Missing Blue Sky and Clouds

[Ischtara]

I have duplicated the map 'airplane' and created it as airplane_02 in the server. Likewise I have entered the entry in the indoorrswtable.txt in the client. So far so good. But now the blue background and the animation of the clouds are missing.

Can anyone tell me why this is and how I can make my copied map have the same background + clouds as the original maps 'airplane' and 'airplane_01' have? 

Not that it would affect the use of the map, but it just doesn't look nice.

I would be grateful for any hints.

Edited October 14, 2021 by Ischtara
Typo

[Emistry]

may refer this topic

https://rathena.org/board/topic/101222-adding-sky-in-the-map/