You thought that Amazonia was one of the rare area in the world to know a constant sunshine, all the year, with hot temperatures and a stifling humidity? You will be disenchanted.
Indeed, apart from the rainy season (so often between June and September), cold and dry winds came from the highest lands of the Andes make a small deviation to the low areas. The temperature falls suddenly, passing from 27° to almost 9 or 10°, making fall the humidity rate in a drastic way. Moreover, winds are regularly very strong when it pass through these low altitudes. This phenomena can cause a big problem for half-constant cultures like the cacao, coffee or tea (supporting neither cold nor lack of humidity), as well as week populations (children and old persons) unaccustomed to the cold generally speaking. But, on the other side, it would seem that it would be benefic for the biodiversity and the biological endemism of this area.
At any rate, if you think about travelling all around Peruvian South Amazonia between June and September, you will need something to put on your back and protect you; thongs and little short won’t be enough if you are closer to one of this cold snap.