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Here's the basic gist of this (debatably) habitable Earth-like desert planet:

Size: Same as Earth

Rotation: 30 hours (three extra hours of daylight followed by three extra hours of night)

Revolution: To be determined, but 304 rotations is the lowest wager

Atmospheric thickness: 480 miles (160% as thick as Earth's)

Atmospheric content: 0.88% carbon dioxide (that's an awful lot), 0.5% ozone (that's even more of an awful lot), 25% oxygen (though this is suspected to be artificial in origin), 0.1% water vapor (again, suspiciously artificial in origin)

Land: 90%

Water: 10%, consisting of freshwater pools 30-100 vertical meters deep, but those pools are actually cenotes, the flooded openings of underwater cave systems, so really, surface water makes up only one percent of the planet's overall water supply

Terrestrial terrain: 79% plains, 19% shield volcanoes, 2% divergent rift valleys

Axial tilt: 19.01-28.28 degrees on a cycle lasting 205,000 years

So this planet is habitable only in the sense that liquid surface water is possible. But days are so hot that the water vapor in the atmosphere can't cool down to bring in the shade or the rain. Ergo, condensation and possibly precipitation is strictly a nocturnal global occurrence on this desert planet. True or false?

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    $\begingroup$ The "atmospheric height" probably needs a rework. Look up scale height - the atmosphere of a planet will increase in pressure by a factor of e every so many kilometers. Assuming the mass of your planet is vaguely Earthlike, adding "60%" to atmosphere height actually means adding e to the (180 km / 8 km) more pressure at the surface, which is a lot more than you want. Caveat: if the planet is a very low density indeed, the scale height really could be that much taller, and you could have the same pressure after all. $\endgroup$
    – Mike Serfas
    Commented Nov 7, 2021 at 1:02
  • $\begingroup$ @MikeSerfas Titan would like to have a word with you. $\endgroup$
    – JohnWDailey
    Commented Nov 7, 2021 at 1:43
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    $\begingroup$ Hey, I didn't have room for a treatise ... scale height depends on gravity (directly proportional to density squared at the same mass, directly proportional to mass at the same density, in this case directly proportional to density at the same radius) but also the density of the atmosphere, which depends on its composition and temperature ... in the case of Titan, it's less than 2 Moons worth of mass, so it has a high scale height for that reason. But this planet is Earth-sized. $\endgroup$
    – Mike Serfas
    Commented Nov 7, 2021 at 1:53
  • $\begingroup$ You can use this calculator to estimate pressure: keisan.casio.com/exec/system/1224562962. The number you get for the additional 292 km are insane. Even with 6 or 7 km more of atmosphere you would get a pressure of 2.33 atm and a temperature of about 50-60°C. $\endgroup$
    – Aristeus
    Commented Dec 3, 2021 at 21:38

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How deep is your writing going into the science? If you are looking for plausibility rather than hard science, it seems fairly simple: Warm air on a cold object causes condensation. So, if the air mass generally moved in directions not exactly the same as the rotation of the planet, then the hot (I assume) desert air would contact cold mountains and rocks on the dark side. This colder area would have cooled down once passing through the day-night terminator as the planet rotates.

Again, as a reader, I'd give that a pass on plausibility but I have no idea about the actual hard science for this.

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Rain on the mountains

Even if the atmosphere is very hot, it will still be colder higher up. If a current of saturated (hot) air moves towards a mountain or volcano, it will rise over the mountain and cool. Some of the water will condense and you get rain.

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One can't do any actual calculations without knowing the amount of light energy hitting the planet. You should be able to get that number from the average temperature you want your planet to have.

Yakhchāls on earth are used to make ice in the Sahara for millennia. So it would be perfectly possible for dew to form on the ground during the coldest time of the night.

I have two concerns. The first is how well the sand and rock can radiate the warmth away through the gap in the absorption spectrum of CO2. Us humans have been able to engineer such materials and some of those can occur naturally.

The second is about where the water would end up. The travel speed of water vapour is faster than that of water on average. And ice moves much slower still. The polar regions of any planet are the coldest so that is where water will condense first. Over time this would gather more and more water there. Your underground rivers can help with this. But it is a long way for the water to go from the pole all the way to the equator.

I think it is a plausible world. So long as this weather dous not uniformly cover your entire plant. You will need to pick your orbit around your sun carefully to get it to where your scenario would be commonplace on planet.

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