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Since there's wind you can see and hear and this is a tower, I suspect that this level is actually on the surface.
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Correct. More than suspicion, too - here's a quote from tvtropes website, for example: "The Utility Tower found in Upper Underrail heavily implies that this was what forced humanity underground centuries ago. It takes you up high enough to feel and see wind and snow, both of which are so deathly cold that they can kill you in seconds without protection".
So this is at least a "heavy implication".
I can shed a bit more light on it based on what i know about possible future for our real Earth, here, for those interested to better understand the lore of this game. And, perhaps, the possible real future for those few of us who may end up surviving this kind of thing, too - and no, this is not a joke.
So, the actual real-world science, in a nutshell and most simple terms - is this:
- as was 1st calculated in 1980s and since then confirmed and re-confirmed by multiple scientific studies and Earth climate computer models, any large nuclear war which ignites many modern megapolises (large cities) on Earth - will trigger a nuclear winter. This means several years of greatly reduced temperatures everywhere on Earth, due to lots of soot in upper athmosphere;
- as was discovered by geologists and many field studies, Earth suffered at least three long periods of near-complete or complete glaciation, when ice and snow covered whole Earth, in distant past: 600+ millions years ago and over a billion years ago. At least two of these lasted for many millions years each;
- whenever, for any reason, much of Earth gets covered by snow even for a brief moment (like, even a few days) - Earth surface starts to reflect so much sunglight (which otherwise would mostly get absorbed and warm up Earth surface) that net temperature nosedives. The more snow and ice it is - the more sunlight gets reflected - thus the faster further temperature drop occurs - thus still remaining clouds drop as even more snow even faster. This goes on until there's no open water left. Then, practically all athmospheric moisture drops down as snow, and almost no new moisture gets into athmosphere, because there's no open water left anywhere on Earth. This is called "Snowball Earth" in actual science - because whole Earth is (and, was in the past during above mentioned events) like a snowball: complete ball of snow and ice, all around;
- in the past "Snowball Earth" events, paleo evidence suggests only a few single-celled organisms survived these events. These were the ones able to resume life functions after being frozen rock solid for millions years, like certain species of Algae. No complex life known to science (read - multicellular, like land plants or humans) can exist on the surface of 24/365 frozen world. This is known with complete practical certainty simply because this is exactly what happens on top of the huge Antharctic ice sheet: nothing lives there "on its own", only a few research stations humans supply from other continents and some few birds migrating over it;
- to end "Snowball Earth" state, one of main hypotheses is that after many millions years, very slow accumultation of various greenhouse gases in Earth athmosphere, which gradually occurs as a side-effect of volcano eruptions (which have the power to penetrate otherwise frozen-solid surface) results in accumulation of enough heat in the athmosphere itself that surface starts to melt, more and more places (starting with equator) lose ice and snow cover and start to absorb most of sunlight once again, and Earth gets back to non-frozen state;
- due to astronomic amount of heat trapped within Earth's liquid magma and her metallic core, even if her surface is frozen - there will always be a thick layer of rock / crust which is above water freezing point but below its boiling point. It's guaranteed for at least few more billions years, unless something breaks Earth apart (which is quite unlikely, as so far Earth managed to remain in one piece for over 4 billions years).
Bottom line:
- it is reasonable to believe that events of Underrail happen few centuries after a global nuclear war, which resulted in complete freezing and extreme, near-space cold setting everywhere on Earth surface, thus rendering it uninhabitable by any complex life;
- it is reasonable to believe that should such a war ever happen in real world, then such surface conditions will indeed happen. However, it is not reasonable to believe tiny remnants of mankind - perhaps a few thousand people, like it's shown in Underrail - would manage to survive under Earth surface for at least several generations more. There is no known to science method to maintain actual effective agriculture underground for such long time (multiple human generations). In real-world science, that is. Certain actual practical attempts i'm aware of - were done, but results are, in short, pityful and disappointing. In this regard, Underrail is a work of fiction which may well be way too optimistic.
