Full っている vs shorter ってる: slang or official tool


I know, that I can skip い in building continuous form of a verb.

> For example:  
>  雪が降っている。  
>  雪が降ってる。
>
> are equal and mean: [It's snowing.]

Question:  
how legal is it to skip い in case like this?  
Is it slang?  
Is it error?  
Or maybe it is a part of official Japanese language already?




Dropping of い is a very common _colloquialism_. It is heard in all sorts of
informal situations, and kids probably learn how to say 降ってる before 降っている. You
should use the long version in formal settings. There are [similar contraction
patterns](https://japanese.stackexchange.com/a/18159/5010), and てる and ちゃう are
especially common among them.




Dropping of い is a very common _colloquialism_. It is heard in all sorts of
informal situations, and kids probably learn how to say 降ってる before 降っている. You
should use the long version in formal settings. There are [similar contraction
patterns](https://japanese.stackexchange.com/a/18159/5010), and てる and ちゃう are
especially common among them.

