Why does Port Moresby receive much less rain than the southward-facing Highland slopes?

The disparity in rainfall between Port Moresby and the southward-facing Highland slopes is primarily due to the influence of the southeast trade winds and the specific topography of the region.1

During the period when the southeast trade winds prevail (roughly May to November), these winds blow parallel to the coast near Port Moresby, leaving the area relatively parched and resulting in an annual precipitation of less than 50 inches (1,300 mm). In contrast, where mountainous terrain lies directly athwart—or perpendicular to—this airflow, such as on the southward-facing slopes of the Highlands, the air is forced upward. As this air rises and cools, it releases significant moisture, leading to extremely heavy rainfall that frequently exceeds 300 inches (7,600 mm) annually. This phenomenon highlights how site characteristics, particularly exposure to prevailing wind systems, act as a primary determinant of local climate patterns in Papua New Guinea.1