霂 Mù is composed of the 雨 radical on top and the character 沐 (Mù) "Bathe" on the bottom. 沐 (Mù) "Bathe" is the phonetic component of this character and also distinguishes it from other forms of rain such as: 霖 (Lín) "Downpour". 沐 (Mù) is composed of the 水 Shuǐ radical, written in its radical form as: "氵", and 木 (Mù) "Tree, Wood".
The flow of water while bathing is similar to the gentle flow of rain in a drizzle.