Is an area of localized reduced visibility considered to be a cloud?
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.