well, if you can make the fan work depending on speed you can make the humidifier start when rain starts in game. You just have to put it in front of the fans. You could test before the effect just turning on an humidifier in front of your working fans to see how it goes. I don´t think the humidifier will generate enough water but testing wont hurt (unless it gets on eletrical parts so its going to be Alonso´s Mclaren pre season experience )
So recently did a DIY wind simulator using one of the fan controllers from Sym Projects, and 3 x Thermalright TY-143 140mm pwm fans (600-2500rpm). In total it cost me around £95 with bits and bobs to mount. Now I'm happy with the positioning, I need to un-mount it and cut the metal down to the correct size (& maybe paint it too) so as to tidy up the visual aspect of it. The fan controller runs on the SImDash software allowing you to set a top speed and a 10% incremental fan speed increase reaching that speed. Each profile can then be saved for individual cars. In VR it's just nice to have wind blowing against your face, especially in the warmth, as for immersion, it feels somewhat of a novelty in all honesty. I needed duct tape on the 'stem' of the fans as I notices extreme vibration from the wheel and my seat (with bass shakers & buttkicker) caused one of the stems to crack. I think these fans come with a poorly moulded 'stem' as one of them I had purchased had already cracked and broken the plastic that hold the wiring whilst in transport. I ignored it and mounted it and then went around Le Mans with Endurance mod , going over the kerb I heard a large crunch and one of the blades came flying off after touching the broken end of the plastic. After that I decided to tape all the stems just to reinforce the weakest stress point.