[DoreenMichele]

I had to think about that for a bit. I've tried to Google it to better understand it. I am not immediately coming up with an elucidation.

Anyone got a link to a quick and dirty explanation of how the vacuum of space would impact this?

[Groxx]

Air pressure on earth literally squeezes the water together, preventing it from turning into a gas until it's at a higher temperature.

More generally: practically all phase-changes depend on both temperature and pressure. Lower pressure almost behaves like higher temperature... within certain ranges (and it varies for every material).

E.g. for water, note that there are three major regions in its phase diagram[1]. At human-normal scales (the red horizontal line is 1 atmosphere) we see water boil/freeze at 100 and 0 celsius. If you lower the pressure though, you'll see there's a spot where all three phases meet, and below that there's no liquid phase separating solid and gas. When it's in near vacuum, you literally can't have water - ice evaporates (sublimates) directly into a gas, with no intermediate phase.

Given the diagram, this seems to happen somewhere around -60c in a serious vacuum. So in space or on the surface of the moon (without an atmosphere / something pressurizing it) you simply can't have liquid water - a portion of it will evaporate almost immediately, which steals energy from the remaining portion, causing it to freeze. After that, the solid portion just sublimates away (because the sunny side is 127c, well above the temperature needed to turn it into a gas).

[1]: https://en.wikipedia.org/wiki/Phase_diagram#/media/File:Phas...

[DoreenMichele]

Thank you.

What I am trying to figure out specifically -- to mental model -- is what impact this has on human comfort, per the original question.

So I have lived in very hot and humid places -- like Georgia, where it is often around 100F in summer -- and I have lived in even hotter places with a much dryer climate -- like the Mojave Desert, where it can get to 115F and I would wait until nightfall to go for walks for exercise at 99F and no sun. Humidity does make a big difference in how uncomfortable a temperature is. If it is hot and dry, you can stay reasonably comfortable if you get enough fluids and electrolytes into you and stay out of direct sunlight.

I'm trying to fit this into a mental model of that sort. Like is a boiling temperature going to burn you? Or, you know "It's a dry heat, man!" only "It's in the vacuum of space, man!"

Thanks to anyone who replies to this.

[Groxx]

Aaah, gotcha. Yeah, that's definitely fuzzier... like, what does boiling feel like if it's room temperature.

I imagine some of it depends on what our heat-sensors actually react to. At the least-interesting case, seems like being above boiling without heat would feel fizzy, just because it's physically doing that to the outer layers of your skin. Or something like laser hair removal, where it kinda feels like a hard slap.

[DoreenMichele]

Thanks.