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Composition: Nitrogen 53% Helium 16% Oxygen 13% Neon 10% Argon 6% Hydrogen 2% Pressure: 2.13 bar

I think the air should be breathable, but I'm concerned about the effects on flammability and wind speed.

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    $\begingroup$ You cannot have both oxygen and more than trace amounts of hydrogen at the same time. Hydrogen and oxygen just love to combine energetically (see the Hindenburg) and make water. Any little pretext is enough to ignite hydrogen. $\endgroup$ – AlexP Jul 6 '17 at 21:54
  • $\begingroup$ Okay. I thought it would just make the air more flammable $\endgroup$ – NeoBourne Jul 6 '17 at 21:55
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    $\begingroup$ Air should not be flammable. What good is a flammable atmosphere? And anyway hydrogen is so light that it just won't stay mixed up with the other gases. $\endgroup$ – AlexP Jul 6 '17 at 21:56
  • $\begingroup$ In hindsight, adding hydrogen was a bad idea. $\endgroup$ – NeoBourne Jul 6 '17 at 22:02
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    $\begingroup$ It is fine to have hydrogen to start. It will all go away shortly. The helium too. $\endgroup$ – Willk Jul 6 '17 at 22:12
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Nitrogen doesn't induce narcosis until about 4 bars partial pressure, so that's no big deal. Neon is also completely inert. Argon has a mild sedative effect at high pressures, but helium has a so-called "anti-sedative" effect, and neither one is really relevant at partial pressures lower than one atmosphere, so that's also pretty much a wash; humans won't notice. That concentration of hydrogen is outside the flammable range, so the atmosphere won't light up on its own, but unless it is being continually replenished by life (like oxygen has to be anyway), it will rapidly disappear.

The average molecular weight of the air will be a bit lower than on Earth, meaning sound will travel a little bit faster; the prevalence of monatomic over diatomic gasses also increases the speed of sound. The higher pressure, however, would contribute to decreasing the speed of sound, and I'm not sure which effect wins. With a higher speed of sound, people's voices will be a little bit higher, and vice-versa for a lower speed of sound.

Wind speeds will be lower for the same amount of energy than on Earth due to the higher density of the air, and slower winds will feel stronger than expected.

The increased heat capacity and higher thermal conductivity of the air will make fires harder to start, even though there's more oxygen around than on Earth, which partially counterbalances the effect. Things will generally burn faster, but flames will be a little bit cooler, and some things that can sustain their own flame in our atmosphere may not be able to in this one.

Adult humans will find the air "invigorating"- slightly higher than normal oxygen pressure makes it easier to get oxygen into the blood, which increases endurance and makes you feel stronger. The effect would most likely be temporary, however, as blood cell production is down-regulated in response. High oxygen pressures can also be dangerous to infants, most notably causing retinal damage and blindness; I can't find a good source for the precise danger limits, but the level of oxygen here should, however, still be safe.

None of these gasses is strongly colored, so the sky would still be blue, for the same reason our own sky is blue. Reddening near the sun would be slightly stronger around sunrise and sunset, due to the higher numerical density of gas particles.

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    $\begingroup$ For speed of sound (C) from Ideal gas law, C is proportional to density**0.5. Avg molecule weight is about 21.7 vs 28.8 (earth) and pressure is 2.15::1, so overall density is 1.613 earth, or C is about 0.787 Earth, so everybody talks with a deeper voice. $\endgroup$ – Gary Walker Jul 6 '17 at 23:31
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At 2.13 bar 13% oxygen is breathable, but 2% hydrogen will make this atmosphere unstable.

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  • $\begingroup$ Do you mean it will go off with bang? Explaining why it's unstable will improve your answer. But surely you already should know that, right? $\endgroup$ – a4android Jul 7 '17 at 2:00

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