Most ofen the haze is lingering smoke from forest fires. Some locations on the west side of the Sierra also get some air pollution creeping up. And then there is the shooting toward the sun (such as trying to get north facing cliffs). I think some of my high altitude photo wash-outs are a UV issue.

Examples:
Unprocessed:

Processed: (colors look strange to me)


Smoke in air: (processed as best I could get it)


Shooting to the south into sun