English equivalent of the Portuguese phrase: "this person's mood changes according to the moon"

In Portuguese there is an expression that says:

"Essa pessoa é de lua."

Literally "this person's mood changes according to the moon", which means that nobody can predict that person's mood.

Is there an equivalent expression or similar to this in English?

When someone is angry one day and very peaceful the next day...

Such a person is said to be mercurial or capricious.

mercurial: likely to change your mood or opinion unexpectedly

capricious: suddenly and unexpectedly changing your opinion or behaviour without any good reason

An equivalent expression might be, "He/she is as changeable as the weather."

Though there are many individual words that might capture the Portuguese phrase, common English phrases for variability are:

changes with the wind

as temperamental as a prima donna

an "unpredictable and volatile temperament" comes to mind.

volatile - adj. (of persons) disposed to caprice or inconstancy; fickle; mercurial.

Examples:

• "Warburg grew up in a conservative home environment. Early on he demonstrated an unstable, unpredictable and volatile temperament."

• "She's unpredictable as weather."

• "Agrippa's kinsman already alluded to above, did somehow run afoul of Nero's unpredictable and volatile temperament"

• " you may need to wear protective gear if your dog has an unpredictable and volatile temperament"

There is also a traditional comparison (credit to tchrist) which fits perfectly: "as changeable as the moon" - "Her mood is changeable as the moon"

• The frequent and obvious changes in the position of the moon, in the area of its enlightened surface, and in the quantity of light reflected there from the earth, are so obvious that they are proverbial; and to say of anything that it is as changeable as the moon, is to mark it with a note of instability in the extreme.