I'm creating a custom (unmodded) world, and I want to understand what I'm doing when I change parameters.

For most of the parameters, there is information readily accessible via the fan wikis, but one setting eludes me:

How does the rain frequency option define the frequency of rain?

It has the following option of which one is self-explanatory:

None, Less, Default, More, Lots.

None means no rain caused by weather at all. But what are the percentages for the rain to occur or how does the game determine when to make rain?

An ideal answer would include percentages and a concise explanation of the rain mechanic.