Surely " foggy" is an adjective . The noun is " fog" ! So why not use "está " for " it is foggy " _
There are specific rules on talking about weather in Spanish. You can only say hay niebla.
See lesson - using hay and estar to talk about weather.
niebla is a noun. Nublado would the adjective. I don't see any problems in "está nublado".
When we say that we use a noun with hay and an adjective with está, we mean these are nouns and adjectives in Spanish, not necessarily in English.
Hay + noun niebla
The translation in English for this is a bit different and you use an adjective, not a noun: it is foggy (foggy is an adjective) - Literally what we say here in Spanish is "There is fog", but that would sound terrible in English.
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