Quote Originally Posted by speedminded
Until it's completely cleaned off and the glass is polished it will always come back. The rain is reacting with the junk that is stuck on the windows.

If it's really bad just get some Etch Cream from a hobby store. It's a really really mild form of hydrofluoric acid. It will take scratches and spots right off glass then you'll have to polish afterwards.
Good info!

I don't recommend the brick cleaner.. your vision gets messed up in the rain. His idea is wayyy better.