[flomble]

>nebulous

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.

[gattilorenz]

I think that another factor is that they have quite different effects in day-to-day life. If it's cloudy you can still do most of the stuff you would normally do, but you wouldn't want to ride your car/horse/chariot/whatever at full speed in a foggy day (and in general, things that require good visibility). It's a useful distinction.

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.

[de_watcher]

There can even be two different words for the fog: for one that's low above the fields and the other one that's everywhere.