I educate my customers to the fact that it is the wind and not the rain that makes a mess of the windows.
When it has been raining you will see the bottom 70% will be clean, the top bit which collects all the dirt will still be dirty.
Also offer them a rain guarantee, that if the windows are dirty because of the rain in the next 24 hours after you have cleaned them, then you will come back and re-do the effected windows for free.
I don't know if the above is true in all areas, but it is with those of us that work coastal rounds.
Spindrift [atomised sea water] blows in on the wind and covers the glass, rain will wash it off again, but it doesn't fall on the top of the window as it's usually sheltered by the overhang.
I did a pub on Conwy Marina this morning, and the wind was blowing off the sea, I could see that the salt was starting to build up again by the time I did the insides.