Rain's a funny old thing - when it drizzles straight down, even more than drizzle then no problem working in it. Same with showers - just stop for a bit when they get heavy.
But ... customer perception.
They instinctively don't like cleaning done in the rain or if it's imminent or in the bright period between showers.
Generally - a few days of on and off rain isn't going to mark the windows - it's low TDS and dries clear but ...
The side facing any of the following, especially if the wind often comes from that direction:
The sea.
Busy roads.
Industrial areas.
Farmers activity/dust from fields.
Occasional Sahara rain with dust in it.
Volcanic Ash (remember that?)
Some numpty with a cutter or kango hammer digging up the pavement after you've gone.
So I train custies - not easy when you are starting out.
When I take them on I say:-
We offer a regular reliable service which means that you can depend on having clean windows all the time. (I sometimes joke with them that we are a "proper business" and that I won't be taking off time to do my "real job", "collect the dole", "sell drugs" "go to prison" etc) In return I ask that you don't tell me "not today" when I turn up and that you allow me to clean in showery or drizzly conditions.
I then explain (but it goes over their head until they ask me to leave it this time and I remind them) that I offer a rain guarantee that if any of their windows are marked by the rain within 24 hours of my clean we will come back and do the offending windows completely free of charge. (remember if it happens it is only likely to be one side of the house due to weather)
To seal the deal during a run of bad weather like now I also staple a note to my payment slips outlining my "rain" callback guarantee.
To date I have been called back three tmes. One was marks on the inside (!) - one was due to a guy cutting up the tarmac next door. Another was a prolonged westerly over Avonmouth, M5 motorway and a dual carriageway onto a custies back windows - I cleaned Friday afternoon, she called on the Monday after a weekend away telling me the front and sides were perfect but the back had grey marks like rain splashes and so I went back out of goodwill.
My worst for this is a car manufacturer in Avonmouth. They are my only fortnightly customer and have white upvc/white aluminium frames.
If the weather is kind the fortnightly clean is a breeze. If like now we have westerly winds, ships unloading coal on conveyors to trains less than a mile away, then a fine black powder gets everywhere and the windows are filthy almost immediately. Mix in occasional salt from the Bristol Channel/Severn Estuary and it's a good 'un!
You learn and adapt and the quicker you do the happier you will be.