This sentence is a question on the surface, but the answer is self-evident. The hot sun is temporary, and the beauty of the rainbow is illusory. Literally, it is a feeling for the passage of time and a nostalgia for beautiful things, but in fact it expresses a kind of regret, sadness and helplessness.
This is a personal signature. Literally, it is a feeling of the passage of time and a nostalgia for beautiful things. The scorching sun is temporary, and the rainbow is illusory and beautiful. It is because some things are beautiful and can't last long that they leave an indelible impact on people's hearts.