Up air is a term used to describe the upward movement or direction of something. Its opposite, or antonym, would be down air, which refers to movement or direction in a downward direction. Another possible antonym for up air would be still air, which describes an absence of movement or direction altogether. Another possible antonym could be negative air, which could be used in certain contexts to describe air that is harmful or toxic rather than uplifting or positive. Ultimately, the antonym for up air would depend on the specific context in which the term is being used.