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Would there any scientifical explanation to a permanent rain (at least 2 years) everywhere on Earth ? Climate-change ?

My best explanation at this point is accelerated ice cap melting. There must be about 25 millions km3 of water, which would be enough for a 3 mm/h rain for two years. But I don’t know what could cause that.

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    $\begingroup$ "everywhere on Earth" is tricky. You would need water evaporating as fast as it was falling, which sort of defeats the point of the rain in the first place $\endgroup$
    – Sol
    Sep 11 '20 at 5:42
  • $\begingroup$ I guess it is necessary to increase the quantity of liquid water in the world at some point. $\endgroup$
    – user21102
    Sep 11 '20 at 5:54
  • $\begingroup$ Just observing that "everywhere on Earth" includes the Sahara, the Atacama and the Namib deserts, and Antarctica, places where rain is between rare and practically unheard of. There are considerable areas in the Namib desert which receive less than 2 mm of rain per year. $\endgroup$
    – AlexP
    Sep 11 '20 at 6:42
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    $\begingroup$ I believe current theory is that this was basically the state Earth was in billions of years ago when it first started cooling from a near-molten ball of iron and rock to, well, habitable temperatures. Water condensed in the upper atmosphere, came down as rain, evaporated nearly instantly on the superheated rock, went back up where part of the heat radiated outward into space so it eventually condensed again and came back down for another round... $\endgroup$
    – Shadur
    Sep 11 '20 at 6:47
  • $\begingroup$ is it need to be natural or man made is fine? $\endgroup$
    – Li Jun
    Sep 11 '20 at 9:49
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A asteroid the size of Greenland, made entirely of loosely packed chunks of ice, in a decaying elliptical orbit.

Every few weeks or so, it brushes past the upper atmosphere and thousands of ~1m fragments break off. Those fragments burn up entirely in the atmosphere, increasing global humidity.

This would also accelerate global warming - water vapour is a greenhouse gas, as well as white clouds becoming dark rain-heavy clouds will decrease planetary albedo.

The extra water vapour will accumulate in the atmosphere, be distributed by high altitude winds, slowly descend, form clouds, and be eventually discharged as rain. Every few weeks the water is topped up by another pass of the asteroid, and whats left of the asteroid looses a little bit of speed.

After about 20-30 passes, the asteroid will entirely enter the atmosphere, but it'll be at a shallow angle, loosely packed, and almost entirely burn up before impact.

That should give the Earth a few solid years of rain.

Why do we need to bring the water in from space?

Using only water on Earth; This is really tricky, over water, you'll need to evaporate water while raining, water going back up and down again at the same time at the same place, carefully calibrated so that it doesn't push up too far (and build hail), or push up not enough (you'll just get a foggy mist, or it'll stop raining). You'd need to maintain this balance of wind and temperature overnight and through winter.

You're not going to be able to get it to rain in Antarctica easily either.

By bringing the water in from space in the form of evaporated ice, we can make it rain regardless of surface conditions.

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    $\begingroup$ Thank you for your very interesting suggestion. I have two questions though : 1. don’t you think humanity would be able to destroy the asteroid pretty soon ? 2. Would a massive rain of little icy meteors (like hail from space) do the same, supposing it lasts long enough ? Of course, it would also need an explanation. $\endgroup$
    – user21102
    Sep 11 '20 at 7:00
  • $\begingroup$ @user21102 It's already in our gravity well, if we destroy it, it will rain more intensely, or chunks of ice will make it to the ground. If anything it'll be better for humanity to prolong it (so the final impact is even smaller) rather than bring it all down at once. I guess. You'd need to explain where they come from, and "broke off a bigger one" is a fairly decent explaination. $\endgroup$
    – Ash
    Sep 11 '20 at 7:05
  • $\begingroup$ An ice asteroid surviving 20-30 times the tidal forces of a close encounter with Earth seems far fetched $\endgroup$
    – L.Dutch
    Sep 11 '20 at 7:08
  • $\begingroup$ So is 2 years of non-stop rain. But it doesn't survive - it breaks a bit each time, and slows down a bit. $\endgroup$
    – Ash
    Sep 11 '20 at 7:12
  • $\begingroup$ Would the increased albedo from the cloud cover counteract the greenhouse effect from the increased amount of atmospheric water vapour? $\endgroup$
    – DrMcCleod
    Sep 11 '20 at 8:25
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Permanent fog

How about a different approach? Reduce the pressure of the atmosphere. In lower atmosphere, water boils at much lower temperatures. This will also increase the evaporation at temperatures lower than the boiling point. This causes the oceans to have a near permanent layer of fog, interspersed with rain if I'm not mistaken. Still, there would be a lot less rain. Lots falling rain would already evaporate before hitting the ground and would require more to start falling. That being said, the air would contain much more moisture. This would also increase the temperature on the planet, making it more plausible that a lot of water evaporates and stays in the air.

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If you could create ideal conditions, then it would be theoretically possible.

The water doesn't come from the area where it is raining. So if you can have winds constantly bringing in clouds from the sea, and geological features and climate which causes the rain to condense from the clouds in the given area, then you would have permanent rain. The rain would then go into rivers, and into the ocean, constantly replenish the source.

I doubt, though, that you will find such a place anywhere on Earth. Making sure that the rain never stops, no matter the local conditions or seasons would be really hard.

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    $\begingroup$ But the OP is asking for permanent rain everywhere, not just in particular locations. There are places that come pretty close to permanent rain even now, e.g. the northern Pacific coast of North America, or northwest Ireland. $\endgroup$
    – jamesqf
    Sep 11 '20 at 17:17

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