A brief “turned out nice, didn’t it” or “can’t believe this rain” to a stranger is a normal way to acknowledge someone without any real commitment to a longer conversation, respond in kind rather than ignoring it or over-engaging.
It also works as a safe fallback if a silence gets awkward, nobody will read a weather comment as odd or forward.
What it’s standing in for matters just as much: with someone you’ve just met, steer away from personal finances, relationships or dating, and politics, especially anything Brexit-adjacent. These aren’t hard taboos, but they’re the topics most likely to make a stranger visibly uncomfortable, where weather never will.