IMHO .... just get out there and do it.
If anyone objects just say fine no problem see you next month .............. if they ask can you come back on Friday when its not going to rain then just say no sorry as will be in somewhereelsetown then .... so ur telling them in a nice way its now or next month.
I have a theory that messers love the rain as gives them (in their eyes) a valid reason to say not today thanks.
You need to bin ALL you not todays if your going to have a great round ... takes time to weed em out once you have ditched all the messers that you have already identified ... you know who they are 
So onto part 2 ............... as you will be culling a few customers you will need somemore .... if only your missus had some spare time ..... oh hang on she will have come August 
i think ians right here. there really is no need for this type of letter.
firstly, great decision to work in the rain. i have never not worked because of the weather even when it is bucketing it down, in the snow, or in the gales we had. there's no need. customers dont say a thing, and if they do they seem pretty impressed that we're so regular and hard working.
customers dont need to know you might be working on saturdays. just turn up and clean.
customers dont need to be pre warned you'll be cleaning in the rain. just turn up and clean.
i worry you re creating problems for yourself by raising it as an issue. customers may now get in touch if they dont like it! if you didnt write the letter most of them wouldnt notice, wouldnt mind, or wouldnt say anything. the ones that do say something, will just be checking it out with you. so long as you give them a good explanatino theyll be fine with it.
also you want to focus your explanations on the benefit to the customer. by the way you write the letter, mentioning financial difficulties, you are saying you are cleaning in the rain because of these difficulties. the reason should be that the process works just as well, or better in the rain (and youll come back if it doesnt).
good luck
