pluvial

pluvial \PLOO-vee-uhl\, adjective:

1. of or pertaining to rain; rainy.
2. Geology. occurring through the action of rain.

noun:
1. Geology. a rainy period formerly regarded as coeval with a glacial age, but now recognized as episodic and, in the tropics, as characteristic of interglacial ages.

Swimming in the pluvial waters, or inert and caked over by the torrid mud, he would have discovered what he would certainly have regarded as lowly, specially-modified, and degenerate relations of the active denizens of the ocean—the Dipnoi, or mud-fish.
-- H.G. Wells, “Zoological Retrogression”, 1891

Nothing enters her tomb save a little moisture, pluvial in origin, and, it may be, certain mysterious effluvia of which we do not yet know the nature.
-- Maurice Maeterlinck (1862-1949), translated by Bernard Miall, The Life of the Ant, 2001

Pluvial is from the Latin pluvia meaning "rain, water." It shares the Proto-Indo-European root pleu meaning "to flow, to swim" with Pluto, the name of God of the underworld in classical mythology.