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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?

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  • $\begingroup$ You could use existing weather data to see if there's a correlation. However, you may be confusing cause and effect here. Rain causes higher humidity, not necessarily vice versa. $\endgroup$ – Barry Carter Oct 10 at 13:24
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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.

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  • $\begingroup$ Humidity doesn't affect whether or not it rains. Rain clouds form on their own. Maybe I'm misunderstanding your point, but the idea that cloud formation and rainfall are unrelated to humidity is, well, nonsense. Can you explain your reasoning a bit more? Thx. $\endgroup$ – Deditos Oct 7 at 16:25
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    $\begingroup$ Oh yes, I can. So whe I say humidity, I mean the humidity of the air close to the ground. That part of the humidity affets wheter or not raindrops touch the ground. I am not talking about thw humidity in the sky, where the clouds form. In that case, yes, you'd be right, you need humidity for clouds to form. But I'm referencing the humidity that's near thr ground. Sorry for the confusion $\endgroup$ – Fuzzy Squid Oct 8 at 1:02
  • $\begingroup$ Oh, and keep in mind that winds can blow clouds to other places... $\endgroup$ – Fuzzy Squid Oct 8 at 1:04
  • $\begingroup$ Sure, so you're talking about (stratiform) rainfall into a dry air layer, i.e., virga. I think you need to make it clear that you're talking about that specific phenomena, because in trying to clarify your other answer you've made here a lot of broad statements that aren't true. I suspect that similar broad statements in your other answer lead to this question by the same OP. $\endgroup$ – Deditos Oct 8 at 8:46
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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.

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