You just got to explain to the customer is there any difference if I clean them in the rain and it clears up in the afternoon and possibly rains again tomorrow or when I clean them in the in the dry and it remains soon after!!? The answer is no, its unavoidable and your doing a maintenance clean which is what they pay for. If they would like you to constantly stop and start when its raining or possibly going to rain, drop them as quick as, you won`t survive long like that living in a country where it rains often!!
As already mentioned if the rain is horizontal thats the only time it should stop you, which is thank fully quite rare