Hola Rohan, 

foggy and sunny are indeed adjectives but "niebla is a noun in Spanish meaning "fog", not foggy; this is why we say "hay niebla" (hay + noun). Soleado is actually an adjective in Spanish coming from the noun "sol":

sol = sun / soleado = sunny

so here we need está, "está soleado".

I hope this clarified it for you. If you keep reading in this thread, you can see that a very similar question was clarified to Jim, Goya and Rohan.

Saludos,

Inma