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If there an area containing visible moisture in an otherwise clear sky and that area of visible moisture is transparent is it considered a "thin cloud" or simply a "localized area of reduced visibility"?

What about areas of mist and haze with no definite border below an overcast layer that present in the sky as large areas of reduced visibility still above VFR visibility limitations for aircraft within them?

Ryan Mortensen
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    Does this answer your question? How do the FAR's define a cloud? – Dave Feb 25 '20 at 01:48
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    @Dave it does not because that answer does not address the spectrum of visibility between what would be considered haze and what would be considered a cloud. If any "aggregate" of moisture particles constituted a cloud, then any time haze was reported, it would be considered IMC, but it isn't, so there's clearly a spectrum. You did answer what a cloud is. I am asking what form of visible moisture isn't a cloud if any. – Ryan Mortensen Feb 25 '20 at 01:53

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This might be an answer that works: In meteorology, the definition of fog is less than 1000 m visiblity and mist/haze more than 1000 m visibility. I think you could reasonably infer the same definition to apply to higher cloud as well. To the extent you are looking for a go/no/go definition, I think that might be all there is.

John K
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  • Your source actually says, "By international agreement (particularly for aviation purposes) fog is the name given to resulting visibility less than 1 km." There's the answer. If you look at the answer Dave is referring to that defines a cloud in the AIM, it says the difference between a cloud and fog is that fog is on the ground. So correlating these, then a cloud would also be something resulting in less than 1km of visibility, but simply not on the ground. – Ryan Mortensen Feb 25 '20 at 18:22
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    The thing is the linked answer only refers to ground level - fog. You have to infer the same definition for above-surface layers but I couldn't find anything explicit. – John K Feb 26 '20 at 00:38