I believe that in most rainy conditions that it won't effect the windows to any noticeable degree.
Occasionally, tho' you get that "sandy" rain which leaves sandy marks on cars etc when it dries and if that is accompanied by wind it can blow on the glass and cause spotting.
Most householders instincts are that they don't want to have their windows done in the rain - it doesn't "feel" right, same as they often initially don't want wfp and they wanted you to collect and they only wanted you to clean when they were in and opened the gate and, and, and...
So you have to train them to your way of working. You do this by having confidence that you offer them better than their perceptions ... you're regular, you'll turn up on time, you do a good job etc etc etc.
Personally, I hate working in the rain and that's why I tell my custy's that if it's dry I'll clean'em and if I've started and it comes on to rain then I'll finish that house. And if it's just patchy drizzle then I'll prolly continue. I also tell'em I have very full books and that if they turn me away cuz it's grey or cloudy or "it might rain" then I will have to find another customer from my "long, so long, long, long"
waiting list.
(Not that long mind cuz I've taken a day off today!
)