ephemeral
Lasting for only a very short time; very temporary.
Ephemeral describes something that lasts only a very short time before disappearing or fading away. A soap bubble floating through the air is ephemeral: it shimmers beautifully for just a few seconds before it pops. A rainbow is ephemeral: it appears after a storm, glows brilliantly across the sky, then vanishes as quickly as it came.
Today we use it for anything fleeting or temporary. Morning dew on grass blades is ephemeral, evaporating as soon as the sun warms the ground. Cherry blossoms are famously ephemeral: they bloom in spectacular clouds of pink and white for just a week or two each spring before the petals scatter in the wind.
People sometimes describe moments as ephemeral too. The perfect sand castle you build at the beach is ephemeral because the tide will wash it away. That feeling of excitement on the last day of school is ephemeral: intense and wonderful, but gone by the next morning.
Understanding that something is ephemeral often makes us appreciate it more while it's here. When you know a sunset will fade in minutes or snow will melt by afternoon, you pay closer attention to its beauty right now.