capricious
adj.adj. changing your mind or your behavior very quickly and without a clear reason. You use this to describe someone who is unpredictable or someone who makes decisions based on their mood.
adj. governed by impulse rather than by reason or calculation; subject to sudden and unaccountable changes of mind or mood. Often used to describe legal or administrative decisions that lack a consistent basis.
The weather in this valley is very capricious.
The manager's capricious nature made it difficult for the team to plan their projects in advance.
The court ruled that the board's decision to terminate the contract was capricious, as there was no evidence of a consistent policy or a legitimate business reason for the change.
Borrowed from Middle French capricieux, from Italian capriccioso, from capriccio. By surface analysis, caprice + -ious.