... the script doubles the [starting] cloud settings for the whole game, but it runs only once per game. So, if you save your game, switch the 100 to 200 and start your game from the savepoint it will not triple your clouds because the script had already run at +100. If you start a new game, it will use the +200.
The original cloud \ rainfall settings were established by the "Vegetation" setting when the map was first created. That number likely falls on a scale of 0 to 100. The 'AddTo' adjustment is a % of that number. So 100 doubles it and 200 triples it. See here for more:http://www.the-nextlevel.com/tropico/cafe/index.php?topic=1173.0
Also be aware that more rain does not necessarily equal better. I was playing Mike Cresswell's map 'The Ancient One' which includes an offer that boosts CloudRate by 100% -- instantly the whole island was a swamp! The ground was so wet I had to feed my people pineapple because I couldn't grow corn anywhere! (On the good side, you should have seen my farms pump out sugar after that - too bad it wasn't a rum scenario.) So be careful when messing with CloudRate, small Effects can have big affects!
Also, you weather meddlers out there should be aware that changing CloudRate does not have an *immediate* effect. It changes how thick the clouds are, but the clouds still have to generate at the edge of the map, move over your island and do that rainy thing. So a change in cloud rate will take a few months before you see any effect -- and a few months more before any change in your crops' growth will become noticeable.