I’ve been thinking about the four elemental planes from Dungeons and Dragons of late, and what’s struck me is how different the elemental plane of earth is from the others. The other planes, air, water, and fire, are, or could be said to be, relatively uniform; I am speaking of the material of the plane itself, not the inhabitants. Consider that you have rocks, soil, sand, crystals, metals in the elemental plane of earth, but there aren’t descriptions of other gasses in the plane of air, like methane or carbon dioxide both of which can be produced in sufficiently high quantities, naturally, to kill people exposed to them. The plane of fire and water are similarly barren of variety within their theme.
As a result, I have been thinking about alternatives to the as described elemental planes. These are all as I have envisioned them, so I would not be surprised if there is considerable different between this interpretation and those in other media.
The Plane of Air
The plane of air would be composed of everything that makes up air, so Nitrogen, Oxygen, water vapour, and in trace quantities, Argon, Carbon dioxide, Neon, Helium, and Methane. There might be an argument whether water should be present, as the planes are supposed to be primarily composed of their respective element, with pockets of the other elements. These pockets would be the exception, not the norm, so there is a good argument that there shouldn’t be water spread evenly throughout the plane of air, even in gaseous form. I would counter that the idea of the elemental planes is one that presupposes that it would be like the air we breathe, which has some water spread throughout it. In a similar vein, dust should be present in the plane of air; it should range from undetectable to sand or dust storm levels.
The temperature and pressure of the air itself should also vary. Pressure should range from near vacuum conditions to 37 atmospheres of pressure (the critical point at which air starts to turn to liquid, but only at a very low temperature). Temperature similarly from searing hot to freezing temperatures; using heat alone, the former could be as high as 174,000 Kelvin (313,000 Fahrenheit) before it becomes a plasma, while the latter can go down to 132 Kelvin before the air starts to liquefy (that’s -140 degrees Celsius, -220 Fahrenheit). These are, of course, the extremes, but still representative of the nature of air. I would suggest that the general range should stay within the terrestrial limits, with a maximum of 54 degrees Celsius and a minimum of -90 degrees Celsius (129 to -130 Fahrenheit). Wind speed should top out at 480 km/h (300 mph), where they would gust from areas of high pressure and low temperature to areas of low pressure and high temperature (in general, of course).
Lighting of the plane of air is said to be uniform, unchanging, and from all sides, but I don’t think that properly represents what we would consider our sky. It changes colours and brightness throughout the day and as a result of weather conditions; skies often change to the colour of approaching tornados as it picks up material from the ground. I would suggest that instead of a singular globe or disk of a visible star, that ‘sunrise’ occurs in some set but otherwise arbitrary direction perpendicular to the relative gravity field (more about gravity below). It then traces an arc through 90 degrees (until it is parallel to the gravity field) where it reaches maximum intensity, and then ‘setting’ directly opposite of where it ‘rose’. Imagine being inside a giant translucent sphere and someone has a flashlight on the outside that lights up half of the sphere. To your left ‘sunrise’ begins and the left half of the sphere is bathed in a feeble light that grows stronger and stronger until the top half of the sphere is brightly lit. The opposite sides that are not lit are not black, but are darker, reflecting the light that is already inside the sphere. Night would be a vast blackness, punctuated by aurorae, streaks of fire similar to falling stars but likely brief openings into the positive material plane or the plane of fire, and distant points of light in all directions. These points of light could be portals to other planes, inhabitations on floating rocks, or unreachable spots on an ever moving sphere shaped scrim centered around each viewer.
Gravity would have to be universally directional, not defined by each user. If it was user defined, then rain could never fall from clouds and fire would burn in perfect spheres; objects would behave as if in freefall or zero gravity. Further, there would be no need for creatures from the plane of air to have wings, as they would just will themselves in a particular direction. So just like the plane of fire requires fire resistance, the plane of water requires the ability to breathe water, and the plane of earth requires some means of extracting oneself from being entombed, the plane of air requires the ability to fly. If a character enters without the ability to fly, they reach terminal velocity after two rounds and fall until they are stopped, quite likely by running into something. Unattended objects would similarly fall, and so getting hit by an object of any size would result in taking 20d6 points of damage, unless the object was dropped within the past few rounds. Their maximum speed would be limited, however, and since objects under 0.5 kg (1 lbs.) do not deal damage no matter how far they have fallen, there wouldn’t be the similar issue of micrometeorites that travellers into space have to deal with. Assuming objects max out their speed at low end of human terminal velocity (unrealistic, but more dramatic if your gear falls as fast as you do, or you can adjust your speed like Gandalf did to catch up to his falling sword in “The Two Towers”), that would put them at 195 km/h (120 mph). A character would then have one round to observe and react to an object starting 300 meters away from them (about 1000 ft.) before it crossed their path.