2016-03-29, 20:05:33
Let me split my answer in two parts:
1.) Generally, you are right: the visibility above clouds and/or in high altitudes is usually not so much affected by fog or low visibility on the ground. But then FSX/P3D comes with its limitations which do not play very well with this fact (*). This is because FSX/P3D only can reduce the visibility around your viewpoint uniformly in all directions, i.e. when you are sitting on the ground with 1 mile visibility and then climb to 15.000ft with 10 miles visibility you have everything in sight on the ground. And thats's somehow odd, too, isn't it? The only way to alleviate the problem a bit is 1. to generate a low cloud layer that block your view on the ground and 2. accept a compromise on lower visibilities in higher altitudes. FSXWX uses both methods depending on the circumstances. And indeed, it could be the case that your cruising altitude (15.000ft) still has an 'artificially' reduced visibility due to ground fog. Full upper visibility is only restored at around 20.000ft.
* For the sake of completeness: This is not true for P3D with volumetric fog enabled.
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2.) When you use the route option the exact visibility of your ADEP airport is used, if not, it is an average of multiple surrounding stations which might lead to a higher visibility value. But I would like to check myself (with historic weather) if you tell me the time of your observations.
1.) Generally, you are right: the visibility above clouds and/or in high altitudes is usually not so much affected by fog or low visibility on the ground. But then FSX/P3D comes with its limitations which do not play very well with this fact (*). This is because FSX/P3D only can reduce the visibility around your viewpoint uniformly in all directions, i.e. when you are sitting on the ground with 1 mile visibility and then climb to 15.000ft with 10 miles visibility you have everything in sight on the ground. And thats's somehow odd, too, isn't it? The only way to alleviate the problem a bit is 1. to generate a low cloud layer that block your view on the ground and 2. accept a compromise on lower visibilities in higher altitudes. FSXWX uses both methods depending on the circumstances. And indeed, it could be the case that your cruising altitude (15.000ft) still has an 'artificially' reduced visibility due to ground fog. Full upper visibility is only restored at around 20.000ft.
* For the sake of completeness: This is not true for P3D with volumetric fog enabled.
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2.) When you use the route option the exact visibility of your ADEP airport is used, if not, it is an average of multiple surrounding stations which might lead to a higher visibility value. But I would like to check myself (with historic weather) if you tell me the time of your observations.
Regards