This Southeast Asian country is about three times the size of Spain, and consists of a large number of islands between 6°N and 10°S of the equator, extending over 45° of longitude. The largest islands from west to east are Sumatera, Java, Borneo, and Sulawesi but there are more than 13,600 smaller islands (over 6,000 of them are uninhabited), of which Bali and the Moluccas are the best known. Indonesia also includes the western portion (Irian Jaya) of the large island of New Guinea. The climate of Irian Jaya is described under Papua New Guinea.
Most of the islands are very mountainous, with numerous volcanic peaks and other mountain ranges exceeding 3,000 m/10,000 ft. There are consequently many sharp local differences of climate within Indonesia; not only are temperatures much lower in the hills, but the amount and season of maximum rainfall vary with the different exposure of the islands to the two main seasonal wind systems. The whole archipelago is alternately dominated by the north monsoon, blowing from China and the north Pacific between November and March, and the south monsoon, blowing from the Indian Ocean and the Australian continent between May and September. For a few weeks around April and October the winds are light and variable in direction; this is the period of transition when the Doldrum belt, or intertropical convergence, moves north or south across the islands.
Apart from the reduced temperatures on the higher mountains, the weather and climate of Indonesia are typical of equatorial regions. Rainfall is almost everywhere heavy and well distributed around the year. Most places receive 1,500-4,000 mm/60-160 in of rain a year. Many places have two wetter periods during the passage of the Doldrum belt; but south-facing coasts and islands south of the equator tend to be wetter during the period of the south monsoon, and north-facing coasts and the northern islands are wetter during the period of the north monsoon. Compare the climatic tables for Jakarta on the north coast of Java with those for Ambon in the Moluccas and Balikpapan on the southeast coast of Borneo. The table for Ujung Padang represents conditions in the south of Sulawesi.
Much of the rainfall is heavy and accompanied by thunder. Some parts of Indonesia have more thunderstorms than anywhere else in the world. In spite of the heavy rainfall, sunshine hours are abundant in Indonesia. During the wetter months sunshine averages four to five hours a day, rising to eight or nine hours a day during drier periods. Jakarta, one of the drier places in the country, receives three times as much rain as London, but it falls on fewer days per year and for only half the number of hours.
Temperatures remain high throughout the year and there is very little difference from month to month. There are only two types of weather in Indonesia: fine and sunny or cloudy and wet. Only the extreme southern islands, such as Sumba, are affected by tropical cyclones; but local wind squalls may occur during thunderstorms. On the coast the daily range of temperature is small but this increases inland and in the hills. The cooler nights inland and the daytime sea breezes and strong monsoon winds afford the chief relief from the heat and humidity on the coast.
The weather may often feel muggy and oppressive to the visitor but heat stress is not severe. Dampness and humidity are the worst features of the climate and this may trouble the elderly or those who are not in good health.
The tables for Medan and Padang represent conditions on the island of Sumatera.
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