If it's raining, you say "It's raining" or if it's about to, "It's going to rain/it's about to rain". If it's foggy, you'll say "There's fog" or "It's foggy". If it's thundering, you'll just say "it's thundering" or "there's thunder". There are many ways to rephrase these statements, but for the most part, they are the ones you'll hear most often. Also, when the clouds disperse and you see the sun, you'll say "the sun came out" instead of "a sunny day came".

Hi Robin, You could say 'it's getting foggy' or 'it's starting to thunder'. A nice way of saying the clouds are dispersing is to say 'the sun has come/is coming out'. We wouldn't really say a sunny day came.

Hi Robin, The next time the clouds disappear, you should just pick up your guitar and play this song: https://www.youtube.com/watch?v=C5E_zXbmrlM

The sky cleared up.