The Art of Rain Making

The twigs of Punarnava are offered in the Ahavaniya Fire and the resulting smoke is consecrated with the mantra, Hiranya Kesho Rajaso...

It means that the smoke of Punarnava bellowing out along with the golden flames reaches the clouds quickly and produces such a heavy rain that even the sun rise will not be visible due to the dense coverage of the black clouds.

Up in the smoke goes the undue credit to modern science for rain-making. The Vedas had already done it.

Source: The book Science in Samskrit by Samskrita Bharati