Rain contains vitamin B12.
False - there’s the technical interpretation and the practical interpretation of this claim. The technical would say: yes, vitamin B12 is a water soluble vitamin that is produced by bacteria and ends up in some (tiny) concentration in bodies of water and therefore when evaporated, it is not out of the question for some B12 makes its way into rain clouds. The practical interpretation would say: no, there is no measurable amount of vitamin B12 in rainwater.
From the studies we were able to find, the amount of vitamin B12 in pond water ranged from 60 to 2,000 nanograms per liter and seawater ranged from undetectable (less than 10) to 200 nanograms per liter. A nanogram is 1.0x10^-9 grams, or 0.000000001 grams, and a liter of water weighs 1000 grams (or 1,000,000,000,000 nanograms), and in comparison the Recommended Daily Allowance (RDA) of Vitamin B12 is 2,400 nanograms (2.4 micrograms). So potentially, in a liter of very potent pond water, the typcal human adult could take in their RDA of Vitamin B12 (not a recommended method), but we’re talking rainwater, not pondwater. Now a single rain drop ranges in mass from 0.5 mg to 70mg with larger drops upwards of 100-300mg, since we’re working in nanograms: the average raindrop ranges 500,000 nanograms to 70,000,000 nanograms; you’ll quickly see that we’re talking about some very minute amounts here. Even if we were to assume that all the Vitamin B12 from a potent pond were to move into the clouds with the water vapor, but that’s not how the cycle works. Without veering too far off track, this myth is fueled by the fact that Vitamin B12 is not something that is synthesized by plants or animals (rather by bacteria), and with such concentrations in ponds and the sea, it must come from somewhere. Studies have shown that runoff from soil is the significant source of Vitamin B12, the rain aids in this cycle after it has hit the ground.
original claim: @OMGFacts; sources: Bioassay of Seawater. III. Distribution of Vitamin B12 in the Northeast Pacific Ocean, by A. F. Carlucci and S. B. Silbernagel © 1966American Society of Limnology and Oceanography. page 642, Monthly Survey of Vitamin B12 Concentrations in Some Waters of the English Lake District, by K. W. Daisley © 1969 American Society of Limnology and Oceanography. page 224, Rain as a Source of Vitamin B12, Dietary Reference Intakes: Vitamins [PDF], Mass of a Raindrop