I spent some time in Southern California today (taking off from KSNA), where the seasonal Santa Ana winds are blowing hot dry clear air from the northeast today. This should result in crisp clear visibility, as any haze and low clouds are blown out to sea.
Instead, every city with an airport reporting clear visibility of 10sm was covered in a layer of haze.
I headed northeast past Ontario (KONT) towards Las Vegas, and found that as soon as I left the cities and airports behind, the air cleared right up. Haze only appeared in the distance in areas that I could see on my map were near airports reporting weather. Approached Henderson Executive close to Vegas, and the haze was very much back in action.
It really seems to be taking 10sm visibility to mean “lots of haze”, regardless of whether there even could be haze in that situation. This is very wrong, and means you can never get a clear-visibility approach at an airport that reports METAR observations without using custom weather!