If your looking to double your customers tell each new one you work in the rain.
The rain normally only affects bottom windows on one side of the house.
Clean your own when its raining, see how they dry.
People may not like the thought of you working in it, but you may only clean them two or three times a year on wet days.
Tell them its your only form of income and its the only way you can offer a reliable service
Good waterproofs, look as if its normal to be there.
If they still don't like it, they're crap custards and dump the buggers
Finally, all you can be is honest, reliable and do a reasonable job. These are all achievable in the rain.
All that said if you don't like to work in it, thats fine, don't