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In a (marginally) cursed world rains of blood are an occasional occurrence. The water in the clouds is magically transmuted into blood at the point it starts to coalesce into droplets (no need to worry about how to evaporate blood), and will transform back into water after a short while on the ground (so don't worry about the effects on the ecosystem).

In low temperatures I'm unsure of whether such magically transmuted water will form snowflakes. To the best of my knowledge the water in moist, low temperature air has a short liquid phase before it freezes onto the snowflake proper, but I don't know if magically transmuting it at this point would lead to blood snow or blood hail, as I don't know what the impurities in the water will do to the formation.

For reference the freezing point of this magical blood is ~-2 degrees C.

In freezing temperatures should I expect flurries of blood or hailstorms of blood?

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    $\begingroup$ only one way to know for sure... $\endgroup$ – Topcode Oct 29 '20 at 16:26
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    $\begingroup$ Side question: what happens if someone or something eats this blood and digests it before it transmutes back into water? I imagine this could be deadly to insects like mosquitoes if they try to drink the sky-blood $\endgroup$ – Dragongeek Oct 30 '20 at 12:18
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    $\begingroup$ @Dragongeek: That’s a question I’m probably going to answer with ‘magic’! $\endgroup$ – Joe Bloggs Oct 30 '20 at 12:30
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    $\begingroup$ No, blood forms Khorne flakes. $\endgroup$ – Harabeck Oct 30 '20 at 20:57
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    $\begingroup$ @Harabeck: My good sir/madam: That Was Magnificent. $\endgroup$ – Joe Bloggs Oct 30 '20 at 21:20
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It's hard to believe but I have some RL experience with freezing blood (cow blood!). It freezes rather well, but at lower temperatures (about -5C). When it freezes blood "essence" (this red-yellow substance) happens to be trapped inside ice crystals.

When it freezes as snow — it is more like thin hailstone, like groats. Normal snow is sometimes like that. Since snow gets its white color from multiple reflections and refraction on/in crystals — this "blood snow" is still white with very light dirty-yellow tint (never eat yellow snow!). When you rub it between your fingers (or fall in it) — it stains fingers (or clothes) with black-red-yellow colors (depending on the frozen blood concentration).

Blood snow would be a very dirty weather phenomenon (but clearer than bloodrain!).

P.S. btw — "true yellow snow” is also yellow because of blood; it gains its color from the process of recycling old blood cells.

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    $\begingroup$ I’m torn between intrigued and concerned. Why do you have experience making cow-blood snow??? $\endgroup$ – Joe Bloggs Oct 30 '20 at 11:00
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    $\begingroup$ Some troubles with piping on food plant in polar region. Thin fontain of blood into -30c environment for couple of days. When this snow pile riched some cables, we, IT department, discoverd it. IT happens, you know... $\endgroup$ – ksbes Oct 30 '20 at 11:05
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    $\begingroup$ That is both disturbing and amazing. $\endgroup$ – Joe Bloggs Oct 30 '20 at 12:08
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    $\begingroup$ Is graupel the "like groats" kind of snow you mention? $\endgroup$ – Phil Frost Oct 31 '20 at 5:29
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    $\begingroup$ @PhilFrost yes, smth like that but more fine. $\endgroup$ – ksbes Nov 1 '20 at 10:03
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Probably not. Snowflakes - at least the pretty multi-branched ones - take a rather long time to grow, perhaps 25 minutes or so, as they drift down from the upper atmosphere: https://www.noaa.gov/stories/how-do-snowflakes-form-science-behind-snow

Unfortunately for your bloodflakes, normal blood takes about 2-8 minutes to clot: https://en.wikipedia.org/wiki/Clotting_time though the time does vary with temperature, and there are anti-clotting agents.

But you have yet another problem. When the water in a solution freezes, it tends to separate into ice crystals and everything else. (Reference putting stuff in your freezer :-)) So you'd likely wind up with ice crystals and freeze-dried blood, which might be handy for vampires on long backpacking trips - "Just add water!" :-)

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  • $\begingroup$ Hadn’t even considered clotting. $\endgroup$ – Joe Bloggs Oct 29 '20 at 17:26
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    $\begingroup$ Umbrellas up, lads! Looks like clots today. Hope you wore your galoshes. $\endgroup$ – elemtilas Oct 29 '20 at 20:37
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    $\begingroup$ I don't see where your link gives a time to grow. And it doesn't seem like the growth period really matters anyway; most of the flake is added as it falls, but the core (made of blood here) would be frozen before it even starts falling (as your link notes "A snowflake begins to form when an extremely cold water droplet freezes onto a pollen or dust particle in the sky."). So it starts as a small frozen (and therefore clot immune) drop of blood, and water vapor freezes onto it as it falls normally. Seems like that would lead to snow that's mostly normal, but with a bit of blood in the middle. $\endgroup$ – ShadowRanger Oct 30 '20 at 14:31
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Blood is water based solution. As all water based solutions, they don't freeze particularly well, meaning that some sort of separation/precipitation is expected to happen, leaving ice crystals with something else around.

In the case of blood, having red and white cells plus various proteins, I expect them to make some goo around the crystal, producing something akin to dirty snow.

Plus, not being pure water the freezing temperature will be lower than 0 Celsius. Thus, it might even be that you won't get any solid at all.

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    $\begingroup$ I noted the lower freezing point in the question. I'm anticipating some nights getting very cold indeed in this world. It is marginally cursed, after all. $\endgroup$ – Joe Bloggs Oct 29 '20 at 16:24
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Snowflakes are Crystals

They often have hexagonal symmetry because of the bonding angle and polarity of H2O. Because "blood" is formed of numerous macromolecules, it will not form anything resembling a finely structured crystal. It's like tossing a bunch of bean bags into a pile and hoping they form a rhomboid prism or something. As others have noted, if any crystalline structure emerges from frozen blood, it will actually be due to the water inside the blood cells (or the plasma) freezing and forming crystals. However, there is no reason to believe that the water inside distinct blood cells will freeze into a coherent meta-crystal that is visible at macroscopic scales.

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Snow crystals grow in clouds not from liquid water but from the direct sublimation of water vapour. There is often a supercooled water phase, but the ice crystal that forms as this freezes is very small. It forms the nucleus of the snowflake. It grows as water vapour crystallises out of the air onto the nucleus.

When water droplets freeze you get hailstones, since water has a strong surface tension, it will pull itself into a sphere and not form an externally hexangonal shape.

You can't have "blood vapour" the water can evaporate, but it would leave the non-volatiles behind (probably as a little ball of dark red gunk) If this were to from snow you would get regular water-ice snow, that could then pick up some of the dark red gunk and fall with it. The effect would be mixture of ice crystals and partially frozen bloody gunk.

So your blood clouds can produce blood hail not blood snow. You might get what we used to call "rimestones" (though googling suggests that this was just my family) Small balls of "snow" formed by rime frost coating a nucleus. Not hexagonal, but not hard ice like a hailstone.

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    $\begingroup$ ‘Rime’ is the name for the fluffy ice crystals that grow on things when there’s freezing fog, I think. Sounds like your family use that in a portmanteau with rhinestones?? $\endgroup$ – Joe Bloggs Oct 30 '20 at 12:06
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Your World is Magically Cursed

I only feel comfortable taking this direction because there are so many good science-based answers already. Whether or not it's scientific, I would step back one level and ask myself this question: what role does the scientific have in the world I'm building? Do I need to explain everything?

It's a popular trend in fiction to explain magic with science and keep things wholly consistent with a naturalistic world. Thing is, if true magic happens in a universe, natural assumptions go out the window. There isn't a natural explanation for when magical things happen, and seeking to explain certain interactions naturally leads you down the rabbit hole of keeping your universe consistent.

You already said the reasons it doesn't have trouble forming in the atmosphere and doesn't harm the environment is magic. Is science really going to stop you from having magical snow? If you want it to snow blood, then let it snow blood. If you need it to hail blood, let it hail blood.

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    $\begingroup$ Strangely it’s precisely because I have no strong feeling on whether it should be snow or hail that I asked the question. Knowing which it might be from a scientific viewpoint helps decide the flavour and feel of the curses. Do we have whimsical flurries of pink snow or lumps of red ice? Turns out it’s vaguely yellow piles of fine hail that melt into horrifying brown/black goo. That’s an even better form of cursed weather than I expected, which is incidentally why I love this site!! $\endgroup$ – Joe Bloggs Oct 30 '20 at 21:26
  • $\begingroup$ @JoeBloggs Good to know, and I'm glad you got your answer :) That's why I love this site too! The first question I viewed years ago was "What if God mandated that warfare be done entirely with wood and nothing else?" Apparently some trees can kill you just by touching them (and of course they grow in Australia). The things you learn about the real world when building another is fascinating $\endgroup$ – automaton Nov 2 '20 at 14:14
  • $\begingroup$ I once asked a question about snakes living in bones only to discover that there’s a species of marine worm that lives exclusively in bones. WB.SE: AKA Mother Nature Did It First. $\endgroup$ – Joe Bloggs Nov 2 '20 at 15:29
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I think you are making this more complex than needed. You don't need to transmute rain into blood, then back into water. We do have blood rains on Earth (albeit rarely). You can easily copy those explanations and get rid of magic for this rain. It is not important if it is blood, sand or algae, only that your peasants believe it to be blood. Which is a simple conclusion, actually.

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