When, do you think, the knowledge about the formation of rain was expiscated? 19th Century? Or may be 18th Century?
When I was browsing through Vishnu Puranam in Sacred Texts, I found the following piece of information about Rain as Narrated by Parashara to his disciple Maitreya. Vishnu Purana is often considered the important purana of the 18 puranas. The following excerpt is from the translation work done by HH Wilson on Vishnu Purana. This comes in chapter IX of Book 2.
” In what manner the world depends upon the sun, be attentive, and you shall hear.
During eight months of the year the sun attracts the waters, which are the essence of all fluids, and then pours them upon earths (during the other four months) as rain; from rain grows corn; and by corn the whole world subsists. The sun with his scorching rays absorbs the moisture of the earth, and with them nourishes the moon. The moon communicates, through tubes of air, its dews to the clouds, which, being composed of smoke, fire, and wind (or vapour), can retain the waters with which they are charged: they are therefore called Abhras, because their contents are not dispersed. When however they are broken to pieces by the wind, then watery stores descend, bland, and freed front every impurity by the sweetening process of time. The sun, Maitreya, exhales watery fluids from four sources, seas, rivers, the earth, and living creatures. The water that the sun has drawn up from the Gangá of the skies he quickly pours down with his rays, and without a cloud; and men who are touched by this pure rain are cleansed from the soil of sin, and never see hell: this is termed celestial ablution. That rain which falls whilst the sun is shining, and without a cloud in the sky, is the water of the heavenly Ganges, shed by the solar rays.
The water which the clouds shed upon earth is in truth the ambrosia of living beings, for it gives fertility to the plants which are the support of their existence. By this all vegetables grow and are matured, and become the means of maintaining life. With them, again, those men who take the law for their light perform daily sacrifices, and through them give nourishment to the gods. And thus sacrifices, the Vedas, the font’ castes, with the Brahmans at their head, all the residences of the gods, all the tribes of animals, the whole world, all are supported by the rains by which food is produced. But the rain is evolved by the sun; the sun is sustained by Dhruva; and Dhruva is supported by the celestial porpoise-shaped sphere, which is one with Náráyan?a. Náráyan?a, the primeval existent, and eternally enduring, seated in the heart of the stellar sphere, is the supporter of all beings”
Vishnu Purana is considered to be composed in First or Second Century AD. It has more details like the distance of each planets from each other, diameter of the Sun, number of years in each Yuga etc. More information about RAIN are explained in Matsya Purana, Linga Purana and Vayu Purana.