Weather

Well it certainly seems to be less frequent after a maintenance (from my experience anyway), and despite one homestead having rain, another may not at the same given time. If it were a pattern it'd have to be different for every single player. My assumption is that it's based on probability and you're more likely to "roll" a rain bonus when you instance your homestead if the server's uptime is higher

Also I don't quite believe it works like regular weather since it follows the shadow realm's qualities,especially the part in which if you never leave your homestead the rain will never go away.

That and I seem to get fixed rain bonus rates, similar to alchemy ovens (the level of rain doesn't matter, either it's not raining, raining, or there's a thunderstorm)

Really though, give it a try? It doesn't take much work, just enter your homestead every few minutes with a potion making kit or something before and after a maintenance and compare how often you get a success rate boost

Just be sure to exit your homestead and make sure nobody is in your homestead after you check so the instance can close