Hola David

when we added this note in this lesson:

we mean adjective or noun in Spanish, not in English.

Sometimes the phrase used in English has a different structure to its equivalent in Spanish. The "foggy" example is one of them, as you say in English: 

It is foggy (foggy = adjective) 

but the way we say this in Spanish is:

Hay niebla (niebla meaning fog, not foggy; niebla = noun)

We are literally saying here "There is fog".

So, you need to look out for adjectives or nouns in Spanish, not in English.

I hope this clarifies it.

Saludos

Inma