'cloud.' This may cause problems for anyone trying to live there.

As a start, the individual droplets in clouds are actually denser than the air, but are held aloft by colloidal suspension. In short, water droplets in clouds have an extremely low mass to surface area ratio and, while they are trying to fall, the combination of friction with the air around them as well as updrafts hold the clouds aloft. If you start making clouds too dense, however, this phenomenon stops working unless you likewise amp up atmospheric density to keep up the friction (increasing updraft strength too much may just scatter the clouds). It's when the weight and density of a cloud overcomes the colloidal suspension that we end up with rain.

So, looking at 'Earth' clouds, a measurement made by Millersville University figured a density of $0.5 g/m^3$ for a Cumulus cloud (the fluffy ones). The density of liquid water at room temperature is $1,000 kg/m^3$ (A bit more in the ocean), or roughly 2 million times denser than a cloud. So, equivalently, your cloud-ship would need to be 2 million times less dense than an ocean-going ship. As it stands, the only way we presently have to manage this is by using "lighter than air" gasses (which are, actually, gasses that are less dense than Earth's standard air mixture, and thus float on top of air). And, if you take a look at any lighter-than-air craft, you can see how much lift-gas it takes relative to the size of the cabin in order to get it off the ground (the air envelope is vastly larger than what it is carrying).

So, as for inventing a world where this is possible...you are going to need a world with a much denser atmosphere, and are going to need to make ships out of very flimsy materials (or add big air envelopes for them to carry lighter-than-air gasses).

Now, the problem you run into is well addressed in this question's accepted answer. Having an atmosphere dense enough to hold up your clouds, while not being so dense that it kills anything that tries to live in it...or ceases to be a gas, and becomes a liquid. And, ultimately, you are going to have to increase your air pressure, because to keep clouds made of a denser material aloft, you're going to need a denser atmosphere to produce more friction on the droplets of it.

So, I can't give you a 'hard and fast' answer here, but can give you a few constraints on pressure.

If their atmosphere has the same mix as ours, you are restricted to increasing sustained atmospheric pressure to no more than about 2.3 atmospheres of pressure (2.3x normal pressure at sea level). This is because oxygen is actually toxic to humans in too large of quantities and can cause severe lung damage, convulsions, unconsciousness, and damage to the eyes.

If you alter their atmospheric mix to contain a smaller percentage of oxygen so that, at the higher density, we still get about the same amount of oxygen into our lungs per breath as well as... Reduce Nitrogen to prevent inert gas narcosis but not so much as to cause High Pressure Nervous Syndrome, and replace helium with hydrogen for lower density to make expelling CO2 easier (careful of explosions here), the upper limit that has been tested, short-term, is 50 atmospheres, and prolonged exposure caused bone necrosis. They have not tested for a 'maximum long-term survivable pressure' with a carefully controlled atmosphere mixture

The other variable you can adjust is temperature. Colder temperatures mean denser gases but, here again, you are going to run into problems. If you get too cold, water cannot maintain a gaseous state, and water clouds will not form. However, like on Saturn's moon Titan, you could end up with clouds made of some other gas, such as methane. Titan's atmosphere is dense enough that, if you strapped wings to your arms, you could fly by flapping. The problem is, Titan is so cold that Oxygen would be a liquid there. Kinda hard to breathe that. And long before that, water can no longer maintain a liquid state, and survival on that planet just got a LOT harder.

So, there are the constraints you have to work within...you can adjust atmospheric pressure and temperature to try to create a dense enough atmosphere to support clouds that are dense enough to support a ship.

If you do end up with a setting on those that works, bear in mind that your ships are probably going to be very, very flimsy in order to have a low enough density to float. To be fair, I don't believe it is going to be possible.

So, let's look at the actual numbers (somewhat simplified, assuming Earth Atmosphere for Specific Gas Constant). If you are very careful and dress very warmly, humans are known to survive sustained temperatures of -60 degrees Celsius (-73 Fahrenheit), though they can have practically none of their body exposed to the elements at any time. So, let's look at that as our lower limit for temperature. Lets then take that upper limit that has been tested in atmospheric pressure of 50 atmospheres with a 'perfect' atmosphere. Air Density of 'dry' air can be calculated using the Ideal Gas Law, like so...

$$ ρ=\frac{p}{R_{specific}T} $$

p is pressure: 50 atmosphere, or 5066250 Pascals R is the specific gas constant, assuming dry air, that is $287.058 \frac{J}{kg·K}$ T is temperature, at -60 C or 213.15K

Plug in all my numbers...

$$ ρ=\frac{5066250}{287.058·213.15} $$

And solve

$$ ρ=82.8 kg/m^3 $$

So, making a (probably incorrect) assumption that Atmospheric Pressure and cloud density increases linearly, atmospheric density at sea level on Earth is $1.225 kg/m^3$ so we have an increase of 67.59 times atmospheric density. Therefore we can assume that clouds can now be supported of a density of $33.795 g/m^3$. Which is still 29,590 times lower than the density of water. You could have smaller air envelopes on your airship, but you're still gonna need them, and at that point, you'd probably do better to not 'sail' on a cloud, but just fly...it's safer because there's no chance of running out of cloud and falling off.

If you are willing to deviate away from human-survivable conditions, and force your people to wear pressure suits, live in sealed environments, and otherwise not be able to survive at all outside, you might be able to still make this work. But, as it stands? No.

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  • $\begingroup$ Thank you so much, that's a fantastic answer. One more thing, what if these large, dense "clouds" of gas did not float themselves but rest and move across the surface. Keeping in mind that the planet does not have to be habitable at or near the surface but at a higher altitude, and that they don't have to be "Earth" clouds, could such a phenomenon be somehow explained? (I am sorry for such a broad question but my lack of knowledge on this subject is pretty evident :P) $\endgroup$ – Niko Mar 2 '16 at 22:27

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