The mists gather, that is the hinkâra; the cloud has risen, that is the prastâva; it rains, that is the udgîtha; it flashes and thunders, that is the pratihâra; it stops, that is the nidhana. That is the Vairûpa Sâman, as interwoven in Parganya, the god of rain.

In plain words: The five parts of the chant are matched to a rainstorm: mist gathering, the cloud rising, the rain falling, the lightning and thunder, and the rain stopping. This is the Vairûpa chant found in Parganya, the god of rain.