Why does it rain before and after the arrival of the 24 solar terms?

Spring Rain Starts Spring and Bright Valley Days: beginning of spring, Rain, Sting, Equinox, Qingming, Grain Rain.

Summer is full of mountains and summer is hot: long summer, small full, mangzhong, summer solstice, light summer and great summer.

Autumn dew autumn frost autumn: beginning of autumn, summer heat, dew, autumnal equinox, cold dew, first frost.

Winter snow is slightly cold in winter: beginning of winter, light snow, heavy snow, winter solstice, slight cold, severe cold.