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