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A well-known pop song talks about setting fire to the rain, and I'm wondering if that's possible.

My first thought was that if you had oil droplets in the clouds, then those could be ignited as they fell, but I have no idea if it's possible to get oil droplets in the clouds.

Using real-life physics and chemistry, is it possible to set rainfall on fire?

Clarifying notes:

  • By "set rainfall on fire" I mean, there are liquid drops falling from the sky, and those drops end up being on fire by the time they hit the ground. I would prefer it if the rain appears to be natural rain until it lights on fire. Ideally it is natural rain, but I'm not sure how to light natural water on fire.
  • Assume current technology levels, but not necessarily existing technology. So if you need to invent some specialized machinery to make it work, that's fine as long as the invention is currently plausible.
  • you can ignore financial and legal limitations. I'm only asking whether it's physically possible.
  • This question touches on setting fire to the rain, but it is not a duplicate because it is asking about manipulating atoms with your mind, not real physics.
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    $\begingroup$ Highly pedantic and mostly unrelated "um actually" point - Adele is from Tottenham in London, England. en.wikipedia.org/wiki/Adele lol $\endgroup$ Commented Aug 18, 2021 at 10:34
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    $\begingroup$ This isn’t really a worldbuilding problem since you’re just curious about this, not building a world where this is true, so take my upvote — but tentatively. $\endgroup$ Commented Aug 18, 2021 at 11:53
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    $\begingroup$ @James Geddes Good point. What's funny is that the reason I specified "in the US" is because I was worried someone would post an "um actually" comment saying that the song isn't well-known where they're from, and I'm confident that it's at least well-known where I'm from $\endgroup$
    – T Hummus
    Commented Aug 18, 2021 at 14:45
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    $\begingroup$ @THummus Much lolz :D $\endgroup$ Commented Aug 18, 2021 at 15:26
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    $\begingroup$ " is it possible to set rainfall on fire". Yes. Just replace the atmosphere with pure Fluorine. Water will burn. So will sand. And asbestos. And especially humans, like fireworks we would flare! (p.s. you need to replace all the air. Even plain old N2 nitrogen will burn happily in Fluorine!) $\endgroup$
    – PcMan
    Commented Aug 18, 2021 at 15:46

12 Answers 12

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Alkali metal dust

The Alkali metals lithium, sodium, potassium, rubidium, cesium, and francium, all react violently with water when in their elemental pure form. If a dust storm consisting of small particles of these elements were in the air when it rained, there would be fire and explosions. Additionally some of the Alkaline Earth metals are also reactive enough for this purpose (not beryllium).

In a story setting, a mad scientist could pulverize tons of elemental potassium and drop it from several airplanes above a rainstorm.

The problem is that if you've got a dust storm with these raw elements did happen, it would have much worse consequences than the rain being on fire. They are all highly reactive with other things as well as the water.

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    $\begingroup$ Mostly explosions, really. Powdered combustibles are known for that. $\endgroup$
    – jdunlop
    Commented Aug 17, 2021 at 21:24
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    $\begingroup$ Yes, but fire and explosions are really just different sides of the same coin, the difference being the rate of reaction which is highly reliant on stoichiometry and concentrations of the reactants. Which could be managed through purposeful planning on the part of the perpetrators. $\endgroup$
    – Mathaddict
    Commented Aug 17, 2021 at 21:28
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    $\begingroup$ Francium, while an alkali metal, is EXTREMELY radioactive (half life of 22 minutes) and would probably cause a nuclear reaction before causing fiery rain through a chemical reaction (which I guess could technically still fulfill the requirement) $\endgroup$ Commented Aug 18, 2021 at 14:34
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    $\begingroup$ Could you micro-encapsulate the dust in a soluble coating to delay the reaction until near ground level, and use it to seed clouds so that the raindrops form around the encapsulated dust? $\endgroup$
    – William
    Commented Aug 18, 2021 at 15:37
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    $\begingroup$ Might this not combust BEFORE the rain hit? Like I'm fairly sure that atmospheric moisture would be enough to trigger this with that kind of fine dust spread, and then you end up with a fireball instead of flaming droplets. $\endgroup$ Commented Aug 19, 2021 at 0:25
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WASP-76b is a planet where it literally rains molten iron and glass.

In May 2020, it was discovered what the previous spectra of WASP-76b, taken by the Hubble Space Telescope, were distorted by the light from a suspected stellar companion. Therefore, updated atmospheric model is cloudy hydrogen-helium envelope, non-detection of alternatively reported neutral iron (including "iron rain"), and only upper limits on oxides of titanium and vanadium. By 2021, the controversy was resolved by demonstrating that the tentative iron condensation signal may also appear due to the temperature asymmetry between leading and trailing limbs, although existing data does not allow distinguishing between the two scenarios. Combination of data from the Hubble and Spitzer space telescopes has allowed the detection of titanium oxide and traces of water in the atmosphere of WASP-76b though. A later, higher resolution spectrum, has features of ionized Li, Na, Mg, Ca, Mn, K, and Fe, but no ionized Ti, Cr, Ni, or molecular oxides of titanium, vanadium or zirconium were found.

Anyway droplets there should be red hot.

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    $\begingroup$ Not sure it meets the intent of the question, but have to upvote for the educational value. Interesting that "WASP-76b seems to be the only planet in its solar system" - it's so unpleasant that even other planets don't want to hang out in its vicinity! $\endgroup$ Commented Aug 17, 2021 at 23:08
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Water mixed with petrochemicals.

The island where it rained oil

ST. CROIX, U.S. Virgin Islands — Two hours after midnight in this island paradise, a cloudy vapor rose from a massive oil refinery and floated over nearby homes as quietly as a ghost.

The fine mist of oil and water from Limetree Bay Refining rained down on the community of Clifton Hill, showering the slick mix onto cars, gardens, rooftops and cisterns filled with rainwater that residents use for daily tasks.

According to a company report, when water gushed into a drum holding hot coke — an oil byproduct — the reaction triggered a safety valve that relieved the pressure. Refinery flares usually release a mix of water vapor and carbon dioxide: In this case tiny oil droplets entered the air, drifting as far as three miles away.

Of course water and oil does not mix. The oil would stay on the surface of the drop. That is good because the water will keep the oil cool and keep it from evaporating off the surface. Your oil could come from terrestrial sources shooting up in the air and being carried by wind as was the case here. Or maybe spaceborne naphtha rains down on the planet and is captured by the rain on the way in.

Getting it lit would be tricky unless it were really volatile stuff coming from a nearby source. It might have to be really hot. But once it got lit it might be self sustaining. Probably not that romantic.

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Hypothetically, rain is liquid water falling from the sky. If what's falling is not water, then it is not rain.

If we bend that a bit, oil falling from the sky could certainly burn. When the Iraqi army pulled out of Kuwait in 1991, they decapped many oil wells, and ignited almost all of them. A literal scorched-earth policy.

This left geysers of hydrocarbons, in the form of oil and natural gas venting into the atmosphere. Most of this ignited at the vent.

enter image description here

While this could be correctly be described as "a lot of fires", there are stories from troops of soot, ashes, and still-burning "globs" falling from the sky, that had been caught in the winds and carried a distance.

It is only a small step to call that fire raining from the sky

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    $\begingroup$ If it's just fire from the sky you want, most any major forest fire will do. Of course what falls is not liquid, it's solid embers lifted by rising air, but to the casual observer the difference is really immaterial. $\endgroup$
    – jamesqf
    Commented Aug 18, 2021 at 17:27
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Oil Producing Algae

You would need some form of additive in the rain because water doesn't burn. Water is essentially the ashes of H2 burning. Water also makes a good heat sink so any additive will have to overcome the heat soak of the water as it absorbs the heat and/or boils off.

Maybe have some form of algae that can float up where the clouds are that produces an oil that is flammable. The trouble is that oil would make the algae heavy, so it might fall out without the rain.

On the plus side, the algae may act as condensation seeds which would help rain begin.

The trick now is getting the algae to ignite. Lightning would do the trick. Otherwise, you are looking at some unlikely chemical reactions that would be hard to justify with algae.

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A high-energy Microwave source directed upwards, could theoretically, split the falling water in to Hydrogen and Oxygen which could then burn. Hydrogen burns with a blue flame, but if there were any impurities in the air, such as sodium from salt the colour could be different.

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  • $\begingroup$ Nice solution Stano. Welcome to worldbuilding, take our tour and refer to the help center as and when for guidance as to our ways. Enjoy the site. $\endgroup$ Commented Aug 18, 2021 at 6:00
  • $\begingroup$ would microwaves affect raindrops? They are very small compared to the wavelength. $\endgroup$
    – JDługosz
    Commented Aug 19, 2021 at 7:08
  • $\begingroup$ I led it with "high-energy", just how much energy is open to conjecture. Basically a Microwave oven works by the heating of water due to the resonance of the radiation with the bonds in the water molecule, already there are some examples of hydrogen production using Microwaves, see: anthropocenemagazine.org/2020/11/… $\endgroup$
    – Stano
    Commented Aug 23, 2021 at 19:26
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Apologies in advance, this should really be a comment but this is not a SE I've contributed to before. However, my 'someone is wrong on the internet' instincts kicked in!

Alkali metal powders, as referenced in this answer, are very pyrophoric solids. making a powder out of these metals hugely increases their reactivity as it exposes a much greater surface area of metal to the atmosphere. You'd not need to get them near a shower of rain for them to ignite, opening the container in the air would do! In my former life as a chemist I did make lithium powder (lithium sand) on occasion (under an argon atmosphere at all times). Sodium and potassium are more reactive and I wouldn't want to try isolating them as dry powders. Cesium and rubidium are many times more reactive, and liquid at near room temperature so it'd be impractical and extremely dangerous to try making powders of them. As mentioned elsewhere there's never been a sample of francium in existence big enough to consider for this purpose.

This article gives an overview of the method and precautions required to make and isolate lithium sand.

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Fluorine (etc)

Fluorine is the most electronegative element and will react with just about every other element (apart from the light noble gases). This means it and its derivatives have the potential to oxidise (ie burn) things that are generally considered already fully oxidised. Chlorine trifluoride for instance, sometimes referred to simply as the "nope" chemical, is a colourless, odourless (because it sets your nose on fire), heavier-than-air gas which burns literally anything apart from a whitelist of a handful of chemicals, most of which are things that have already been oxidised by fluorine. In particular, it will oxidise water into an exciting concoction of hydrofluoric and hydrochloric acids which will then go on to burn other stuff.

A layer of this gas blanketing the ground would set the rain on fire, along with everything else. If through some weird atmospheric conditions you could concentrate it in a layer above the ground and then drop raindrops through it, they would indeed be on fire (explosively so) by the time they fell out the other side. Kind of like turned-up-to-eleven acid rain, that's also on fire. And exploding. While the sky was simultaneously on fire.

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  • $\begingroup$ The only hard part about this would be that you could never prove it... because the gas set fire to any cameras you sent down to witness it... and any lab techs you sent down to witness it as well! $\endgroup$
    – Cort Ammon
    Commented Aug 20, 2021 at 4:38
  • $\begingroup$ @CortAmmon - Phd students are a renewable resource, I'm sure you could send one down with a sufficently protected camera :p $\endgroup$
    – lupe
    Commented Aug 20, 2021 at 10:49
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Can I set fire to the rain?

No, because water is the product of combustion. IOW, it's what happens when you "burn" hydrogen and oxygen together. (The Space Shuttle Main Engines produced a lot of water vapor...)

My first thought was that if you had oil droplets in the clouds, then those could be ignited as they fell, but I have no idea if it's possible to get oil droplets in the clouds.

Droplets -- by the nature of being small, which is why they're droplets -- have so much surface area that they'd quickly be consumed in the fire.

Any combustible liquid would have the same problem, because of what fire is, and what it requires:

  • fuel
  • oxidizer
  • heat
  1. It's cold in the upper atmosphere, removing one edge of the fire triangle.
  2. The falling droplets are, of course, falling, which makes a wind, which blows the fire out.
  3. Anything that is burning heats the atmosphere: enough of it, and you'd have a firestorm up where it's burning, consuming the fuel, and pulling in oxygen, heating everything, and thus making it rise (because that's what hot gasses do).

Thus... if by "fire" you mean what people think of as "fire" when you ask them what "fire" is, then... no.

enter image description here

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Hypothetically...

There isn't much of a LIKELY situation where this happens, but that shouldn't stop us from having fun, should it? If the Cuyahoga river can burn, why not rain?

Imagine a situation where a really massive rocket powered by an alcohol (or another liquid stable at STP) rocket fuel begins to fail shortly after take-off in a rain storm. The engines cut out just as the fuel tank fails, splattering liquid rocket fuel into the air. The fuel doe not immediately ignite, but forms droplets. But as enough rocket fuel begins to volatilize, the still hot engines ignite the fuel, resulting in an explosion. Meanwhile, the droplets (which are mostly between 100-200 proof) are ignited by the explosion, and a rain of burning rocket fuel falls to the Earth (to be promptly extinguished by the rain, but we can't have everything).

So the water itself is kind of burning, but definitely the rain (of mostly rocket fuel) would be. The key would be for the rocket fuel to form droplets first, and not be immediately vaporized in an explosion.

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For flamable rain we need some process in the upper atmosphere that is making liquid fuel and not retaining it. this is wasting energy on a massive scale.

This is a risky proposition because a finely divided fuel is a fuel-air explosive. something that went around doing that accidentally would not last long.

So I'm forced to assume that it's intentional, some sort of flying or floating life-form hunting or fighting using a fuel-air explosive.

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The question specifies the rain as "liquid drops" falling, and does not demand that it be on earth.

If you're willing to move to Saturn's moon Titan, you might be able to do it. On titan, it rains liquid methane. There isn't any oxygen to combust with - but that could be the answer to your question:

How do I set fire to the rain (of methane on titan)? You provide Oxygen, and a bit of initial heat. Titan's atmosphere is 95% nitrogen and 5% methane, with the methane mostly in the upper part. So you might not cause an immediate explosion from gaseous methane in the "air", getting a nice burning rain going.

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