This time of year in California people get nervous about having their windows cleaned because of the rain. Should we hold off till another day? Should we wait till spring? Generally speaking the answer is no, rain generally does not effect windows. Most people think of their cars when it comes to rain and how it would effect their windows, but your car is horizontal and travels 70mph down the freeway so it collects dirt very fast. Your windows are vertical and stationary so its apples to oranges comparison. Not to mention for many homes the windows are under overhangs and rain rarely even touches the glass. Even if the rain does touch the glass there are 2 reasons this doesn’t matter, first is rain pure water and pure water does not leave behind spots(Think spotless car wash, or using distilled water for like a CPAP machine). If the rain does hit the glass it just moves around the dirt that is already there, if you have the windows clean there is no dirt for the rain to move around. The exception to his is usually overgrown bushes or trees overhanging windows, the rain hits the bush or tree and transfers the dirt from the tree to the window and then you have some spots.
This is why I offer a 7 day rain guarantee. In the off chance rain does mess up your windows within 7 days of service I will come out and clean the effected window for free. That way you can still have your windows cleaned for the fall and holiday season without worry.