A lot of clients have asked me this question, especially the ones who live in the Edmond area. What causes it is usually a sprinkler hitting the glass with hard water or when water splashes from watering nearby plants. In time there are these tiny white water drop marks on the windows that are difficult to clean off. What I usually do is use a spray bottle of lime scale remover, you can get this at any home improvement or home and garden store. Spray the affected area of the glass and let it sit for a few minutes. Then scrub it with a low abrasive scrub pad, they are usually color coded so look for the white ones and do not use green as this is too abrasive and could scratch the glass! Or you could use #0000 steel wool, the least abrasive kind. Afterwards rinse with clean water (not hard water, that’s what got you in to this mess to begin with). Depending on how bad the windows are you may have to repeat this process a few times. With a little time and a lot of elbow grease you’re windows will look as good as new.