How is that a general rule of thumb that less humidity equals more rain? Deserts don’t get much rain and humidity is also very low. Rainforests get a lot of rain when the humidity is very high so how is that that less humidity equals more rain?
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.
I wouldn't say that low humidity equals more rain. The key to the conundrum is the Hadley cell. There are two of them, one on either side of the equator, and they are huge air masses which encircle the globe. What happens is that the equator is heated by the sun, and this huge mass of humid, water laden, tropical air rises to a height of about 7 or 8 miles. As it rises, the pressure falls and the air cools. Cool air at low pressure can hold less water than warm air at high pressure, so the water condenses and falls as rain on the tropical rainforest and tropical seas.
Thus relieved of most of its moisture, the air starts to descend, moving away from the tropics and into the sub-tropics as it does so. Warming and compressing as it passes over the desert areas which lie north and south of the tropics, it becomes much less humid and rainfall is scarce. There are other circulation cells north and south beyond the Hadley cells, but they are much less relevant to your question. Here in Hereford UK I have noticed that rain is more likely when there is high humidity.