# If the rain all fell at once, how would Earth be different?

If all the rain for the next 24 hours fell in one go, say over 5 seconds, how would the world be different?

We're assuming the same volume of rain falls in each place on the Earth as does now, for example the U.K. gets around 3.6mm per day, of course varying throughout the year.

My main interest is how plants would be different, and how human infrastructure would differ.

• Obligatory XKCD reference
– PTwr
Jan 5, 2017 at 12:23
• @InbarRose the latter - it rains all the rain every day at, say, midday.
– Tim
Jan 5, 2017 at 12:29
• Look at desert regions, where the infrequent rains tend to come as brief but torrential downpours, and extrapolate. Jan 5, 2017 at 19:31
• @Tim: Not true that all deserts are hot. Look up "cold desert". FTM, I live on the edge of a desert, and it's snowing at the moment :-) But what I was suggesting has more to do with the hydrological effects of sudden downpours, like severe erosion, flash flooding, &c. Jan 5, 2017 at 22:26
• @RonJohn “we are assuming the same volume of rain falls in each place as does now”. I think that clearly covers the no rain situation
– Tim
Feb 19, 2018 at 11:13

# Things will be flattened

Most places in the Eastern US will see an inch of rain in a day at least once a year, and often as much as two inches. Here is last May in DC, where we got 1.24 inches (31 mm) on 5/2.

31 mm is a lot different from 0.8 mm. That means that every square meter of surface area will get hit with 31 kg of rainwater. Even if your body's upwards facing surface area is 0.1 m$^2$, you will still be hit with a 3 kg weight at terminal velocity. Ouch. But its worse for your house. Your 150 m$^2$ of roof just got hit with two tons of water, also at terminal velocity. Housing insurance will cost more in this world, to say the least.

Loose soil will be churned up and run off. Leaves will be stripped from trees. Grass and flowers will be crushed. The falling sheet of water will destroy all flying bugs.

In short, this would be a bad thing, and neither plants nor human infrastructure would exist as we know it.

• I'd argue if you dropped a 31mm thick "plate" of water from 1000m height, its would be no different from ordinary heavy rain by the time it reaches the ground. Water does not stick together when falling in air. The terminal velocity of large rain drops is just 9m/s, less for smaller drops. Jan 5, 2017 at 20:38
• I agree with @Durandal. Even if you released a sheet the water would break up into large drops and spread out a bit unless you released from small height but would still break up a bit. Jan 5, 2017 at 20:49
• except it would still deliver significant force, the volume of rain would still be much higher thus the force delivered on site would also be higher. the destruction of soil is mostly caused by the volume of rain at a given time anyway, so either way say goodbye to your topsoil.
– John
Jan 5, 2017 at 20:51
• @Paparazzi the volume is much higher becasue the time is much shorter, I see, I am talking about the amount of water per unit air. You are squeezing a day of rain into a few seconds, that means more water per cubic meter of air. The effect will be similar to a waterfall not rain.
– John
Jan 5, 2017 at 20:58
• In irrigation it is used in terms of high volume vs low volume high volume pipes deliver more water per unit time, it really doesn't matter for the point the amount of rain per second is so high it will cause drastic erosion and create a significant force. Dispersal rules don't matter much in rain since the stream takes up the whole sky so the weight of the rain is relatively conserved.
– John
Jan 5, 2017 at 21:18

Massive erosion.

Less plant life as most of the rain runs off before it has a chance to be absorbed into the soil. This would increase erosion. As plant life gets thinner even more erosion.

Depending on the terrain and volume could get local flooding.

To follow up on a comment from Durandal. If you instantaneously released a 3mm sheet of water from altitude it would break up into drops of about 5mm and a terminal velocity of about 10 meters / second (20 miles per hour). The individual drops would not hurt anymore than a heavy rain storm. I have been out during a hurricane with 12 inches in an hour and you are drenched but it does not hurt. Trees did not lose leaves. The primary impact is erosion and flooding.

• Is it really such a massive change? Can soil not hold 0.7mm of rain per day?
– Tim
Jan 5, 2017 at 12:40
• @Tim It does not rain 0.8 mm per day everywhere. Yes 24 hours of rain in 5 seconds is a massive change. Jan 5, 2017 at 12:43
• @Tim it about how it arrives normally rain is spread out giving it time to percolate down into the soil, all at one it does the opposite and runs off taking a lot of soil with it. We see that just in rainfall patterns now. Heavy rains can strip soil quickly. crops.extension.iastate.edu/cropnews/2008/06/… water absorbion takes time.
– John
Jan 5, 2017 at 20:55

i imagine a prairie might evolve well in that situation, lots of organic material to soak up and hold all that moisture, fibrous roots to hold soil and grass type plants are very resilient to agitation/cheap to rebuild from the plans perspective. as for human infrastructure, early groups would have coalesced around natural cachements. agriculture is hard to figure, most of our current crops wouldnt be great, maybe a grazing culture? or perhaps, because the torrential rains wash everything off the surface, humans live in caves and subsist primarily off of fungiculture (like https://en.wikipedia.org/wiki/Ant%E2%80%93fungus_mutualism)

taking the ant thread farther, its a mystery how ants predict rain but they do so incredibly accurately, much of "our" lore and tech would probably center around predicting the periodic cataclysmic catarachts

• Please, use a bit more time to write your answers. A bit more than just your imagination - some facts, references to back it up, maybe? Feb 19, 2018 at 8:29
• @molot , dont forget to police frank's post as well Feb 19, 2018 at 9:18
• It's your post that landed in Very Low Quality review queue. If his post will end up there, I'll review it as well. If you have a problem with it, then you should take action. Telling others how they should perform their part of community moderation is kinda rude, unless you think they made mistake. Feb 19, 2018 at 9:21
• oh okay, there is a reply directly above mine that is far sparser, i was very confused by the selection of my answer as the problem. but, regardless, ive fleshed out the ideas, i hope this is up to community standards Feb 19, 2018 at 9:28
• @PatrickZissou May be worth taking a look at the tour and help center if you haven't already. It takes some time to get used to the expectations around here, you'll get the hang of it. Feb 20, 2018 at 15:15

Wet deserts, areas above a certain rainfall threshold, I'm not sure what the limit would be exactly sorry, would consist of either clay pans or sandy gravel both without any plantlife as all the silty sediments were washed away by each downpour. Each daily rain dump would strip all the fine sediments and any plant litter that had built up during the rest of the day. Clay bonds together tightly enough to stay together in the face of heavy rainfall and course sand and gravel are too heavy to move but silt grains and light organic debris will be washed off continuously. More river sediment also means the sea will be better fertilized, for good and ill. In short you'd lose a lot of vegetation on land and gain a lot of seaborne vegetation in wet climate areas. Not a total answer by any measure but some things to think about. Also you need to think about how this effects seasonal rainfall variation, in places where there is such a thing, so not the UK, and storm systems and cycles such as hurricane season and the monsoons, both of which would be weird.