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Geography & Climate

Geography

Indonesia is the largest archipelago in the world with the total number of 17,508 islands according to the Indonesian Naval Hydro-Oceanographic office. The archipelago is on a crossroads between two oceans, the Pacific and the Indian ocean, and bridges two continents, Asia and Australia. This strategic position has always influenced the cultural, social, political and economic life of the country. The territory of the Republic of Indonesia stretches from 6°08' north latitude to 11°15' south latitude, and from 94°45' to 141°05' east longitude. The Indonesian sea area is four times greater than its land area, which is about 1.9 million sq. km. The sea area is about 7.9 million sq. km (including an exclusive economic zone) and constitutes about 81% of the total area of the country.

The five main islands are: Sumatra, which is about 473,606 sq. km. in size: the most fertile and densely populated islands. Java/Madura, 132,107 sq. km: Kalimantan, which comprises two thirds of the island of Borneo and measures 539.460 sq. km; Sulawesi. 189,216 sq. km; and Irian Jaya, 421.981 sq. km, which is part of the world's second largest island. New Guinea. Indonesia's other islands are smaller in size. 
The archipelago is divided into three groups. The islands of Java, Sumatra and Kalimantan, and the small islands in-between. lie on the Sunda Shelf which begin on the coasts of Malaysia and Indo China, where the sea depth does not exceed 700 feet. Irian Jaya which is part of the island of New Guinea, and the Aru Islands lie on the Sahul Shelf, which stretches northwards from the Australian coast. Here the sea depth is similar to that of the Sunda Shelf. Located between these two shelves is the island group of Nusa Tenggara. Maluku and Sulawesi, where the sea depth reaches 15.000 feet. Coastal plains have been developed around the islands of Sumatra. Java. Kalimantan and Irian Jaya. The land area is generally covered by thick tropical rain forests, where fertile soils are continuously replenished by volcanic eruptions like those on the island of Java. The country is predominantly mountainous with some 400 volcanoes. of which 100 are active. Mountains higher than 9,000 feet are found on the islands of Sumatra (Mt. Leuser and Mt. Kerinci), Java (Mt. Cede. Mt. Tangkubanperahu, Mt. Cirernai. Mt. Kawi. Mt. Kelud, Mt. Serneru and Mt. Raung). Sulawesi (Mt. Lompobatang and Mt. Rantekombala). Bali (Mt. Batur and Mt. Agung), Lombok (Mt. Rinjani) and Sumbawa (Mt. Tambora). The highest mountain is the perpetually snow-capped Mandala Top (15,300 feet) in the Jaya Wijaya mountain range of Irian Jaya. Many rivers flow throughout the country. They serve as useful transportation routes on certain islands, for example, the Musi, Batanghari. Indragiri and Kampar rivers in Sumatra: the Kapuas, Barito. Mahakarn and Rejang rivers in Kalimantan; and the Memberarno and Digul rivers in Irian Jaya. In Java rivers are important for irrigation purposes, i.e.. the Bengawan Solo, Citarum and Brantas rivers. 
A number of islands are dotted with scenic lakes, like the Toba, Maninjau and Singkarak lakes in Sumatra; the Tempe, Towuti, Sidenreng, Poso. Limboto, Tondano, and Matana lakes in Sulawesi: and the Paniai and Sentani lakes in Irian Jaya.

Climate

The main variable of 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. Split by the equator, the archipelago is almost entirely tropical in 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 ranges between 70 and 90 percent. 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 Indonesia waters; the major danger comes from swift currents in channels, such as the Lombok and Sape straits. 

The extreme variations in rainfall are linked with the monsoons. Generally speaking, there is a dry season (June to September), influenced by the Australian continental air masses, and a rainy season (December to March) that is the result of mainland Asia and Pacific Ocean air masses. Local wind patterns, however, can greatly modify these general wind patterns, especially in the islands of central Maluku-Seram, Ambon, and Buru. This oscillating seasonal pattern of wind and rain is related to Indonesia's geographical location as an archipelago between two large continents. In July and August, high pressure over the Australian 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 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, 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. In part, this moisture originates on strategically located high mountain peaks that trap damp air. The city of Bogor, near Jakarta, lays claim to having to world's highest number of rainstorms per year. On the other hand, the islands 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 per year. To complicate the situation, some of the islands of the southern Malukus experience highly unpredictable rainfall patterns, depending on local wind currents. 

Although air temperature changes little from season to season or from one region to the next, cooler temperatures prevail at higher elevations. In general, temperatures drop approximately 1° per 90 meters increase in elevation from sea level with some high altitude interior mountain regions experiencing night frosts. The highest mountain ranges in Irian Jaya 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 and the shortest day of the year is only forty-eight minutes. The archipelago stretches across three time zones: Western Indonesian Time-seven hours in advance of Greenwich Mean Time (GMT)-includes Sumatra, Java, and eastern Kalimantan; Central Indonesian Time-eight hours head of GMT--includes western Kalimantan, Nusa Tenggara, and Sulawesi; and Eastern Indonesian Time-nine hours ahead of GMT-- includes the Malukus and Irian Jaya. 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 central and eastern time zones runs north from the eastern tip of Timor to the eastern tip of Sulawesi.

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