I'm writing a graphic novel on the premise of a world that has a liquid or semi-liquid atmosphere, separate from a true liquid sea. But since I don't want 'Magical Underwater Adventure!' I've decided (tentatively) on a 'gaseous semi-liquid with non-newtonian-fluid-like physics' - i.e. under low pressure, it takes on more gas-like properties, and vice versa.

(From what I understand a gaseous NN-fluid is technically not possible, because there’s no ‘shearing’ involved… but I'm just looking for something with that particular, similar effect. Because of what I think it will result in. And the idea is kind of cool.)

For practical purposes, however, I want it to mostly maintain the visual and chemical characteristics of an Earth-like atmosphere (burnable, breathable, stable, insulates, transparent, etc.) You know. So you can see more than 100 meters or so at most. Also, I want it to be practical to roam around 'normally', run, jump, fall, throw stuff, shoot arrows, etc. Though some fudge is acceptable there.

The ideal is to have a pseudo-sciencey basis or maybe justification for things, and artistic license can come in after that, for visual/practical purposes. (Considering it's a visual media.)

I'm mainly struggling to visualize the physics and fluid dynamics, though. More specifically, how it would affect/create weather patterns and phenomenon, climate, and the like. Beyond that, I'm looking at ecology; and what it might mean for developments of societies. But I'll probably split those topics off into separate questions, and then link the whole thing together. I've got lots of notes. Any feedback here would be awesome, especially if it helps me reasonably justify or create (or improve!) my ideal.

So I'll just go through what I've kind of worked out / guessed at so far:

1. Basic Physics
   • At high pressures, and to some degree cold temperatures, it will become more viscous and dense; and at low pressures, it will become more gaseous and thin. (It could go the other way, but I don't know that it'd create the effect I want.)
   • I figure it will be somewhere in-between normal air and water in density, so it will create surface tension with either. i.e. True gasses (Side-note - where would you notably see these??) would be slowish bubbles, and water would behave as if in a slightly low-gravity environment, (more 'globby'? Look up water-rag in space videos.) and the gradient between sea and 'air' would be slightly less stark; depending on pressure and temperature, of course. Heavier materials would more or less function the same, except perhaps for large surface-areas and aerodynamic forms.
     • This will also (ideally) create a slip-stream effect easily, so you could run around normally; (or maybe it'd be harder at first, but life would be adapted to that.) but at a certain speed you get crouching-tiger-hidden-dragon type leaps; arrows would travel straighter and longer, provided they don't run into a current; and the like. Also, this may create a more pronounced 'contrail' effect on very quickly moving objects.
   • Sound will travel similarly to normal? I don't know. I'd rather this didn't impede the ability to communicate. (Note to self: Google "How does water affect sound-transmission"?)
   • It will evaporate at higher temperatures, however, so while burning it will sort of 'boil' and look like a rolling lava lamp of plasma as it cools upward. Think prototypical mushroom explosion. Less heat will be radiated, more light and pressure. (?)
   • It will freeze at a lower temperature than water will, instead just becoming more viscous; but it will also make an easier structure (with particles) for water to freeze in midair and on objects; resulting in delicate, near-invisible 'lattices' that fracture with shearing (i.e. weird noises and slightly refracted, broken visual patterns) which will eventually build up to larger ice crystals/object layering.
   • It can be saturated with both fresh and salt-water, depending...? Like a saline air. I'd just imagine this would have interesting effects?
   • Explosions would oscillate like they do underwater, if more quickly and 'rolling' more vertically; transferring much of the force to pressure-waves rather than to heat. (Owing to cavitation and a wall of pressure-increased density) Although, accelerant explosions may expand in a sort of 'fire-wave', I imagine. (I'd rather the sky medium not be a super-accelerant itself, even though it is more dense... though I can see this technically being a byproduct. But I'd prefer not to have a burning-sky apocalypse.)

Before I get into climates and such, I do have a basic states-of-matter question: What, really, is the difference between a gas and a liquid? I don't feel there is a static line to judge; just relative benchmarks. The reason I feel this is the case is while you might consider a liquid a fluid because it will (a) take the shape of its container (b) without expanding to fill it completely, like a gas would; this doesn't take into account the effect of air pressure on top of it. Effectively, atmospheric pressure is part of the 'container' keeping it in its current volume. In other words, in a vacuum, water will expand (boil) immediately to fill its container, i.e. fulfilling the second criteria. Also, consider when you 'pour' carbon dioxide (i think?) into a container and it will stay there, allowing for the 'magically extinguishing match!' party trick.