In addition to changing the "cloud rate" by using a script which changes an existing map, it is possible to set the relative base level of rain when setting up random maps at the beginning.
When "starting from scratch" to set up the attributes of an island, the first setting is "Vegetation"
- Normal (the default)
.These settings also set the base rainfall level at a parallel amount.
Therefore, you need to set the "vegetation" level to match the rainfall for the crop you wish to be primary on the island, e.g. 'overgrown' for sugar.