What does the clear sky mean in the solar term Song Dynasty?

According to Tianqi Encyclopedia, Qinggutian refers to Qingming and Grain Rain. Valley-clearing Day comes from the 24-year solar term Song Dynasty, and the full text says: "Spring rain starts the Valley-clearing Day in spring, and summer is full and summer is hot. Autumn frost, winter snow. "

"Twenty-four solar term song" is a poem compiled for the convenience of remembering the twenty-four solar terms in the ancient calendar of China. Many versions have been circulated and sung everywhere. Twenty-four solar terms originated in the Yellow River Basin.