If you want to significantly darken a blue sky, remove reflections and glare, and you want to saturate color -- there is only one filter choice: The polarizing filter. In this article, I'll be discussing the pros of using a circular polarizing filter.
Light polarization happens occurs in open blue sky and from shiny surface reflections. When you rotate the filter, you can incrementally prevent the polarized light from passing through to the camera. The effect is removal or reduction of reflections and darkening the sky and increasing contrast of clouds. (thereby darkening the sky or removing the reflections), while at the same time allowing normal light to pass through.
There are two types of polarizing filters: circular and linear. Circular polarizers are the most common and will be the preferred to most amateur photographers.
If you're not already using a polarizer, you need to get one today. These will have a remarkable impact on the quality of your outdoor shots, particularly if you shoot scenic/landscape photos.