Originally Posted by
tramsan
Weather is not a mob. It was discussed as a possible - but undesirable - workaround to create dynamic weather. At the moment they are statically defined properties of the region you are in.
Until this is taken care of, however, you can easily remove the thunder (as well as ran and smoke puffs, if you wish), by editing the file region_rain.def in the horizons\resources\definitions\regions directory. Simply open it with notepad. At the very bottom, you will see three blocks of "envEffect" - remove whichever block corresponds to the effect you want gone, and renumber the remaining ones, if needed, so they start at 0.