The main variable of Indonesia's climate is not temperature or air pressure, but rainfall. Split by the equator, indonesia has an almost entirely tropical climate, with the coastal plains averaging 28°C, the inland and mountain areas averaging 26°C, and the higher mountain regions, 23°C. The area's relative humidity is quite high, and ranges between 70 and 90 percent.
The extreme variations in rainfall are linked with the monsoons. Generally speaking, there is a dry season (June to September), and a rainy season (December to March). Western and northern parts of Indonesia experience the most precipitation, since the north- and westward-moving monsoon clouds are heavy with moisture by the time they reach these more distant regions. Western Sumatra, Java, Bali, the interiors of Kalimantan, Sulawesi, and Irian Jaya are the most predictably damp regions of Indonesia, with rainfall measuring more than 2,000 millimeters per year.
Typhoons can hit the Islands of the Indonesia between September and December, and can cause rainstorms and heavy winds. However, not every Typhoon that hits Indonesia is a strong one, and in some years only a few Typhoons occur durind the tropical storm season.