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Okay, so imagine a situation where most of earth's atmosphere is lost, the pressure at sea level is somewhere around 0.3 atm. If it matters, let's say that this is the result of the gradual escape of air during a period of a few years. Disregarding the prospect of life under such conditions, I'm more interested in what would happen to the climate in such a situation.

So in this scenario, the total amount of water vapor in the atmosphere would be expected to rise a bit due to the decreased vapor pressure (at least that's what I think would happen, though this could probably be negated by things such as temperature changes).

What I'm looking for is mostly an idea of the most general effects on Earth's climate. How would such a change affect the average temperature, day/night temperature cycle, winds, rainfall, and anything else you might eventually think of?

Oh, and hi.

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  • $\begingroup$ Percentage of water vapor might increase, but the total amount of it would not. Temperatures would definitely be lower. Higher temperature gradients would lead to stronger winds. $\endgroup$ – Alexander Jul 19 '17 at 0:04
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    $\begingroup$ Welcome to Worldbuilding Stack Exchange! To learn more about our site, please take the tour and check out the help center. That said, this is a pretty good first question, though I'd recommend limiting it a bit - "anything else you might eventually think of" is a bit broad. $\endgroup$ – HDE 226868 Jul 19 '17 at 1:18
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I would start by considering how the atmosphere on Earth behaves at the altitude where there is 0.3 atm of pressure, and then adding in surface effects with the oceans (mainly) and the ground (secondarily). According to this calculator (https://www.mide.com/pages/air-pressure-at-altitude-calculator), the earth has an atmospheric pressure of 0.3 atm at about 9076m, so roughly at the top of the highest mountains. At that altitude, several types of clouds exist (https://scied.ucar.edu/webweather/clouds/cloud-types), and the interesting ionospheric effects are still above you.

For the same energy input from the sun, the atmosphere will be colder because less infra-red will be blocked. The surface water would likely freeze, so there would be less surface wave effects. Water would enter the atmosphere more by sublimation that evaporation, and the air at sea level would be much drier.

With dryer air, there would be simpler atmospheric phase-change dynamics. Cyclonic storms may be less frequent and severe because there would be less of the cycle where condensing rain heats the atmosphere, which becomes warmer and less dense, supporting the inrush of surrounding moist air. This last statement is a guess. It may be that snowing would, in a less dense atmosphere, cause the same cyclonic storms as we have now.

There would still be an equator-to-pole-to-equator cycle, as the poles would still be colder from the less energy input per square meter. Movement of air away from the equator would still cause Coriolis forces to induce latitudinal movement.

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Your pressure is lower, so you rightly infere that there will be more evaporation. The problems start now...

Having a thinner atmosphere also means you have less shielding of UV radiation, which has the ability of cracking molecules. So your water vapor is going to be split into O2 and H2.

Well, H2 is not going to last in the earth atmosphere: it will escape to space soon. Once all the hydrogen is gone, you end up with more Oxygen or oxydes, but no water clouds. Just winds...

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