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I'm intrigued by the notion of life on a planet where rain is a constant presence. Imagine a world where precipitation never ceases, where the skies are perpetually gray, and the sound of raindrops provides the soundtrack to daily life. What would life be like on a planet where it rains constantly, assuming conditions similar to Earth? How might plant and animal life adapt to the perpetual moisture?

Thanks!

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What would life be like on a planet where it rains constantly, assuming conditions similar to Earth?

The Netherlands and the UK manage it pretty well, they just complain a lot about the weather.

Jokes aside, there was a period of time on Earth where rain was constant, it lasted 2 million years.

many [findings] suggested one thing: around 232 million years ago, the Earth left a dry spell and it began to rain. In fact, given that the gray sandstone and siliclastic sediment was deposited over a long, long time, it was [evident] that right at the beginning of the age of the dinosaurs when their numbers and diversity exploded, there was an unusually wet period lasting 1-2 million years.

In fact, since the discovery, there has been growing evidence that the wet period may have been the "trigger that enabled dinosaurs, and possibly the other members of the modern terrestrial fauna, to diversify and dominate the land".

In those times, due to concurrent huge volcanic eruptions, the rain turned into acid rain and killed many species, leaving space for the emerging dinosaurs to diversify.

In your case this doesn't seem to be the case, so there seems to be no big deal with life under constant rain. I can imagine plants and animals will adapt to a lower light intensity, considering that the cloud cover will make for a dimly lit environment.

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assuming conditions similar to Earth?

I think you have to assume that it won't be globally that similar to Earth.

The problem you have is that planets are big, and the climate is very variable across their surfaces. Even in the middle of something like the Carnian pluvial episode that L. Dutch proposed, there will still be things like rain shadows, because that's just how geography rolls... the water has to come from somewhere, and landscape is an excellent trigger to make it come out again, and once it has come out the downwind landscape is going to be less rainy. Certainly, there were deserts in the late triassic.

(as an aside, there's a possibility that the Carnian rains were wholly or partially a megamonsoon, and thus seasonal and at least partially localized, but I'm not really equipped to chase that reference for you).

Some of the wettest-all-year-round inhabited places on Earth are sandwiched between mountains and sea (people might complain about the Netherlands being damp, but I assume they've never visited the Pacific Northwest, or the Atlantic coast of Norway). They can have substantially less annual rain than areas which experience monsoon climate (also a result of convenient mountainous geography), but monsoons are seasonal and so must come hand-in-hand with a dry season, which presumably doesn't quite fit with what you wanted.

What I would suggest, then, is not quite that the entirety of the planet's surface is continually rained upon (because that's a bit impractical), but instead that the inhabited bits of the planet might be (almost) continually rained upon. High ice-capped mountain ranges with high-atmosphere desert plateaus are simply not very inviting, and the lack of warmth and potentially harsh weather make living there inconvenient. Lower altitude coastal landscapes can be more temperate, and the fact that they can support considerable plant and animal life makes them potentially a lot more appealing to live in... both interesting, but also providing a potentially useful source of materials and food (or work animals, etc). Careful positioning of landmasses and tweaking of climate limits the amount of welcoming land without frequent or near-continuous rainfall.

It might be possible to arrange an even more unusual setup, with a tidally locked planet with large oceans and carefully placed continents. World-encircling wind patterns can drive heavy rain and towering clouds over the sun-synchronous point. Sure, the dark side of the world won't be that rainy, but it'll also be colder, very dark, and have a distinct lack of plants and a very different (possibly microscopic) ecosystem. There's more information in Atmospheric Circulation and Climate, from a book about Red Dwarfs, but I don't have a convenient unpaywalled source for that right now. Worldbuildingpasta has a nice collect8ion of information on habitable tidally locked worlds, if you wanted to read more on those.

How might plant and animal life adapt to the perpetual moisture?

I would have a look at how things live in cloud forests and related biomes like laurel forests, which are perpetually damp if not necessarily perpetually subject to rainfall (but they get at least twice the rainfall of some of the wettest cities on Earth, and don't experience significant seasonal changes in precipitation). Similarly, rainforests of both the tropical and termperate kinds can be extremely wet, but even they can have day-night cycles of rainfall, as morning sunlight drives moisture into the air.

Such places support a very wide variety of plants and animals, including birds and fluffy mammals. You also get a lot more plants and animals that require high levels of moisture to survive. Your rain world might do well for amphibians and mosses, for example.

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  • $\begingroup$ Also, it sounds like a brilliant environment for fungus. $\endgroup$ Apr 2 at 18:13
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It washes out the ground over time, similar to australia which is so old it has seen alot of rain. https://en.wikipedia.org/wiki/Australian_Plate

So very little nutrients and those usually captured in the flora and fauna. It also sweeps away the soil you want over time https://en.wikipedia.org/wiki/Erosion so you end up with rocky planes and wadis https://en.wikipedia.org/wiki/Wadi - and as it always rains, the wadis are always filled. Agriculture will move were the soil is and try to shore up the flowing away riches.

Imagine terraces similar to https://en.wikipedia.org/wiki/Banaue_Rice_Terraces were the swept away soil is captured and redistributed up hill. Mold and fungi are a tremendous problem, as everything is constantly wet. Thus important buildings are made not from wood, but burned ceramics or stone.

Evolution also rewards those keeping dry and thus keeping the heat together - or if its a https://en.wikipedia.org/wiki/Wet-bulb_temperature it rewards turning into a dinosaurcrocodile and eating mammals working in the terraces.

Hydro-electric power is a good source of energy- the oceans of such a world would be bathed in sunshine and steamy (the distilled water from the sky has to come from somewhere) - and thus very hard to navigate. Lots of moisture also means hurricanes. Big ones. https://en.wikipedia.org/wiki/Tropical_cyclone

Mining would be hard, as you have to pump ground water constantly, then again- hydro mining would be easy- just fill a damn in the mountains, then cut with a water jet into the mountain and collect the rewards at the outflow. Or just fish the metals out of rivers sandbanks.

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Some additional thoughs re.

Agriculture

Food production and thus diet would be very much different from what we know on Earth. From my dabbling in vegetable gardening, I know many water-logged crops don't flourish and become "drowned" easily, or else become sickly and weak, being attacked by pests. Too much rain can turn the soil anaerobic (compacted and the water displacing air), where it is during normal drier times pretty aerobic (air in pockets in between the soil particles). This influences microbial life in the soil, shifting the preference to the anaerobic species. While many aerobic bacteria, fungi etc. live in symbiosis with plants and provide nutrients (e.g. minerals gained from rocky material) to plants in exchange for some sugars (manufactured via photosynthesis), ANaerobic species are often involved in processes of putrefaction. (I try to aerate the soil with the tines of a gardening fork after prolonged rains.)

So I expect that plant species on the planet, as consumed by native life and colonists, will favor more moisture-loving and aquatic plants (in Earth terms, think of species favoring swamps, waterways, lagoons, etc. etc.)

For agriculture, especially if you look at a colony from Earth, maybe something similar to hydroponics may become the preferred method. However, with constant rain you get the problem of leaching of nutrients out of the ground - small quantities but constant and over geological timeframes. So the runoff will not contain sufficient to feed plants (except maybe if they have super developed filtering systems), while rain itself contains only distilled water.

I can also imagine plants having very large leaves (to compensate for lower sunlight due the constant overcast) but maybe turned outwards (to direct some rainfall away from the plant as opposed to help it collect water).

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Preconditions

Rain exists as part of a cycle that involves heat transfer. Without evaporation (energy moving into water), and condensation (energy moving out of water), you can't have rain.

On Earth, we get evaporation from sunlight on water. If your skies are constantly overcast, however, you may not get enough energy transfer to cause evaporation. So you have two obvious choices:

  1. The planet's ground is very hot
  2. The planet is close to the sun

A young, volcanic planet would have acid rain due to the presence of high quantities of sulfur.

A planet close to the sun would likely just be hot all the time. Because the sunlight is directional (i.e there's a dark side to the planet as it rotates), without enough heat, you'd get higher rainfall past the terminator and the possibility of a break in the rainfall as the atmosphere loses enough moisture.

Perhaps the exact mechanism is unimportant or hand-wavium, but it's still good to keep in mind

Flora

I'm assuming that rainfall isn't constant light sun-showers, but rather a steady downpour. Without light, most species of plants will have trouble growing, with the possible exception of fungi. Other non-photosynthesizing plants are parasitic, and without a host, couldn't exist.

Even aquatic plants, like kelp, need sunlight.

Without plants, higher-order life won't be able to survive. So your planet will be mostly barren.

What about a colony?

Let's say your planet has been colonized to mine its resources, for example rare earth minerals. It's feasible a small crew of miners live on imported food and are under constant downpour. What would that life be like? My guess? Something similar to submariners + miners. With a decent supply of alcohol or other sedatives and plenty of entertainment.

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