I asked my customers how they would feel about me cleaning their windows in the rain, and their responses told me all I need to know.
I don't work in anything more than light drizzle, and even then, if it's not hitting the glass.
I'm in the south west of Ireland, so we see plenty of rain here!
I use the rainy days for doing gutter cleans, so unless it's hammering down I can usually keep going.
John