Confused over 'hay' and 'hace'

Question

Hay and hace both seem to use nouns but they don't seem to be interchangeable.  I'm puzzled as to why, for example, it's hace calor but hay humedad.

Answer

Hola Hazel
This lesson talks about idiomatic sentences used to describe the weather. 
There is no strict rule for it I'm afraid - some use hace, some está, some hay. 
We tried to give some tips so they don't get so mixed up when memorising them, but it's true that both hace and hay are followed by a noun. After that there is a bit of memorising to do. The good thing is that está is always followed by an adjective in Spanish. 
Saludos
Inma