1.
moody person; fickle person; someone whose mood changes unpredictably
A person whose mood or attitude changes frequently and unpredictably, like the weather. Used to describe someone who is cheerful one moment and grumpy the next.
A colorful metaphor comparing a person's unpredictable mood swings to changeable weather. The お is a polite/softening prefix, 天気 means weather, and 屋 is the suffix for 'person characterized by.' The word has a mildly critical but often affectionate tone.
Usage
Can be used both as a noun (お天気屋だ) and as a na-adjective-like modifier (お天気屋な人). Often used with a tone of exasperation tinged with fondness.