100% humidity means the air contains as much water vapor as it can hold. That means no additional water vapor can be added. That does not mean existing water vapor will spontaneously condense and fall out as rain.
Imagine an empty box. It can fit so many things. Once it’s full, it’s full, but that doesn’t mean the things that are in it are just going to magically jump out of it just because the box is full.
In order for rain to happen, that water vapor must be condensed into water droplets, which usually happens as result of a drop in temperature (usually because the water vapor has risen to a certain altitude), those droplets continue to condense and combine until they are too heavy to be held aloft upon which they fall as rain.
On a hot and humid day near the ground, that’s not going to happen.
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