Just imagine, how we bunch our body up when it gets cold, and how we relax/release our body when it gets warmer, and this is actually the same for almost everything in the world.so, 날씨가 풀리다, literally 'the weather gets released', means the weather's getting warmer, the cold snap is coming to an end etc. The phrase is mostly used in spring.,but not limited to spring. You can use this when cold and warm days alter.