Sure, you could point out that one touches the ground and the other doesn't, but that feels like a rather nebulous distinction to make.
Incidentally, the same lack of distinction exists in Portuguese.
Delightful.
I think you get right to the reason for the distinction in language. The two things weren't unified in your conception until you were inside a cloud, which until recently was impossible except by climbing to high altitudes. Fogs and clouds are usually experienced in very different ways, so it seems natural to me that people came up with different words for them.
I'm not familiar with the Portuguese climate, but is fog somewhat common? If not, that might be a reason why you never felt the need for a word expressing the concept.
Depends on the idiom. In Rio Grande Do Sul, where fog is a lot more common, we used to say "tempo cerrado", while "tempo nublado" means cloudy wheater.
A better candidate for an ambiguous term in portuguese is "tempo". It means both "time" and "wheater".
That's interesting, when I lived in SP I only ever saw nublado for either. But I've only been around SP and Brasilia, so I haven't been to Rio Grande Do Sul at all.
Humid is usually related to I weather and a lot of the time hot weather but not always. It’s so humid today it feels like a jungle outside.