Good find ! I figured descr_disasters was meaningless, but that's because I checked the /data one, not the /date/world/base/ one.

But there's gotta be more to it than that... not only is 20 not a percentage, but a min number of turns between events (check the comments at the beginning of the file), but health/sewers/poplevel isn't taken into account, and that goes against empirical evidence.

The file is interesting though. Anyone ever got locusts ? :)