The highlands sit at around 1,600 metres and the climate is cool year-round — daytime temperatures in the low 20s°C, nights cold enough to need a jumper. The drier season runs roughly May to October and is the best window for trekking and for reliable Wamena flights.
The Baliem Valley Festival in August is the single best time to come if you can plan ahead. Avoid heavy rain months (December–February) if you can — paths turn to mud, and small-aircraft cancellations spike.