Climate of Indonesia

The main variable of Indonesia's climate is not temperature or air pressure, but rainfall. The area's relative humidity ranges between 70 and 90%. Winds are moderate and generally predictable, with monsoons usually blowing in from the south and east in June through September and from the northwest in December through March. Typhoons and large-scale storms pose little hazard to mariners in Indonesian waters; the major danger comes from swift currents in channels, such as the Lombok and Sape straits.

Indonesia experiences a number of climates, mostly tropical rainforest (highest precipitation), followed by tropical monsoon and tropical savanna (lowest precipitation). However, oceanic climates and subtropical highland climates are found in a number of high-altitude regions in Indonesia, mostly between 1,500 and 3,500 metres (4,900 and 11,500 ft) above sea level. Regions that are above this level (mostly in the Papuan highlands) fall into the tundra climate category and the subpolar oceanic category.

Monsoons

The extreme variations in rainfall are linked with the Australian-Indonesian monsoons. Generally speaking, there is a dry season (April to September), caused by Australian continental air masses, and a rainy season (October to March), caused by air masses in Asia and the Pacific Ocean. Local wind patterns, however, can greatly modify these general wind patterns, especially in the islands of central Maluku—Seram, Ambon, and Buru. This oscillating annual pattern of wind and rain is related to Indonesia's geographical location as an isthmus between two large continents. In September and May, high pressure over the Gobi desert moves winds from that continent 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 January and February, a corresponding low pressure system over Asia causes the pattern to reverse. The result is a monsoon which is augmented by humid breezes from the Indian Ocean, producing significant amounts of rain throughout many parts of the Malay Archipelago. See also monsoon trough.