I too have been thinking about weather to implement it in Darkmana. While it might be easy to picture, to imagine, it requires a good dose of faking, since modeling a proper weather model could be way beyond a system's resources.
Wind is hard to cluster precisely, imho. I think it would affect many world regions, with different strenght the more you get further from the most windy point. Imagine a sine function and its waving. This leads me to think that it requires a coarser grained form of clustering. You can also "lerp", so to speak, the wind intensity via a formula. I don't know if you want to take in account the "temperature" of the air, whether it's a cold wind or a warm one.
In any case, you could send the most windy point, and let each client interpolate the wind intensity in the nearby areas, perhaps even reaching a negative factor, which would simulate the pseudo-random movement of the air.
As for rain... I could say the same. However it happens that rain might stop more abruptly all of a sudden, and start in the same fashion.
All of these are not "local" events. They affect nearby areas. Knowing that you have a specific range where the weather should affect the world, you can send a "weather effect start position", along with its intensity and some kind of tuning value to allow you to customize the effect. If the player is just too far from this event, he / she won't receive it, whereas a player in the range of the effect will see its client interpolate the values the server sent and modify the weather accordingly. It has to be deterministic, in the sense that every client in the same position has to see the same weather.
That's all I can come up with at the moment, I'm sure it can be done better. One thing is to design, another to implement.
Hope this helps anyway!
