Sounds pretty interesting but personally, I'd make those particles HitMOs = 0 and remove sharpness and increase mass on the water based weather. High mass prevents piling and no sharpness makes it look like water and not acid. Sand storms can have a little sharpness though.

In any case, I'd keep minor weather more as an aestetic effect/addon to the map rather than an interactive thing. This applies mostly to rain and clouds. Something that could be interactive however is lightning storms with electricity and the wind being the interactive things, but the rain as a something to show that player to be aware of the storm.

Also can we has wind control? : 3
(Either Lua based areas/zones or global gravity changing)

And how far is it possible to take this system?

-Can we have earthquakes? (powerful changes in gravity within a small period of time)
-Can we have tornadoes that lift and spin actors and object and then chuck them away from the top?
-Can we have hurricanes? (Powerful wind that blows away things on open land)
-Can we have meteor showers? (That counts as nature imo. ehehee. Giant flaming MOSRotating raining down from the sky.)
-Can we have plant growth? (Invisible seeds that fall from the sky that spawn plants/vegetation where they landed)

Umbrellas for machinimas or as a modded umbrella shield/weapon with offencive capabilities could be something interesting though.

Though, with Lua their's probably a less hacky way.

can we have some decent shadowing code then?
like, clouds that darken everything, and the code for making something just darken the surface of the ground?
perhaps?
if so, id love love love to see that implemented in that desert night one, just all over gamma reduction., maybe a spot of rain.
ambient effects, go tim, go!

A whole algorithm? o_O