I thought we agreed I wasn't giving Ryohei the Rain jutsus. ~_~
Rain is the clouds crying.
No jk, uhh... I'm not entirely sure what rain is to us in the Narutoverse. It's a commodity in Amegakure, I know that for sure. Rain in real life is basically the sunlight absorbing the rain, which then form clouds and when they get heavy enough, it rains. I'm not too confident in my knowledge of the scientific side of everything to do with rain. In the Narutoverse, Rain as I said is basically native to Amegakure, and the requirement for using rain is for one to spend a nice amount of time in said village, soaking it up in order to embrace it, and then eventually learn it. Rain can be either falling naturally or artifically with our chakra, using the "Rain Tiger at Will" technique which is common for us to use in order to use our rain techniques. Also because this artificial rain is already infused with our chakra, it makes it much easier to control than naturally created rain already on the battlefield.
Rain created artificially through Rain Tiger at Will is chakra-based, and is also linked to our senses. The rain can feel chakra resonating within other shinobi and thus sense their location.
That's all I got.