In just three short lines, the speaker describes a moment when their inner world and the weather outside seem to meet. The "small black thoughts" sound heavy but also tiny, like worries that have shrunk the speaker's mind to something cramped and gloomy. Then they "collide with the cold rain," a word that suggests a sudden, almost physical impact rather than a gentle mixing.
The last line surprises us. Instead of ending in misery, the speaker names a "joy of becoming one." The poem seems to suggest that merging with the rain brings relief, as if the speaker's private gloom stops feeling lonely once it joins something larger and natural. There's no punctuation to guide us, so the joy arrives without explanation, which makes it feel quiet and a little strange. The mood is moody and contemplative, but it holds a small spark of release, a sense that giving in to the darkness and the cold can feel like belonging.