There are research papers which mention that panchangs are more accurate than IMD in terms of climate prediction. Moreover, I read that there are multiple predictions possible, over and above rainfall prediction. Can I know if there is any document/book which discusses how this is calculated? I would like to know how it is calculated, and not the links where predictions are available.
It comes under mundane Astrology. Sometimes a horoscope is delineated considering location such as Sumeru or Vindhyā at other times, specific verses from Brihat Samhitā, etc. are interpreted. You can look at this research paper to know how it is carried out in the later case.