Climate and timing
The climate of West Papua varies from equatorial with high rainfall and diffuse seasons, however commonly involving a double rainfall peak, to monsoonal in the Trans-Fly zone, where a short concentrated wet season and prolonged dry season is normative. Highly complex local variations exist in accordance with elevation and exposure to the prevailing winds at different times of year, obviously defying quick characterization in a few words here. Certain regions, such as the Raja Ampat archipelago for instance, have also been proven to be particularly vulnerable to the influence of ENSO-events (El Niño Southern Oscillation), with restricted rainfall during El Niño warm phases and more abundant rainfall during La Niña cold phases.
In many locales, the mornings are clear and bright, with clouds building up by mid-day, and rain showers commonly falling in the early afternoon, often only towards dusk or overnight. Generally the drier season, and perhaps the optimum time of year for visiting, lasts from June through October, when southeastern trade winds can be rather gusty however. Remember that rain falls throughout the year in the highlands and is probably inherently unpredictable.
At sea level daily minima range between 21-24°C throughout the year, whereas daily maxima, somewhat more variable due to cloudiness, average 30-34°C. On average, temperature decreases at a rate of roughly 0.5°C per 100 m elevation's rise. At 1,000 m above sea level expected minima and maxima are roughly 17°C and 26°C. Snowfall and frost is regular only above 3,800 m elevation. In topographic depressions, however, ground frost may occur much lower, in the Anggi Giji lake basin of the Arfak Mountains for instance as low as 1,860 m above sea level.
Relative humidity is constantly high, falling from above 90 % at dawn to 50-60 % in the afternoons. In the perhumid cloud-forest zone humidity rises to 100 % on most nights. In contrast, in the Trans-Fly region it commonly drops below 30 % at the height of the dry season in July-August.
Birding or diving?
The optimum time for birding in West Papua is essentially reversed to that of diving. Birding is most productive during the normal prevalence of southeastern trade winds from June through to September, when displays in the charismatic birds-of-paradise and bowerbirds are at their peak. Diving the fabled Raja Ampat Islands, on the other hand, is best undertaken during the normal northwestern monsoon from November through to April, when seas there are generally calmer and visibility better. The transition months of May and October are conducive to both activities. In fact, over the past ten years, September and October invariably have proven to be the optimum months for displaying birds-of-paradise and bowerbirds in western New Guinea.