Grammar 3 了 in the second sentence indicates that it wasn’t raining before and it just started, so the situation has changed. http://chineseffect.com/grammar/past/finished-action/ http://chineseffect.com/grammar/change-of-state/
Today we’ll learn the radical that is related to “rain, or other weather symbols“: 雨 yǔ - 雨 This radical is placed on the top of a character. Most characters that have this radical are related to rain, or other weather symbols. The following are a few examples: 雪 xuě (snow), 霜 shuāng (frost), 雾 wù (fog), 雷 léi (thunder) etc.