For the last 3 years I have worked in light to moderate rain and waited for it stop and carry on rather than come home and waste a day.
But I do admit it is difficult to clean them to a good standard when it is raining, cloths get soaked just from a couple of windows. Difficult to detail.
I would think WFP can carry on as you do not dry the frames or glass.
Even though it was not raining when I cleaned a few this morning I had a couple of customers say "oh, I did not expect you to come today", and "in future I do not want my windows cleaned if it is going to rain later that day". I have a few that have told me, "after you came last time it rained".
Difficult to please everyone, all the time.