The climate of Papua New Guinea is chiefly influenced by altitude and by the monsoons. The northwest or wet monsoon prevails from December to March, and the southeast or dry trade winds from May to October. Annual rainfall varies widely with the monsoon pattern, ranging from as little as 127 cm (50 in) at Port Moresby to an average of 584 cm (230 in) in the western river basin. Most of the lowland and island areas have daily mean temperatures of about 27°c (81°f), while in the highlands temperatures may fall to 4°c (39°f) at night and rise to 32°c (90°f) in the day-time. Relative humidity is uniformly high in the lowlands at about 80% and averages between 65 and 80% in the highlands.