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In a fantasy world I've been working on Dragons are venerated by many for their ability to seemingly summon rain where ever they roam. It's common for the peasantry of the south to leave offerings of both animals and incense to not only attract dragons to a region, but also as thanks for the blessing of rain.

What kind of gas or chemical would a Dragon have to exhale to cause rain? The effect doesn't have up be spontaneous, but anything quick (a few hours) would be preferable.

Note: don't worry about how a dragon produces the chemical too much. That will be the topic of another question.

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  • $\begingroup$ Just Sharing a thought, even if you manage to find a reasonable substance that causes rain, a single dragon won't be able to produce large scale quantities of it to cause rain, unless its a really really huge dragon or if you are planning on a whole pack of dragons. $\endgroup$
    – V.Aggarwal
    May 15, 2019 at 4:10
  • $\begingroup$ @V.Aggarwal I'm planning on packs of dragons (broods), but also a few titanic dragons here and there. $\endgroup$
    – Celestial Dragon Emperor
    May 15, 2019 at 11:43
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    $\begingroup$ How about exuding .... water? $\endgroup$
    – Carl Witthoft
    May 15, 2019 at 12:02
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    $\begingroup$ @Carl Witthoft Water cooled dragons for your bitcoin hoarding needs! $\endgroup$
    – Muuski
    May 16, 2019 at 21:14

9 Answers 9

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Salt. Also, ice. But not the normal kind.

Cloud seeding is a pretty cool technique, and involves blasting ions into clouds to make them rain. Normal salt (sodium chloride) is effective, but there are a few others which make for more effective draconic breath weapons, like calcium chloride (eye irritant), silver iodide (also an eye irritant) or solid carbon dioxide aka dry ice (can cause frostbite on contact).

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    $\begingroup$ I love the idea of dry ice breath! $\endgroup$
    – Celestial Dragon Emperor
    May 15, 2019 at 3:54
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    $\begingroup$ I suspect that hurled microcrystals of dry ice are far less effective in causing frostbite than holding one in your hand. The contact time is minuscule. $\endgroup$
    – John Dvorak
    May 15, 2019 at 7:45
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    $\begingroup$ In a cloud of dry ice you'd have trouble breathing though. $\endgroup$
    – Borgh
    May 15, 2019 at 9:46
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    $\begingroup$ @Baldrickk Perhaps the dragon doesn't breathe dry ice, per se. It exhales carbon dioxide (much like humans), but does so at such a high pressure that adiabatic cooling flash-freezes it, and so the dragon appears to be breathing dry ice. This would also create a region of air cold enough to cause frostbite even if the dry ice itself did not. $\endgroup$
    – The Spooniest
    May 15, 2019 at 12:48
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    $\begingroup$ @TheSpooniest that would be a cool way of doing it. Pun definitely intended $\endgroup$
    – Baldrickk
    May 15, 2019 at 13:20
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Frame Challenge: It's irrelevant what they breathe

If the dragon was the size of a Boeing 747, the exhaled breath wouldn't be enough to cause more than a second worth of rain over a 500-population town. If the only thing the dragon exhaled was pure silver iodide or CO2 it wouldn't be a large enough quantity to cause substantial virga.

If you ignore that inconvenient truth, then I'd vote that your traditional money-grubbing dragon, having slept so long on a huge pile of precious metals located in an undersea cave with surface access (iodine can be derived naturally from brine), spews silver iodide.

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  • $\begingroup$ This doesn't count as a frame challenge. You're saying that no substance would work, which is a fairly direct answer to the question (but also wrong - if no chemical works, you could always resort to something crazier like nanobots). $\endgroup$
    – Brilliand
    May 15, 2019 at 21:50
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    $\begingroup$ @Brilliand, Only the OP could approve "something crazier" like nanobots. Stack Exchange doesn't permit questions that allow for any old answer. Within the context the OP provided, I consider this the best answer, and it's a frame challenge because without some form of magic (fantasy or Clarkean), dragons can't make it rain. $\endgroup$
    – JBH
    May 16, 2019 at 0:21
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Pollens

Some time ago, someone told me that Sugarcane farms cause rain, I am not sure if it is just some kind of belief or an actual fact. But if we assume that sugarcane generates some kind of pollens (again, not sure if they actually do) that when comes in contact with clouds may cause cloud seeding and hence causes rain.

You can use something like this. Lets say that your Dragons like to spend a lot of their play time in grasslands that cause a lot of pollen to get stuck into their body scales and a lot more to go inside their respiratory system (also assume that it doesn't cause the dragons to sneeze a lot, like the way it does to me) and when these dragons go high up to the cloud altitude the pollens comes out and acts as cloud seed.

You can also add a twist to it, by assuming that these pollens have some kind of bacteria that flourishes inside of your dragon's respiratory system and produces some micro mucus particles that also adds up to the required quantity of cloud seeding.

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    $\begingroup$ ooooh I like the idea of dragons have a niche in the environment like spreading pollen (since I was going for a hint of realism) $\endgroup$
    – Celestial Dragon Emperor
    May 15, 2019 at 11:46
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Dragons are reversed heat engines, similar to refrigerators and air conditioners: they breate out hot air to cool down their bodies. This causes moisture in the air to condense around them and fall down in the form of rain and snow.

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  • $\begingroup$ Welcome to Worldbuilding.SE! We're glad you could join us! When you have a moment, please click here to learn more about our culture and take our tour. This is a clever idea! The water is condensing on the dragon and dripping off it's body. Cheers! $\endgroup$
    – JBH
    May 15, 2019 at 16:18
  • $\begingroup$ I like this idea, I imagine that a powerful dragon has quite the heat build up. Hence, steam and tons of humidity. $\endgroup$
    – nostalgk
    May 15, 2019 at 18:24
  • $\begingroup$ This works really well because I wanted my dragons to have a kinda symbiotic relationship with the dwarves. Dwarves make intricate and fancy jewelry for dragons and dragons heat the blast furnaces the Dwarves need to smelt mithril/adamantium equivalent. $\endgroup$
    – Celestial Dragon Emperor
    May 15, 2019 at 18:49
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Dust.

Rain droplets originate with a bit of dust that the water droplets collect and condense upon, until it gets too heavy and falls.

Exhaling (or otherwise excreting) lots of ash would be a good choice and appropriate to dragons. http://volcano.oregonstate.edu/how-do-volcanoes-affect-atmosphere-and-climate:

The main effect on weather right near a volcano is that there is often a lot of rain, lightning, and thunder during an eruption. This is because all the ash particles that are thrown up into the atmosphere are good at attracting/collecting water droplets.

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    $\begingroup$ Maybe dander would work too. $\endgroup$
    – Willk
    May 15, 2019 at 13:24
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They could exhale then burn hydrogen gas for their flame breath - the direct result of burning hydrogen is water

Not familiar with biological processes that generate hydrogen (I imagine it would be very energy-intensive) but regardless, it can also help keep them aloft by storing the hydrogen in gas-filled lung-like sacs between their internal organs

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  • $\begingroup$ I think exhaling methane is more plausible, since living animals actually produce it. $\endgroup$
    – Michael Ivko
    May 15, 2019 at 16:29
  • $\begingroup$ On the subject of dragons exhaling hydrogen, I would HIGHLY recommend reading Peter Dickinson's "The Flight of Dragons" (amazon.com/dp/0879518391, ISBN 978-0889025691 or 978-0879518394) which attempts to offer a biologically plausible explanation. $\endgroup$
    – Matthew
    May 15, 2019 at 16:44
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Soot can also act as a cloud condensation nuclei (worse than what others mentioned, but you probably don't need 100% reliable rain generating dragons), so one way would be for the dragons to exhale flames with the incomplete combustion of something producing lots of soot. Interestingly, propane itself is a cloud seed so if your dragons run on propane they could exhale this combination of seeds :)

They might also shed skin/scales while flying in large quantities which have high concentration of non-organic seeds others mentioned. I know you said you didn't care, but one reason might be that they need/enjoy lightning and have evolved to increase the probability that thunderclouds will form around them.

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  • $\begingroup$ I love the contribution at the end I only said I didn't care how so I wouldn't get swarmed with "not possible" answers. $\endgroup$
    – Celestial Dragon Emperor
    May 16, 2019 at 1:00
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Just a thought: do you allow semi-magic? If a dragon can belch out a large thermonuclear device, that explosion will definitely produce rain. You did say "don't worry about how a dragon produces chemicals..." but unless you're going to present dragons as having supernatural-chemical-lab powers, that isn't going to happen either.

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To stray slightly from your specifics - what about non-chemical rain seeding? Such as a sonic-boom type "bark", or perhaps lightening based?

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