What comparison do the English use when they want to describe a person whose mood and opinion change very often?

The English often use the phrase "like a weather vane" to describe a person whose mood and opinion change frequently. This comparison highlights the idea of inconsistency, suggesting that just as a weather vane shifts direction with changing winds, the person's feelings and views can be unpredictable and fickle. Another common expression is "having a face like a wet weekend," which conveys a similarly variable disposition.