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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.
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.
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.
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.
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.
