Open up KotOR tool and expand the tree of the game you want, be it either KotOR1 or KotOR2. Expand the rims directory and scroll down until you find the module identifier for the module you wish to edit. (A list of what module identifiers correspond with which modules can be found here: KotOR 1
, KotOR 2
Once you've found the module you wish to edit you should see two examples of the identifier. (I'll use tar_m02ac as an example.) One should be tar_m02ac.RIM and one should be tar_m02ac_s.RIM.
In order to find the ARE file you'll need to to expand one of the three trees contained within the tar_m02ac.RIM tree. However, at this time I cannot recall which one exactly it is, but a guessing game consisting of choosing between 3 trees should hardly waste your time.
Anyway, once you've found the ARE just extract it to your desired location and use the K-GFF editor, (I recommend this) or any of the other GFF editors, found here
to edit the file. Then once you've edited the ChanceRain field you'll have to save the ARE file and pack up all the files from the module into a .MOD file, including your edited ARE. Once the .MOD is created, stick it in the modules folder and your done..