Hola Russell

Spanish expressions about the weather are very idiomatic and use sometimes verbs that you don't expect in English. There is a bit of memorizing to do in this topic as there is no specific rule for it. Bear in mind though that when using estar, you need an adjective or a present participle (in Spanish): está nublado, está soleado, está lloviendo... but with hace you need a noun: hace sol, hace calor...

Saludos cordiales

Inma