Sometimes, different substances get on windows. If it's left on the window for a long enough time, a stain is created that won't come off. There is a chemical reaction with the glass that occurs, called etching.
The glass being used nowadays is softer than the glass in older windows. It gets etching more easily.
* Neglecting windows for more than a year may cause etching into glass from the minerals in dirt and rainwater, causing permanent stains.
* Salt water spray (for homes near Puget Sound) has lots of minerals in it. These windows need more frequent cleaning to preserve quality.
* Sap from plants and trees in late spring and summer can be very acidic.
* Plants, bushes and trees that stay in contact with windows may cause etching.