Deserts and Humidity
Humidity doesn't affect whether or not it rains. Rain clouds form on their own. Humidity does control whether or not the rain will touch the ground or not.

Sorry if I wasn't clear in my other answer.

Deserts. They don't get a lot of rain because rain clouds never make it to a desert. This can be due to a factor of many reasons. You can read more about it here: Why don't deserts get a lot of rain?

Rain forests. Like I said before, humidity doesn't really affect whether or not it will rain. Even though there is high humidity, rain clouds can still form, and it will still rain. Like I said in my other question, the surrounding air can't hold more water, so there will always be rain in a rain forest if rain clouds are present.

What I was trying to say in my other question, is that the surrounding humidity could play a role in whether or not the rain drops actually fall to the ground or not. But rain cloud formation is completely unrelated to humidity.

I'll be sure to edit that question to make it more concise. Sorry for the confusion.