At the risk of posting a Nathan like post 

Educate them guys and you can work in the rain too.
well done mate.thats exactly what i would of texted too.

also what difference does it make if you clean their windows during a dry spell/day and it rains hard a couple of hours later or overnight?(which happens at times all year round)....no difference whatsoever.you just cant run a business like that or you d hardly ever work!
most of my customers are very well trained that they dont even mention the rain when im working in it and just pay me(and sometimes a coffee).im truly blessed!

occasionally i get a customer who doesnt want them cleaning in the rain.thats fine.they get missed until their next due date.