Had one of these on Friday. Big property, lock gate. I usually call for arranging the clean. She said don't worry if it rains. When I turned up that morning it was sunny. When I finished the clean I simply said, if it ever rains during or after I've cleaned the windows, you can rely on a 48 hour rain guarantee. So if your window are ever dirty after a heavy downpour let me know. I've cleaned hundreds of properties in the rain over the last 6 years, and I've never had anybody call me up. So whatever happens you will get clean windows. Customer was happy as can be, "oh that's good of you, so that's why you clean windows in the rain". Didn't even have to bother explain why 99% of the time windows are just fine.
Simply it's just a matter of having a quick chat why I do clean windows in the rain, some other times you can never win. Those are usually customers that I'd rather replace as they aren't very nice anyway.