'The altar, O Gautama, is Parganya (the god of rain); the fuel is the year itself, the smoke the clouds, the light the lightning, the coals the thunderbolt, the sparks the thunderings. On that altar the Devas offer Soma, the king (the moon). From that oblation rises rain.

In plain words: Next the rain god is pictured as a sacred fire, with clouds as smoke and lightning as light. The gods offer the moon into it, and rain is born.