Rainout would only exacerbate the effects of nuclear fallout.

Yeah. So if you detonate a nuclear weapon and there’s a rain cloud above where all the radiological mess is in the ground, what would’ve been fallout is now “rainout.” And there are arguments I find convincing — this appears in computer models and appears to have maybe happened in the atomic bombings in Japan — where the nuclear explosion itself creates what’s called a pyrocumulus like a big forest fire can. The heat source actually causes a rain cloud to form, then it can rain out the nuclear fission products directly. That’s really scary, because it’s so near to the burst time that the stuff is ludicrously radioactive.

If it’s rained out within a few minutes of the burst, the doses that can be experienced on the ground by people are outrageous even for a low-yield device. So there are all these sorts of intangibles, where Russia could be trying very hard to engineer the attack to avoid collateral damage, then just fail.