You can add the effect of SNOW or RAIN over your stories, bringing the scene to life and setting the mood:  

Weather effects must always be applied to the very end of a background command. For example:

EXT. ABANDONED HOME - DAY with effect LIGHT RAIN

If that background has other overlays applied to it, the weather must come at the very end:

EXT. ABANDONED HOME - DAY with TABLE CAFE with effect HARD SNOW

The animated weather effects can be found on the sidebar of your script page:

To turn the effect off, you will need to re-establish your background without the weather on it, repositioning all of your characters:

EXT. ABANDONED HOME - DAY