Aponaptrīyā is a minor ritual conduc-ted during and as a part of Somayāga. Water drawn from a river on the day prior to the extraction of the soma juice and preserved in a vessel called vasatīvarī as also the water drawn on the morning of ritual extraction kept in vessels named ekadhanā are ceremonially mixed with appropriate hymns (Ṛgveda 10.30). This hymn is also known by the same name as the rite itself.
The hotṛ (priest, representing the Ṛgveda) is advised to recite the first stanza (pra devatrā brahmaṇe ...) continuously thrice. This will induce Parjanya, the rain-god, to give rains where and when necessary.
This rite will also help the yajamāna (performer of sacrifice) to complete the sacrifice without any obstacles or hitch.