A lot of the laws of physics I’ve studied, like Boyle’s Law and Charles’ Law, describe the behavior of “An ideal confined gas.”
I’ve had to tell several flight students to unlearn what they’ve learned about that in the meteorology chapter, because, for example, in a confined gas, increasing the temperature causes an increase in pressure while the density stays the same. In the Earth’s atmosphere, increasing temperature does nothing to the pressure and decreases the density. Because the Earth’s atmosphere isn’t “confined,” there’s no lid, the air is relatively free to change volume. Heat the entire planet up and the atmosphere will just get a little taller.
But, I think, even if we put a magical vacuum tight shell around the planet 200 miles up, making the volume finite, I think the atmosphere would still act like an unconfined gas, because 1. it’s so vast that it never homogenizes, parcels of different temperatures, pressures and moisture content take days to slosh across the available space, and 2. the Earth’s gravity will cause a pressure gradient; most of the air is at the bottom and if you heat it up, it may not change volume but the pressure at the top will increase.
So I guess there has to be an upper limit to the volume and/or mass of air that can be “confined” and it’s somewhere below planetary scale.
I’m thinking something the scale of the Earth’s atmosphere, where the air isn’t at high enough pressure or low enough temperature to condense, but the mass and volume are great enough for gravity to cause pressure gradients.
Imagine a 1 liter container with 1 atmosphere of N2 inside, floating in space. Sunlight or something is keeping it constantly warm enough to remain gaseous. The gas laws I was taught in school like Boyle’s Law and Charle’s Law would accurately describe the behavior of the gas in that container.
Now scale it up the container, and the gas inside.
as it approaches the size of a planet, gravity will start being a significant factor in the behavior of the gas, that you’d get an area of relatively high pressure at the center of mass of this vast container, and relatively low pressure near the container wall. Then start playing around with the temperature.