Window washing is one of those things that can build up before you know it. If this is not taken care of windows can look like they have rain droplets on them year round or even become foggy. This can be prevented with a semi-annual cleaning. When rain hits your window it can deposit hard minerals onto the glass. This is why cleaning them is always a good idea. Not only does more natural light come in but will also give a clear picture to what is going on outside.