Rain does many things, but it does not make a clean window dirty. It may make a dirty window look dirtier, but only if there is dirt already on the window. Actually, rain usually will help keep windows cleaner, longer.
Rainwater vs. groundwater
Groundwater contains minerals, chemicals, and particulates usually not found (at least in high concentrations) in rainwater.
Groundwater from sprinklers will leave spots on windows and cars for two general reasons. First, the chemicals and particulates in the groundwater will remain after the water evaporates, leaving white spots where the bead of water landed. Second, the groundwater will also disturb and loosen any dirt and dust that was already present on the window or car surface. When the water evaporates, a spot will remain.
Rainwater is usually purer (acid rain and pollution aside) and will not leave spots when it dries, unless the item that got wet was dirty to begin with!
How rain keeps windows clean
Rain actually aids in keeping the air cleaner and will rinse dust, dirt, and pollen off of exterior surfaces. Regular rains keep levels of particulate matter lower, and if your windows are clean, it helps keep dirt and pollen from accumulating on the glass and surrounding areas.
If there is no dirt on the window, there is nothing to create a spot when it rains. If the air is clean, less dirt will settle on the surface of a rain drop and subsequently remain as a spot when the rain evaporates. As a disclaimer, dry weather and seasonal pollen will cause dust and dirt to accumulate on windows and surfaces. Also, construction, landscaping, mulching, or mowing in dry weather will generate large amounts of dust and dirt.
But in general, clean windows + regular rain showers = cleaner windows. Take a look at any windows you have that are protected by eaves or a porch and you will see that they are generally dirtier than windows that are exposed to rain.
This leads to another common misconception: windows can’t be adequately cleaned if it is raining or if they are wet. We wash windows with a solution of soap and water….a little more water isn’t going to hurt.
It is understandable that a homeowner would want to enjoy their clean windows on a sunny, clear day. Just understand that should it rain before, during, or shortly after a cleaning – the windows will still shine when the sun comes back out.