了 usage conflict "now" vs "completion" While learning about 了 usages, i confronted a headache conflict between it's usage or maybe it's an error so i need someone please to clarify this issue.

下雨了

If i go by it's use as action completion marker it mean it rained.
while in the course this example is for situation change meaning it rain now.

does it completely depend on context to figure out witch meaning ? 
Thanks for your help.
了 (le / ㄌㄜ˙) has two meanings.


  
  放在动词或形容词后，表示动作或变化已经完成：写完～。
  


When it follows a verb or adjective, it's a perfect or past tense.

In this case we can use 过 or 曾经 to clarify its tense.
For example, 

下过雨了 or 曾经下雨了
It has been rained.

昨天下雨了
It rained yesterday.


  
  助词，用在句子末尾或句中停顿的地方，表示变化，表示出现新的情况：刮风～。
  


(Particle) When it's used at the end of the sentence or as a pause in the sentence, it means something is changing, a new situation.

下雨了
It's raining.