Sorry but in krpano 1.21.1 Snow Plugin (build 2023-11-23) there is no reference to "rain" mode...
Something must have happened in the meantime
Can you give a link to an example with a working plugin? Maybe I can download it from another source.
BDW
When trying to dynamically create the 'snow' plugin, there is a conflict with mode=image which messes up the whole process. It is probably a conflict with the main IMAGE object of the current scene. I point out that I am using the scope=local action.
I changed 'image' to 'graphic' and everything works fine.