Presumably the fan isn’t really exposed to much other than atmospheric air at atmospheric. There’s generally Less dust outside than inside. I’m doubting humidity has much effect on fans.
I suspect biggest failure mode for fans are the bearing’s lubrication drying out. Sometimes this can be serviced (with much difficulty) or take the efficiency hit and use a thicker grease that doesn’t evaporate as fast.
Nice thing is that they’re not a critical part here, and it’s diminishing returns so if you have multiples and 1 fails, you’re still ok.
The combined heat and humidity cycles of being on a roof would be quite damaging to any electro-mechanical machine. Especially one that isn't designed for that kind of environment. I'd be surprised if a PC fan made it more than 2 or 3 years in that environment.
Probably depends a lot on the specific climate though.
I suspect biggest failure mode for fans are the bearing’s lubrication drying out. Sometimes this can be serviced (with much difficulty) or take the efficiency hit and use a thicker grease that doesn’t evaporate as fast.
Nice thing is that they’re not a critical part here, and it’s diminishing returns so if you have multiples and 1 fails, you’re still ok.