What we can glimpse, and these are only glimpses, from descriptions of Valmiki Ramayana, about the climate and geological regions in India, during the time of Ramayana?
We know that, whatever was the season, lunar month of Chaitra, in Ramayana times, was a pleasant season where in forests, trees were full with blossoms – flowers.
We know that ‘Western ghats’ – Sahyadri, Nilgiri and such received heavy monsoon rains.
It appears that temperatures, average temperatures were cooler/colder than they are today. Areas of India, as far south as Nashik (Maharashtra) might have experienced occasional snow during late autumn and during early winter.
Areas of India (North Central, West-Central) had deep forests. It appears significant areas could be grasslands.
River Ganga appears to be a huge river. River Yamuna had stopped feeding its waters to river Sarasvati. River Yamuna had merged with river Ganga near modern day Prayag. Even river Satlaj had already turned west and was no longer feeding its waters to river Sarasvati. Sarasvati did exist in Ramayana time, however we have no additional information about it.
Lanka(Sri Lanka) may have experienced cooler (only relatively speaking) compared to temperatures experienced today. Possibly it had longer rainy season as Is the case in our times.