How the weather would look like on a planet with an atmosphere composite mainly ($H_2O$ vapor 90%> Other gasses 10%) of $H_2O$ Vapor/Steam.

  • Would it rain continuously?

  • With density around 1 $g/m^3$ and pressure of 1.2 atm, how foggy will it be?

  • What about seasons? assuming it has seasons like Earth.

The average temperature at sea level is 16°C. And It's a Water-Planet/World.

And I know it's uninhabitable.

Thanks for Answers 🙂

  • $\begingroup$ Are you sure about the numbers you have put in? Our atmosphere weights about 1 $kg/m^3$ $\endgroup$
    – L.Dutch
    Commented Jan 29, 2021 at 16:20
  • $\begingroup$ Isn't it 1.2 kg/m3? $\endgroup$
    – Khalid
    Commented Jan 29, 2021 at 16:29
  • $\begingroup$ Are you suggesting that this situation occurs because the planet is hot enough to keep surface temperatures consistently above 100°c, or that some other factor occurs that allows water to vaporize at low temperatures? The main problem is electrostatic attraction. Gaseous water at those concentrations really, really wants to collapse into liquid water, so you need something (temperature, pressure, some electrical field) keeping the molecules apart. Otherwise you won't have rain; you'll have a sudden flood of biblical proportions. $\endgroup$ Commented Jan 29, 2021 at 16:48
  • $\begingroup$ @TedWrigley Thank you, but do I need lower or higher pressure? $\endgroup$
    – Khalid
    Commented Jan 29, 2021 at 17:06
  • $\begingroup$ The ideal gas law is PV=nRT; temperature and pressure are inversely proportional, so you either want high temperature or low pressure. $\endgroup$ Commented Jan 29, 2021 at 17:22

2 Answers 2


Same conclusion as Slarty, but just with some numerical consideration, since the ideal gas equation is far from valid for water vapor.

According to this calculator water at 16 Celsius has a vapor pressure of 0.0179 Atm.

There is no way therefore that with the parameters you have given 90% of the atmosphere will be made of water vapor, because it will condense to form liquid water until the pressure drops at the value of the equilibrium pressure.


What you suggest is not possible. Atmospheric temperature, pressure and (effectively) density can be calculated using the ideal gas law nRT=PV https://www.chemguide.co.uk/physical/kt/idealgases.html

Density can be calculated from the volume V and the the number of moles n

Steam would not exist at the pressure and temperature that you suggest. Or at the very least the conditions you specify would be so far removed from equilibrium that the temperature, pressure and density would very rapidly change from those you suggest.

The phase diagram for water might help. This shows what is possible for water at different temperatures and pressures: https://en.wikipedia.org/wiki/Phase_diagram


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