So after putting up with it for a fair while I've decided to investigate and now bring it up online. 

AS2016, if you have minimum visibility set to 0, yet the METAR gives 9999, it irritatingly blocks out the vertical visibility too. I understand you never can see 100% clearly horizontally to horizon as there is so much dust and dirt in the in lower level air, however this doesn't block the beautiful blue sky above you. So how does one retain the setting so that if the weather drops AS will depict lower visibility, but when it's clear, it shows blue sky, not grey?

Hello Oli,

Based on your description is sounds like you have volumetric fog enabled in P3D, can you please confirm. What is your MAXIMUM visibility set to? If you have Vol. Fog enabled can you try setting your Maximum visibility to 199, this will have multiple visibility layers. If you set it to 200 it will force one layer which can increase performance but will cause the vertical visibility issues you are describing. If Vol Fog is off try setting the Max Vis to 100 (default value).

Thanks!