This is a question I often hear as a window cleaner. The short answer is no, rain doesn’t make your windows dirty... dirt does. Many people blame the rain because it leaves spots on the glass but this happens because the rain simply collected dirt that was already on the glass and left it behind after it evaporated. But if your windows have just been cleaned the rain will not leave any water spots because it’s pure water with no dissolvable solids. So it will evaporate clean, no worries.
