moody person; temperamental person; fickle person
Describes someone whose mood or attitude changes easily, like the weather. Often used in a light or mildly critical way. Also seen as お天気屋.
See also: お天気屋
He's a moody person, so he sometimes suddenly gets into a bad mood.
あの人は天気屋で、言うことがコロコロ変わる。
That person is fickle and keeps changing what they say.
Politer variant with お prefix; essentially the same meaning but slightly softer in tone.
Compound of 天気 (weather) and 屋 (person with a certain trait or occupation). The figurative use of 'weather' to describe a changeable personality is straightforward.