Why do you need the rain to stop? Get yourself a decent set of waterproofs and print up some flyers saying something like: "Today I cleaned your windows while it was raining, I do this because it is my only source of income. Rainwater will not affect windows cleaned by WFP, but just in case there are any problems, give me a ring on ............................ and I will return and do them again at no extra charge."
Put a copy through every customer's door just before you start cleaning, that way you don't have to stand and argue when they ask why you're doing it in the rain.
Cheers,
Ian