This is not a description of the “climate”, since that would mean I actually know what the climate is. However, I do know exactly what weather we experienced, which is what I will describe. Recall, we were there in June, so gauge your season. But given that Papua is a few degrees south of the equator, there really isn’t a lot of seasonal variation, particularly in temperature. The main driving factor here is the elevation, which is between 1500 and 2500 m – meaning you are at mid to high elevations.
Interestingly, we never saw a temperature gauge/thermometer, so I am guessing at the temperatures. That said, I am fairly confident that the weather we experienced can be described as warm days, say mid 20s (22-27 oC), and cool nights, say mid, or even low, teens (10-16 oC). Humidity is not a factor, which will be a very pleasant change for those of you travelling through SE Asia. In a nutshell, I would say the weather was perfect for hiking in the mountains, and then sleeping at night.
In terms of rain, we had one day of rain (i.e., we hiked in the rain for the afternoon), and a few evenings of light rain. So ya, be prepared for rain. But, for the majority of the time, the weather was nice and sunny, and/or mixed cloud and pleasant. All in all, no complaints about the weather J.