At matan torah/the revelation at Har Sinai, why were there clouds, and fog? I know about the lighthing and thunder, but why did the sky have to be full of heavy clouds, as mentioned in parashat va’etchanan?

The light is always revealed through the darkness of the clouds since in its full strengh it is too powerful to be perceived (Rabbeinu B'Chayei Shmot 20, 21).