What even is rain?

July 17, 2021 8:06 PM

If less rain falls in one place, does more rain eventually have to fall in another to compensate? Or could there be one year that was dryer than another everywhere on earth? What would a year like that look like? Where would that extra moisture go?
posted by nebulawindphone to Grab Bag (14 answers total)

Maybe not a direct answer to your question, but something to consider:

The current loss figure is equivalent ~25,920 liters per day, or 9,467 m3 per year. And the reference of that figure seem to be the paper Escape of O+ through the distant tail plasma sheet, that used measurements from the STEREO‐B (Solar Terrestrial Relations Observatory) spacecraft.

How much water is the atmosphere losing to space?
posted by slater at 8:28 PM on July 17, 2021 [1 favorite]

Best answer: The water that falls as rain got into the atmosphere through evaporation. There is no reason to expect that the exact same amount of water would evaporate from oceans, rivers, etc. every single year. On the contrary, you would expect the amount of evaporation to vary from year to year depending on variations in temperature, wind, ocean currents, etc. So, sure, in theory there could be a year where less rain than average fell everywhere on earth, because less water vapor than average got into the atmosphere everywhere on earth. The extra moisture that didn't fall would be in the oceans and lakes and rivers. It would be water that never evaporated.
posted by Redstart at 8:38 PM on July 17, 2021 [9 favorites]

Best answer: Wikipedia actually has a decent diagram on its water cycle article.

You can see that 4.6 + 1.2 = 5.8% of all the water on Earth falls as precipitation every year. Of this, most was evaporation from the ocean (5.1%) and the rest was evapotranspiration from land.

The word "evapotranspiration" is one clue -- a big source of water entering the atmosphere is plants' transpiring -- they take water from the soil and emit it as vapor, which then falls as rain elsewhere. So if the world has less greenery in one year, it would have less rain.

Also, of course water evaporates more quickly if it's warmer, so the mean surface temperature being higher (over the oceans in particular) will result in more evaporation which in turn means more rain. This means that it probably rains less in years when less sunlight hits the earth's surface, for instance after a big volcanic eruption.
posted by goingonit at 8:58 PM on July 17, 2021 [5 favorites]

It's also worth noting that most of the rain falls on the ocean, so it's certainly possible for *every landmass* to be drier all at once and just for more of the rain to fall on the oceans and get ignored by everyone. This would be a change in the 0.56% "ocean to land atmospheric flux" in the Wikipedia diagram.
posted by goingonit at 9:03 PM on July 17, 2021 [4 favorites]

Best answer: I think the last question is really 'Where would that extra moisture stay?' with the answer, In the Oceans.
posted by Rash at 9:28 PM on July 17, 2021 [3 favorites]

What even is rain?

The output of a large solar-powered still. And no, it doesn't run at the same rate every year.
posted by flabdablet at 10:36 PM on July 17, 2021 [7 favorites]

Another place rainwater can go is underground into aquifers etc. According to a pretty recent study, there are 6 quintillion gallons of water underground in this type of aquifer, and this makes up 95% of the earth's fresh water, excluding ice caps and glaciers.

You probably know that a lot of the water we use is taken from aquifers, but others are contaminated with various things (salt), too deep, etc. Just for example, I've read that more water seeps into the sandstone under Lake Powell than evaporates, and that the vast majority of that doesn't go into recharging nice, usable aquifers but rather into salty or contaminated aquifers that will like never be usable. (Can't 100% vouch for that as I can't find a really reliable source for this right now.)

In short, at least some of the water that seeps down into the earth's crust is lost for the reasonable future--too deep to drill, contaminated, etc. It won't seep out, evaporate, etc. It's trapped--maybe not forever, but for a long time.

I assume it might come cycling back up on the scale of hundreds of thousands to millions to hundreds of millions of years so it's not really 100% lost, but definitely for the next some years it's out of the regular water cycle.
posted by flug at 2:24 AM on July 18, 2021 [2 favorites]

Just a comment to say that this question has both a scientific and cultural answer. In the latter camp is the belief that many people seem to have that rain is a sort of karmic entity. Here in wet Scotland we sing "why does it have to rain on me?" and you will also hear people say "we're going to pay for this" after more than about 3 days of sunshine. Dry places see the apparently mendacious distribution of rain in space and time in the opposite way.
posted by rongorongo at 7:12 AM on July 18, 2021

Response by poster: This is in fact why I'm wondering! All my life I've sort of imagined that if we were getting a drought, our rain had gone to somewhere else and I could at least be happy for them! It only recently occurred to me that it might not be that zero-sum at all.
posted by nebulawindphone at 11:22 AM on July 18, 2021 [1 favorite]

Best answer: Variations in ocean surface temperature have a lot to do with how productive the global desalination plant is in any given year.
posted by flabdablet at 11:32 AM on July 18, 2021

If less rain falls in one place, does more rain eventually have to fall in another to compensate?

To compensate for...what? As you note, rain is not zero-sum, there isn't a fixed/set/predetermined amount of rain that falls on Earth every year, thus necessitating a rebalancing or load-shifting if one region is drier than another.
posted by pdb at 3:16 PM on July 18, 2021

I recall reading that the amount of water on earth stays pretty much the same, doesn't separate into hydrogen and oxygen much, nor do they frequently combine to make new water. Some of it gets stored in ice; as earth's ice melts, there's more liquid water - this affects the energy in weather systems, ocean levels, water available for rain, and probably a lot more that we don't understand and probably can't predict. The severe heat events and floods in Europe were apparently not predicted by Climate Crisis models. Weather is incredibly complex, it's amazing it's predictable to the extent it is, but the effects of carbon & heat pile up, we don't really know what will happen.
posted by theora55 at 3:17 PM on July 18, 2021

Or could there be one year that was dryer than another everywhere on earth? What would a year like that look like? Where would that extra moisture go?

There could be such a year, and all that would be necessary (but not sufficient) is that it be hotter than average.

And that's because hot air has much greater capacity to hold water vapor without condensation than cooler air. It's often pointed out in Seattle, for example, that in the rainy season here there's actually less water vapor in the air than in dry Arizona, but it doesn't rain there because the hotter air holds onto its water. So the answer to your final question is that the extra water is in the air.

I looked yesterday for a simple graph which would show this basic property of hotter air, but all I found instead, in ostensibly relevant pages and PDFs, were highly unintuitive psychometric charts which look like they were drawn by runaway spirographs, and seem to be useful mainly for things like making sure the buildings where you feed your herds of "swine" minimize condensation on the walls.
posted by jamjam at 6:48 PM on July 18, 2021

jamjam: here is the chart you're looking for. Indeed, from 20C to 30C, the amount of water a given mass of air holds at saturation almost doubles.

However -- the atmosphere only holds 0.0009% of the world's water! Put another way, every year, it rains 6,000 times as much water as is in the atmosphere right now. So even if you doubled the water content of the atmosphere permanently, that would only reduce the amount of rain that falls this year by 1/6000th. And, once the atmosphere was saturated, unless the rate of evaporation was reduced as well, there would be no continued effect. The oceans would just be veeeeeeery slightly saltier.
posted by goingonit at 6:32 PM on July 19, 2021