Indonesia - Climate

The main variable in Indonesia’s climate is not temperature or air pressure but rainfall. The almost uniformly warm waters that make up 81 percent of Indonesia’s area ensure that temperatures on land remain fairly constant. Traversed by the equator, the archipelago is almost entirely tropical in climate. Temperatures average 28° C on the coastal plains, 26° C in inland and mountain areas, and 23° C in the higher mountain regions. Winds are moderate and generally predictable; monsoons usually blow in from the south and east between June and September and from the northwest between December and March. Typhoons and other large storms pose little hazard to mariners in Indonesia’s waters; the primary danger comes from swift currents in channels such as the Lombok, Sape, and Sunda straits.

Extreme variations in rainfall are linked with the monsoons. There is a dry season (June to September), influenced by the Australian continental air masses, and a rainy season (December to March) that is influenced by air masses from mainland Asia and the Pacific Ocean. Local conditions in Indonesia, however, can greatly modify these patterns, especially in the central islands of the Maluku group. This oscillating seasonal pattern of wind and rain is related to Indonesia’s geographic location as an archipelago between two continents and astride the equator. During the dry monsoon, high pressure over the Australian deserts moves winds from Australia toward the northwest. As the winds reach the equator, the Earth’s rotation causes them to veer off their original course in a northeasterly direction toward the Southeast Asian mainland. During the wet monsoon, a corresponding high-pressure system over the Asian mainland causes the pattern to reverse. The resultant monsoon is augmented by humid breezes from the Indian Ocean, producing significant amounts of rain throughout many parts of the archipelago.

Prevailing wind patterns interact with local topographic conditions to produce significant variations in rainfall throughout the archipelago. In general, the western and northern parts of Indonesia experience the most precipitation because the northward- and westward-moving monsoon clouds are heavy with moisture by the time they reach these more distant regions. The average annual rainfall for Indonesia is around 3,175 millimeters. Western Sumatra, Java, Bali, and the interiors of Kalimantan, Sulawesi, and Papua are the most consistently damp regions of Indonesia, with rainfall measuring more than 2,000 millimeters per year.

In part, this moisture originates on certain high mountain peaks that, because of their location, trap damp air and experience more than 6,000 millimeters of rain a year. The city of Bogor, near Jakarta, has a high rainfall rate of 3,500 to 4,000 millimeters annually. On the other hand, the areas closest to Australia—including Nusa Tenggara and the eastern tip of Java— tend to be dry, with some areas experiencing less than 1,000 millimeters of rainfall per year. Some of the islands of southern Maluku experience highly unpredictable rainfall patterns, depending on local wind currents.

The air temperature changes little from season to season or from one region to the next, but cooler temperatures prevail at higher elevations. In general, temperatures drop approximately 1° C per 90 meters of increase in elevation from sea level; night frosts occur in some high interior mountain regions. The highest mountain ranges in Papua are permanently capped with snow.

Located on the equator, the archipelago experiences relatively little change in the length of daylight hours from one season to the next; the difference between the longest day of the year and the shortest is only 48 minutes. The archipelago stretches across three time zones: Western Indonesian Time—seven hours in advance of Greenwich Mean Time (GMT)—applies to Sumatra, Java, and west and central Kalimantan; Central Indonesian Time—eight hours ahead of GMT—is observed in Bali, Nusa Tenggara, south and east Kalimantan, and Sulawesi; clocks are set to Eastern Indonesian Time—nine hours ahead of GMT—in the Malukus and Papua. The boundary between the western and central time zones—established in 1988—is a line running north between Java and Bali through the center of Kalimantan. The border between the central and eastern time zones runs north from the eastern tip of Timor to the eastern tip of Sulawesi. Indonesia does not operate daylight-saving time in the summer.