Hi Dmitry,

This expression can be use when something is going bad and it can only go better.
Example, someone doesn't get a promotion and either a friend or colleague will say something like this to encourage. (ça va s'arranger) or a break up.
As for the other, it's meant in a bad way. Could be related to weather (disaster), pollution, climate change etc.

Hope it answer your question.

Regards,