Argh! I did take some screenshots of an actual successful witchy activation of the weather stone (I think I remember that it is only Witches that can activate them from some blurb EA put out before Seasons was released by EA - but dunno where I saw that - so don't quote me
), but I forgot I had the UI turned off in the screenshots so I couldn't get the info. There is a "Bewitching Rain" moodlet which lasts 3 hours (?), but I have no idea what it does. Since I don't have the UI in the screens I don't have the descriptions. My witch hasn't been successful at doing the spell again, not to mention he's been rather busy with other problems (namely getting abducted every night - but I think that was due to the Story Progression Mods, since removal of them has stopped that).
Anyway, the spell seems to be the same no matter what the season (I'm in Winter now) so I'll try and do it again after my sim finishes work and get good screenshots of what happens.