One year when I was living in South Africa, the rainy season brought us nine whole days of truly non-stop rain. It was a powerful experience, spiritually speaking, for me. It did seem as if the rain was speaking to me and so I think I comprehend what Merton is talking about here.
Think of it, all that speech pouring down, selling nothing, judging nobody… what a thing it is to sit absolutely alone, in the forest, at night, cherished by this wonderful, intelligible, perfectly innocent speech, the most comforting speech in the world, the talk that rain makes by itself all over the ridges … nobody started it, nobody is going to stop it. it will talk as long as it wants, this rain. as long as it talks I am going to listen.