GothiousRex said:
I'll answer this here, because it raises some interesting issues. Intuitively and qualitatively speaking, this is what one would think these atmospheres ought to mean:
None: No atmosphere. Pretty self-explanatory.
Trace: Think of Mars today. There's enough of an atmosphere for weather and clouds and winds, but it sure isn't breathable and it's extremely thin. Essentially "Trace" is an Exotic atmosphere that is thinner than "Very Thin".
Very Thin: N2/O2 mix that is MUCH thinner than Earth's atmosphere, and not breathable by unaided humans - humans need a respirator (a.k.a. a compressor) to survive in this. Think "top of Mt Everest".
Thin: N2/O2 mix that is thinner than Earth's atmosphere, but still breathable by unprotected humans.
Standard: N2/O2 mix that is similar to Earth's atmosphere, and breathable by unprotected humans.
Dense: N2/O2 mix that is thicker than Earth's atmosphere, but still breathable by unprotected humans.
Now, the problem is that this actually isn't the case. Classic Traveller - for some bizarre reason - defined the atmospheres much more subjectively. Instead of defining them by pressure, CT defined them by 'breathability' and what a human would have to wear to survive in them. And since MGT is based on CT, it's carried that over here. Megatraveller and TNE at least defined them more by pressure though.
In CT, Vacuum (0) and Trace (1) are basically indistinguishable - both need Vacc Suits. Very Thin atmospheres (3) need a Respirator. Thin (5), Standard (6), and Dense (8) atmospheres don't need any aids to breathe. Tainted atmospheres (4, 7, 9) need a Filter Mask, and Very Thin, Tainted atmospheres (2) need a Filter Mask AND a Respirator. Exotic atmospheres (A) need oxygen tanks, but no further protection. Corrosive atmospheres (B) need a "Protective Suit" (apparently you don't need Oxygen Tanks too, but I'm pretty sure that was an erroneous omission), but a Vacc Suit will work too. And Insidious atmospheres (C) need a Vacc Suit at the very least. Dense/High (D), Thin/Low (E), and Special (F) atmospheres don't need any protection inside their habitable areas, but do elsewhere.
Here's the problem though: Say you have a Very Thin atmosphere. In CT terms, you know you need a Respirator to breath it - so there obviously isn't enough oxygen to breathe unaided, right? But that doesn't actually tell you anything about the pressure of the atmosphere, which is important for things like determining lift and drag and air resistance and so on - all it tells you is that it's hard to breathe. So is this atmosphere something with 1 atm pressure at the surface but very little oxygen content? Or is it something with about 0.2 atm pressure and about the same percentage of oxygen as Earth (and therefore still unbreathable without aids)?
Another problem is the Exotic atmospheres - sure, that works for worlds in the habitable zone (primitive Earth would be a type A atmosphere), but what about something like Saturn's moon Titan? Titan is actually listed in CT Book 6 as an Exotic (A) atmosphere... but try walking around on its surface with just oxygen tanks and you'll be a frozen popsicle within seconds - because of course the CT description doesn't account for whether the planet is in the inner or outer zone. So if you really need a Vacc Suit to survive on Titan, shouldn't CT class that as a Corrosive (B) atmosphere? Except it's not actually "corrosive" at all, it's just damn cold. (and while CT Book 3 lists "Cold Weather Gear" as an option, I suspect they actually mean "Arctic clothing" not "clothing that can withstand a cryogenic environment).
The solution (short of breaking an atmosphere down into separate pressure and compositional UWPs) I think is to take the World Builder's Handbook approach. WBH was a book published by Digest Group Publications for Megatraveller that vastly improve the details of world generation. In there, they define the atmosphere types by mean surface pressure instead of breathability, which is much more useful for things like vehicles as well as for visualisation.
Trace was defined as 0.01-0.09 atmospheres.
Very Thin was 0.1 to 0.4 atms.
Thin was 0.43-0.75 atms.
Standard was 0.76 to 1.4 atms.
Dense was 1.5 to 2.4 atms.
Very Dense (D, some A/B/C) was 2.5 to 750 atms.
Thin, Low (E) was 0.23-0.55 atms (at the base of the valleys).
With the surface pressure, one can figure out how breathable the atmosphere actually was by calculating the oxygen pressure (percentage of O2 in the air x surface atmospheric pressure). WBH also helped by defining "Low oxygen" and "High oxygen" taints for atms 2/4/7/9 which meant that you could have a standard pressure atmosphere that had little (or too much) oxygen to be breathable. So the assumption was that if you had atm 5/6/8 then the atmosphere was breathable without aid, and if you had atm 4/7/9 then it needed either a filter mask (for impurities) or respirators or reducers (if there was too little or too much oxygen in the air).
So atms 0-9 were basically defined by a pressure scale (0 was by implication less than 0.01 atm pressure - which annoyingly meant that Mars had no atmosphere according to WBH since its mean surface pressure is about 0.006 atms). Atm A-C could have pressures within a range of thicknesses from Very Thin to Very Dense. Atm D was Very Dense at the surface but thin enough to breathe at altitude (though for higher surface pressures the breathable altitude is way higher than any mountaintops), and Atm E was (Thin minus 0.2 atms) in the valleys.
(and yes, I know CT's atm E wasn't the same as Mongoose's atm E (which is actually CT's atm F). But CT's atm E was silly - you can't get ellipsoidal atmospheres like that).
So to answer your question, I'd say that you should define it by pressure and work from there.
. Done!
