Nexon proved something with the visitor patch.

I've never seen particles in MapleStory, anywhere. Those are not particles, it's probably just 1 - 2 layer(s) with alien ships and they just change the transparency when an event occurs (in this case, time). Same deal with the dark background. Also, that's been possible since a long time ago, the first example that pops into my head is Henesys PQ, when the crescent moon turns into a full moon after activating all flowers.

By the way, have you been in Perion or other foggy places? They have fog. That's a type of weather.

I meant weather that randomly occures. The fog that occures in Perion and inside the clocktower is always constantly recycling across the map.