Assume my kaiju has just been blown open by the military, its organs and intestines spilling out.

What gas, either released from its innards or released during decomposition, could ascend and cause rain?

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    $\begingroup$ When I was in primary school I was told that it is water to ascend and create rain. $\endgroup$
    – L.Dutch
    Commented Oct 17, 2022 at 16:40
  • $\begingroup$ @L.Dutch it's a little bit more complicated. Because otherwise we'd have a reliable way to cause rain. We don't. Right now, we can point a hose up and make water come down over an area but that doesn't qualify as rain. The question is asking for something that's currently not possible using IRL science. I'm not really sure what possible answer can there be to this which doesn't amount to "magic" as this is presumably happening in real-world-like setting. And if not, we're supposed to build everything about it including the agent that will trigger rain. $\endgroup$
    – VLAZ
    Commented Oct 17, 2022 at 17:04
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    $\begingroup$ So... people think this question is asking multiple questions? I don't understand why. It is simply asking what could be emitted by an insanely giant animal that would trigger rain to fall. It's one question, with a clearly definable answer and based in science. $\endgroup$
    – DWKraus
    Commented Oct 17, 2022 at 19:58
  • $\begingroup$ Agreeing with DWKraus, there's one single question, so needing focus isn't a really valid question ^^. For clarity however @ Luke Duffy, can you confirm that besides the size, your kaiju is similar to some Earth organisms? Carbon-based, with (giant) organs you could find on Earth? $\endgroup$ Commented Oct 17, 2022 at 22:05
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    $\begingroup$ @DWKraus the close reason used to be called "too broad" and covers questions that also ask for one unfeasably big thing. E.g., something you can write a book on. SE have reworded the close reason so that it mistakenly sounds like it's only applicable to asking for multiple things. But the guidance for the close reason hasn't changed. The question seems to me to be asking for building the entire world so that rain is possible to be triggered on command and to provide the mechanism for this rain to be triggered on command. That or it's just asking a question about real-world science. $\endgroup$
    – VLAZ
    Commented Oct 18, 2022 at 6:54

3 Answers 3


Water Vapor

The Kaiju's corpse is much warmer than the surrounding climate, and releases large quantities of water vapor (thousands of tonnes -- the Kaiju keeps the water as hot, pressurized liquid which vaporizes when the body is breached and internal pressure lost). This vapor will immediately result in a huge increase in local humidity and produce precipitation -- fog, rain, or snow, depending on the ambient temperature.

This will be very localized, of course (the clouds in an ordinary cold front are hundreds of thousands of tonnes, much too big for a Kaiju to carry around internally).

Don't expect a refreshing rain, though; along with the water vapor will go hundreds of tonnes of various very stinky Kaiju body chemicals; the rain will not only smell like raw sewage or rotting meat, it will be quite toxic as well.


Silver and Lead iodide:

Your Kaiju corpse is burning, releasing clouds of smoke full of silver iodide and other salts that act as seed particulates. This has the same effect as cloud seeding. Other alternatives are if the kaiju is full of liquified propane.

Similar fires are being used on an industrial scale in places like China to control the weather and cause rains to occur when and where the government wants.

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    $\begingroup$ Cloud seeding isn't a "rain spell". It aims to increase the amount of rain overall in an area. Rather than cause rain to happen immediately. And there is still no real scientific consensus whether it does the overall thing reliably or not. $\endgroup$
    – VLAZ
    Commented Oct 17, 2022 at 16:55
  • $\begingroup$ @VLAZ All the answer needs to do is trigger existing moisture to precipitate nearby. It can't actually increase overall rain, just cause the rain to happen where you want (in this case, near the silver-filled kaiju). The Soviets used seeding to trigger rains from radioactive clouds drifting towards Moscow. weathermodificationhistory.com/… $\endgroup$
    – DWKraus
    Commented Oct 17, 2022 at 17:05
  • $\begingroup$ @VLAZ, AFAIK, cloud seeding causes clouds to drop their moisture here instead of there. It doesn't increase overall moisture so much as drop it on a specific area, like where they're growing crops. $\endgroup$ Commented Oct 17, 2022 at 17:47
  • $\begingroup$ @RobertRapplean and the question does not ask for "how do I make my already existing clouds rain" but just for how to spontaneously cause rain. Note that even with cloud seeding, the precipitation is not a guarantee. You do still need other conditions to exist. It's not magic. $\endgroup$
    – VLAZ
    Commented Oct 17, 2022 at 17:51

This is a Frame Challenge

Humanity knows of precious few chemicals that can "cause rain" (or, more accurately, increase the likelihood of rain). A trivial Google search into "cloud seeding" will list them. The most popular are silver iodide, potassium iodide, and dry ice (frozen CO2).

You might be tempted to believe that since (e.g.) potassium iodide is a nutritional supplement (commonly used to iodize salt) that you can use those chemicals with impunity. Not really. In fact, in quantities that could cause rain (even in something as large as a Kaiju), they're down right poisonous. Besides, I dare you to have your dead kaiju decompose into solid CO2 (never mind how it gets into the atmosphere...). Anyway...

That's all boring

Let's look at what could cause the likelihood of rain to increase. Forget whether or not there's an actual chemical that can bring these causes to pass. Think something like this:

Marshall Pentecost, Dr. Geiszler thinks he's discovered why it rains when a Class 6 Kaiju dies.

Well, get on with it, what causes the rain.

I wish it was that easy, sir. Newton insists on calling it "Gottliebium" because, and I quote, "it's always raining on our parade." All he'll tell us is that it's a gas released during decomposition that exacerbates ___ insert mechanism here ___.

That's Newton. Was he kind enough to tell us how to make it stop?

Um... yes, sir. He's perfected a counter-agent gas. He won't tell us what's in it, either, but he has suggested a name....

Let me guess. Newtonium.

Uh, yes, sir. He said something along the lines of, "the sun always shines when Newt's on time!" and then he started playing air guitar.

If you're going to worry about the specific chemistry that's causing the rain, you'll be forced to explain how that specific chemistry is in your Kaijus in the first place, and that rabbit hole runs very, very deep. Don't do that. Pick the mechanism, ignore the chemistry. That's my frame challenge.

So, what mechanisms can increase the likelihood of rain?

  1. Water is naturally cohesive, which is a fancy way of saying water is sticky. If enough water is in the air it naturally bonds together to form droplets. Therefore, your gas increases the cohesion of water.

  2. Water condenses when it gets cold. So your gas could reduce the local air temperature.

  3. The effect of #2 can also be brought about by a low pressure zone. Air rushes in, collides in the center, then some of it forces its way into the upper atmosphere (where it encounters colder temperatures). So, your gas can cause a regional low pressure zone.

  4. If you really want to challenge people, when subjected to a strong enough magnetic field, water is repelled. Yes, yes, yes... it must be a whomping strong magnetic field, but we have Kaiju, right? Your Kaiju is magnificently magnetic, and the gas is the source of the magnetism. When released, it pushes water away from the area of the Kaiju, causing it to momentarily "bunch up" around the perimeter. Which means it rains around the Kaiju, but not on it! (And the Kaiju Cultists go wild!)

  5. You know what our atmosphere has? A lot of free oxygen! About 21% of our atmosphere is free oxygen. And do you know what oxygen needs to become water? Hydrogen! But does that mean your Kaiju is emitting hydrogen during decomposition? NO! Think "Hindenburg" Besides, I've watched hydrogen squirted into the atmosphere (in small, controlled amounts...) and it doesn't precipitate into rain (at least small amounts don't). So what do we need? Well... the Hindenburg... but in a controlled fashion! When you burn hydrogen, what you get is water! So when a Kaiju dies, it burns! Not explosively, but fast enough to inject water into the atmosphere.

But you do have a problem

Any and all of these mechanisms will increase the likelihood of rain. None of them will simply cause rain. I don't believe it's possible to guarantee you'll always have rain. At least not scientifically, which is why I'm advocating that you care about the mechanism and forget the chemistry.

In your world, there's always rain when a Kaiju dies — and we know the mechanism that's causing it.

And if you really do want to know the chemistry, we know that, too. It's Gottliebium gas.


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