Why do people say this "ghost" weather when the weather is bad?

The extended meaning of "ghost" means "hiding".

So "ghost weather" can be understood as "unexpected weather"

Similarly, there is a kind of "ghost idea", which usually refers to hidden ideas.

The mathematical explanation is:

Ghost = bad,

Bad = bad,

∴ Bad weather = ghost weather (equivalent replacement).

In addition, bad weather is always gloomy, which is the legendary environment that ghosts like, so "ghost weather" is "weather suitable for ghosts"

Finally, the word "ghost" means complaining. If the weather is bad, don't blame Zhang San or Li Si, then blame the ghost.

Chinese is extensive and profound. I'm just saying, just for reference.