Use 'impermanent' to describe feelings that change like joy, anger, or sadness. Her joy at winning the trophy was impermanent, as the reality of responsibilities set in.
Pro Tip 2/3
Changeable Weather
Reference to weather or seasons highlights their natural inconsistency over time. The sunny skies were impermanent, soon to be replaced by a blanket of clouds.
Pro Tip 3/3
Not for Relationships
Avoid using 'impermanent' for human relationships unless you mean they're intentionally short-term. Their partnership was strategic and impermanent, meant to last only for the duration of the project.