I "think" the yellowing is cause by some sort on algae. Since the lense is quite porous and rain water isn't clean it leaves the ultimate place for algae to form.
- Polish the lense with any car polish.
- I found a little bit of RainX helps keep it clearer longer, maybe because it repels water better and "fills in" the mircoscopic pores on the lense.
But that's my own non-scientifically proven theory anyway - which seems to work