Hi Lee.
I have been employing pretty much ever since I first went into the window cleaning business and have never stopped working because of the rain, in fact there has only been a few of occasions over the years where the weather has effected us such that we couldn't work.
You dont need any schemes or arrangements for your staff, just make sure they keep some good quality wet weather clothes in the van they are working from, if of course they dont like working in the rain for personal reasons, then I guess you will have to consider whether they are suitable for doing an outdoor job.
Hope this helps