Hola Judy

There is no specific reason why we use hay with some and está with others really. With foggy for example, we use hay because we prefer to use the noun for fog (=niebla/neblina) instead of the adjective foggy (=neblinoso) when we talk about being foggy. As we use the noun "niebla/neblina" we need "hay" which needs to be followed by a noun: "hay niebla". With sunny, we have two options: "hace sol" or "está soleado"; as we explain in the lesson, those that use "está" are followed by an adjective in Spanish (e.g. soleado, nublado, despejado).

There is a bit of memorizing to do with this lesson.

Saludos

Inma