Solar Covers and UV Rays?

I've noticed over the years that when my solar cover is in place, my chlorine levels don't drop as fast. I can think of several reasons for that:
1. The obvious one: Less stuff is falling in that has to be metabolized. It's landing on the cover instead (My 3 weeks of practice-bombing by local birds, for example).
2. The cover is prevented the chlorine in the pool from gassing off, by simple being in the way.
3. The cover is blocking UV rays from breaking down the chlorine.

It's this last I'm most curious about. Does the cover inhibit the UV? Do different colors (like blue) work better than clear? (Obviously, opaque covers block UV).

Just tossing this out there.

Carl

I recently started using my blue solar cover every night - primarily to reduce the heat and water loss from evaporation. I noticed that the
FC levels will climb quite quickly unless I also reduce the ON time of my SWCG.

This brings up a question I've thought about recently: when battling an algae problem would it be best to keep the pool covered to prevent
FC loss from sunlight? Or does the pool need to "breathe" during the epic battle between chlorine and algae?

It's better to expose the water to sunlight, not only to breathe but also to have the UV in sunlight break down some of the chlorine because when it does it produces hydroxyl free radicals that are very powerful oxidizers so can help clear the pool faster. It's the chlorine that gets in cells and kills the algae, but having some of the chlorine lost to sunlight is a good thing in this case, just not too much. If you've got CYA in the water, then you won't lose too much chlorine too quickly.