Use 'transitory' to describe something that won't last long and is not a fixed condition. Their happiness was transitory, as the news they received later changed everything.
Pro Tip 2/3
No Lasting Effect
'Transitory' implies that the impact of the event or condition will not have lasting effects. The economic downturn was seen as transitory, with recovery expected soon.
Pro Tip 3/3
Emphasizing Duration
Stress the briefness of an event by describing it as 'transitory', especially to contrast with longer states. Fortunately, the discomfort from the procedure is transitory.