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I was planning a short story that focuses on a rainy day in a city, and I realized that there's one interesting feature of it: Even though the weather seems bleak, and it affects the actions of those in the story, anyone reading it knows that the next day will be, in all probability, sunny, bringing a lighter mood to the city.1

But what if the next day wasn't sunny? What if it rains then? And the next day, and the next day, and the next day? What if it were to rain forever?

I'm trying to figure out how I can make an earth-like world - very Earth-like world - support a weather system that continuously rains everywhere on the planet. there are some issues with this:

  • The clouds would block sunlight, making evaporation difficult.
  • The shear volume of water would require a large source, and I doubt that evaporation can cover it.

How can I explain the perpetual rain?

I'm not using the tag because I realize that this could be a tough question, and might require some stretches of the imagination. Answers should, of course, be realistic - i.e. obeying all natural laws.

I should add that I'm okay with answers explaining that this scenario is unrealistic. I rather like the current answers, but I'm fine with someone criticizing my idea.


1 Perhaps not in this case, because the city in question is London.

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    $\begingroup$ Read "All Summer In A Day" by Ray Bradbury. It's pretty short, and moving. That's the full text. Synopsis: A planet where rain falls continuously, except for one day a year when the sun comes out. Also, children are monsters. $\endgroup$ – AndyD273 Apr 14 '15 at 16:00
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    $\begingroup$ Maybe you need to think about psychology a little, and why some people see sunny (and often enough, unbearably hot) weather as 'nice', and rainy weather as 'bad'. I live in a place where it doesn't rain much. A few years ago I spent a month's vacation in Britain & Ireland. It rained at least a bit every single day, and I loved it. $\endgroup$ – jamesqf Apr 14 '15 at 19:13
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    $\begingroup$ I think "some stretches of the imagination" is an understatement, if you want it to resemble Earth. In realistic setting, the whole ecosystem would be totally alien, because our plants need sunlight for photosynthesis, which is the energy source of almost all macroscopic life. $\endgroup$ – hyde Apr 14 '15 at 19:50
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    $\begingroup$ @HDE226868 - tried a couple angles, I just can't get enough energy through the clouds to provide this constant rain over the entire globe. The heat transfers get a bit weird...water vapor that condenses releases heat in the upper atmosphere before falling and it creates this heat imbalance where your cloud level is as warm as the surface below. If can justify a particular city receiving nearly constant rainfall, but the entire globe doesn't seem to work without resorting to another source like the geysers style answer by kuhl below. $\endgroup$ – Twelfth Apr 14 '15 at 21:33
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    $\begingroup$ "continuously rains everywhere on the planet" sounds hard. Or do you just need "continuously rains on all land on the planet"? Given that it rains a lot, there might not be a lot of land. Also getting a balancing evaporation from the oceans becomes easier. $\endgroup$ – Keith Apr 15 '15 at 1:28

14 Answers 14

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Not sure if this is what you're looking for, but what about geysers? There'd obviously have to be a ton of them (and maybe some one with more of a science background can speak to what that level of seismic activity would do to the earth), but that'd be a potential way to supplement evaporation. the way I envision it, the world is like ours except for it having a large amount geysers around the earth (or one massive geyser that releases a vapor cloud into the atmosphere). These release superheated steam into the air which will fall back down as mist/rain or be absorbed into the atmosphere.

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    $\begingroup$ Welcome to the site kuhl. Interesting answer. $\endgroup$ – James Apr 14 '15 at 16:22
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    $\begingroup$ I was thinking geothermal heat in the form of marine volcanoes -but a similar idea. $\endgroup$ – Chris H Apr 14 '15 at 19:20
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    $\begingroup$ I love this answer. Great first post. $\endgroup$ – HDE 226868 Apr 15 '15 at 17:41
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    $\begingroup$ I'm not sure if I'm being a stickler here, but isn't water vapor a greenhouse gas? $\endgroup$ – AmagicalFishy Apr 16 '15 at 1:09
  • $\begingroup$ @AmagicalFishy Yes, it is, much more so than CO2. But at a certain point the clouds also reflect the sunlight away, so it kind of evens out. Where clouds come in handy is at night, reflecting the heat down instead of letting it escape into space. It would be a very warm and humid place. $\endgroup$ – AndyD273 Apr 23 '15 at 16:26
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In Stanislaw Lem's 1986 novel Fiasco, the planet Quinta is subject to continual world-wide rain, due to the ongoing slow collapse of an artificial orbiting ring of ice:

The planet was encircled by a ring of ice chunks in an enormous but unstable sheet. [...] Having one large and three lesser divisions due to perturbation caused by Quinta's moon, the ring could last no more than a thousand years, since it increased its diameter while at the same time losing mass. The outer rim was widened by centrifugal forces; the inner, from atmospheric friction, turned into melting fragments and vapor, so that a portion of the water thrown into space by methods unknown returned to the planet in a never-ending rain. It was hard to believe that the Quintans had intentionally provided themselves with a downpour worthy of the Flood. The ring had initially contained three to four trillion tons of ice; each year it lost many billions.

It's suggested, though not confirmed, that the Quintans launched the ring of ice into space because they like the climate that way.

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    $\begingroup$ It might be worth noting that having three to four trillion tons of ice raining off in "no more than a thousand years", with "many billion tons" per year, is pretty mild. That's about how much ice Antartica and Greenland are losing per year right now... $\endgroup$ – DevSolar Apr 15 '15 at 9:15
  • $\begingroup$ @DevSolar Unless the planet was a lot smaller than the Earth, of course. Perhaps it was their way of keeping their atmosphere against the solar wind or something? :P $\endgroup$ – Luaan Apr 15 '15 at 9:22
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Rain Belt

Could be that evaporation takes place closer to the poles where it very seldom rains. Evaporation occurs even if temperatures are lower, and if the poles are ALL water and ice (oceans) than those are very large body of waters and most likely cause loads of evaporation.

It helps if you land is all thin strips (coastal) akin to the Canadian West coast causing weather system to rain the moment they hit land.

It is rather hard to try and explain it scientifically, but then again I think just having more oceans/water in general, would increase the amounts of rain and storms etc.. to the point where it ALMOST always rains in mild areas.

Fancy Space answer

The planet has ice fields in it's orbit causing non stop rain on the side of the planet facing it's direction (the side not facing it gets a reprieve?) If the planet has no rotation or very little, that would make for near perpetual rain on one side. If they aren't aware of land or whats on the non rainy side then you would have a world where it always rains. (Obviously this causes no small amount of water/flooding related issues, but that could be interesting to work with in a short story also.)

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    $\begingroup$ That "fancy space answer" would be a side effect of gentle terraforming. $\endgroup$ – Fungo Apr 14 '15 at 16:44
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    $\begingroup$ Hadn't thought of that but that makes it interesting to. It could be one of the first cities on a planet being terraformed. So the city/colony can get the isolation atmosphere too on top of it raining non stop (for years to come at least), and having to cope with rising water levels and rust etc.. etc.. The people there might want to move but they can't, they have a job to do (colonists) etc.. etc.. etc... $\endgroup$ – Spacemonkey Apr 14 '15 at 17:02
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    $\begingroup$ If the planet has no rotation or very little It's called "tidal locking" and actually exists. In fact, almost exactly this scenario is happening to some moons of Saturn. $\endgroup$ – Cephalopod Apr 15 '15 at 12:19
  • $\begingroup$ That's happening to our moon, too, @Cephalopod. That's why it keeps its face to us. $\endgroup$ – Tony Ennis Apr 15 '15 at 13:10
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    $\begingroup$ @TonyEnnis Yes, our moon is in tidal lock, but with some of Saturn's moons, you also get the "it always rains on the leading side" part (although it's raining ice, not water). $\endgroup$ – Cephalopod Apr 15 '15 at 19:27
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If we can relax the "everywhere" a bit I see a quite feasible way this could happen:

The planet is warmer than Earth and has no tropical land--and the incident energy is even higher than the temperature would indicate. The tropics are extremely hot (quickly lethal to anything but an extremophile, but crossable with adequate technology), there is major evaporation.

Rather than the fast-tropics/slow-pole driven weather we see on the planets of our star system we have a tropics rises/poles drop weather pattern. As everything but the tropics are under perpetual cloud cover they're basically pure white, most of the incoming energy goes into the tropics. The tropics are so hot that they don't have clouds, the air circulation is fast enough that you don't get condensation even as the air rises. Rather, the condensation only happens later when the air cools enough and falls back down in the polar latitudes. You'll need a very slow rotation rate to make this possible.

Note that the weather on such a world is going to be vicious. Also, photosynthesis isn't going to work too well under all those clouds, I doubt you'll have much in the way of plant life outside the tropics. Instead I think the ecosystem in the polar regions will be driven by stuff sucked up in the tropics and brought in by the winds--it's going to be a pretty poor ecology.

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Have a very humid ocean world.

If your world has mostly tropical weather patterns world-wide surface water will evaporate quickly. This goes up, into the atmosphere to form clouds. If you have a world mostly comprised (surface-area anyway) of water. The water will evaporate and create massive hurricanes. These would give you the global rainstorm (and a global warming rate that might eventually make your world into a Venus clone).

Now, this is about the point in my answer where you will be getting ready to comment that you wanted earth-like. Hold your keyboard-happy fingers for a second.

You would want a small continent or group of continents in the northern hemisphere of your world where the temperatures are less humid and more earth-like. Have ocean and wind currents bringing warm water and storms constantly north and south to the poles. This will create a patchwork of clouds across your world and, while not creating a nonstop, endless storm, will give you the closest thing possible.

Your city might experience a rain-free night or day once in a blue moon, but for the most part will have near constant rain. This would lead to some crazy flash floods and a very rough landscape as water will erode the continental surface.

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If the planet was a bit closer to the star, like Venus, had a lot of surface water, and slightly higher gravity, you'd end up with a kind of runaway greenhouse effect, where water would evaporate into the atmosphere at a greater rate, and higher gravity would mean thicker atmosphere which would mean higher humidity and more water in the air.
The water would form clouds and rain, and the perpetual cloud cover would reflect some of the solar insolation, which would help balance the greenhouse effect a bit, making it livable.

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I immediately thought of Manchester, UK, which has a notoriously wet climate due its position on a plain beneath a long hill range which causes water sucked up from the sea to swirl around and dump on the city. Given that I could see a warm planet of mountainous archipelagos in which there were coastal settlements that suffered from this phenomenon, which ranged from light drizzle to heavy rain with little or no dry spells at all. If it was settled there would have to be a good reason to be there. If life had evolved there it could be decidedly more fishy than human.

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    $\begingroup$ This effect has to do with currents in the Atlantic...the pacific northwest of North America gets a similar effect. Warm water in the tropics moves up north and over the UK. Be happy, without the weather pattern, you'd be as cold as Newfoundland Canada. This works for a single spot on the globe to see these wet conditions, but it's not a world wide effect. In any case, good first answer and welcome to the site. $\endgroup$ – Twelfth Apr 14 '15 at 22:38
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Having issues in forming an answer that will work for you in complete earth conditions, so instead of an answer...I'll post the issue and see if comments can't resolve it.

The first is simple...how exactly does water vapor get to the cloud level while it's raining? There would have to be at least one region on the globe where the water evaporation rate is greater than the rate at which it's falling and I'm not sure if I can tell you a setup where it rains at the same time clouds are forming.

Other issue...Rain formation works as heat transfer. Ocean water has energy added to it which evaporates the water (water 'removes' energy from the system by evaporating). The water vapor rises through the atmosphere where it eventually cools and condenses to a liquid state. Conservation of energy...the water vapor condensing in the atmosphere releases the same amount of energy as it took to evaporate it. Assuming there is a mechanism to get rain back up to the clouds, the energy transfer involved here is leaching a huge amount of energy from the surface to the atmosphere without any real mechanism of transferring the energy back down to the surface. I believe the end result here gets into hurricane formation, potentially tornado forming storms if over land. Hurricanes are inherently self destructive, they drain the surface waters of heat and churn up cold water from beneath which ultimately kills its fuel supply...after the hurricane is finished, I have no clue how it could continue to rain without a period of calm cloud free weather to allow everything to warm once again.

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There have been legit scientific studies centered on the idea of planets that orbit their stars much closer than Earth does. In such a close orbit, such planets would often be "locked in" such that one side always faces its star (much like one side of the moon always faces Earth). In one model, it was shown that much of the star-facing side of the planet could feature a permanent hurricane, assuming that the planet has a lot of ocean.

In short: there very well could be hard-science ways for large portions of a planet's surface to be constantly and continually pelted by rain, and some of it severe. At the same time, such a setup also includes a dark side to the planet where everything is in a perpetual freeze. Because of the freeze, very little life would exist on the dark side of the planet much like how very little life exists on Earth's own polar ice caps, especially during winters.

With a perpetual hurricane, any landmasses within the reach of the storm would rarely if ever see direct sunlight, but there are places on Earth where life survives in similar circumstances. Plants can still survive on very little light, and even with constant and permanent cloud cover, some light will still get through the clouds and rain for life to subsist on.

Such a planet would have insanely dense rain forests and, depending on the location of the landmasses, a lot of humidity and heat. Also, it would always be day and never night in those places where life could thrive, so the life on such planets would likely have evolved so that it either doesn't need sleep or it has ways of finding places to hide while resting. On Earth, all life lives according to the rhythm of day and night, as well as the monthly rhythms of lunar cycles. On a planet where one side always faces the sun, no such rhythm exists, so life would possibly be constant noise with no breaks.

Rain may seem to you like a somewhat-depressing state of weather, but on a planet where life has evolved for it, direct sunshine could be deadly simply because a lot of life has never had to deal with it, so rain could be seen as a great protector of life on such a world.

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    $\begingroup$ Everybody assumes “locked”! As I've posted many times, you actually expect a situation like our Mercury, with 3:2 spin orbit resonanc. $\endgroup$ – JDługosz Oct 12 '16 at 0:35
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Either large amounts of fossil fuels are burnt, or a volcano forms where all water naturally flows to, such that global warming in very isolated and uninhabited areas generates enough heat to boil water, forming clouds to replace existing ones. In theory, if water flowed into a centralized location, that location being the heat pit, it would form large amounts of clouds.

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The planet has a large region of exposed magma, due to either tectonic activity or a large meteor impact in the recent (geologically speaking) past. Ocean water pours into this region and boils into steam, then is circulated around the globe.

This would dump a lot of extra heat into the atmosphere. The area near the magma region would probably be too hot to be habitable. But overall, heat from the magma, solar radiation, and heat loss into space would reach some kind of equilibrium. You could have livable areas if the magma region were small enough.

The concentrated heat source would also drive strong, planet-scale convection currents in the atmosphere. In other words, it'd be windy all of the time. High-volume air circulation would make the temperature more uniform around the world. The temperature difference between the poles and the equator might be less pronounced than what we experience.

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In India it rains everyday because of the monsoon. What happens is water evaporated from the Indian ocean travels northward as the normal process but because of the Himalayas acting as a thick barrier this warm air gets pushed up in the atmosphere and loses much of its heat, therefore, the water it carries falls down into rain in torrents.

So to explain the situation in your novel, the location of the city has to have about the same geographical location as New Delhi and there are has to be a mountain range running the whole circumference of the planet where there is land.

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How about rain of a certain molecular mixture, it is transformed at the surface, and evaporates as something else? Some of your energy could essentially come from within the ocean - a tidal heat in the crust can contribute to catalysis on a catalytic surface at the bottom or margins of the ocean. Say it creates an end-product that is volatile and requires little heat to evaporate. Don't get too specific. The rain could be light. -Bruzote

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Have you considered Hydrogen-Oxygen Combustion? $$2 H_2 + O_2 → 2 H_2O$$ Provided a continuous supply of hydrogen were exposed to atmospheric oxygen, combustion of the two would occur spontaenously and create atmospheric water. Since this type of combusion is an exothermic process, it will produce large amounts of heat that may boil the water.

Either way, it'd produce water in the atmosphere. Afterwards, the water would need to clear the hot air mass before it may condense and fall as rain somewhere else. While not everywhere would be raining, it would allow a large portion of the planet to be so, without pesky clouds getting in the way of evaporation.

Notably this type of reaction was used in "The Martian" to make rain inside a section of the base. Watney used leftover hydrogen rocket fuel as the fuel source.

Another fuel source to consider may include Methane; but, it'd produce CO2 as well. That'd trap heat in the atmosphere and may interfere with condensation. It would also require old plant material. N2 doesn't burn, nor do noble gases. Hydrogen is your best bet. Good ol' electrolysis. Or without the electricity, alternately.

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    $\begingroup$ This might be a decent answer, but can you elaborate how this answer the question "How can I explain a planet with perpetual rain?" $\endgroup$ – Vylix Dec 28 '17 at 2:27
  • $\begingroup$ I edited the comment heavily to include Dampendair's intended suggestion of Hydrogen Combustion. That said, I'd like to point out (in the comments) that the suggestion doesn't explain how to cotinuously supply hydrogen, nor how to replenish the oxygen burned in the process. (Evaporated water is still water). $\endgroup$ – KareemElashmawy Dec 28 '17 at 4:34
  • $\begingroup$ To answer the question of how hydrogen would be created constantly: I don't know. That's the question I was hoping someone else would answer. Or, alternately, electrolysis. Solves both. Except this is an earth like planet, and electrolysis requires a constant supply of electrons... like a battery. I don't know. Thanks for the editing. It needed it. So does this. Thanks. $\endgroup$ – Dampendair Dec 29 '17 at 21:41

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