Straddling the equator, Indonesia is a tropical country with a wet, hot, humid climate the entire year, with high temperatures often in the 90s F during the day and the steamy 70s F at night. Cooler temperatures prevail in the highlands. The humidity is usually between 70 and 90 percent. The only relief is air conditioned buildings, which are not as plentiful as in the West. Indonesians have a much higher tolerance for heat and lower tolerance for cold than Westerners. In the mountains, on what seem like a warm day, you see Indonesians bundled in heavy coats. In sweltering places they don’t even use fans.

Indonesia is south of the typhoon belt—which affects the Philippines, Vietnam, China and Japan—and north of the cyclone belt, which affects Australia. Since Indonesia is so close to the Equator and is surrounded by so much water its climate is characterized as marine equatorial or “doldrums” type, with light winds and frequent thunderstorms, which in turn is modified by monsoon winds and the mountains.

Temperatures and rainfall vary across the archipelago because of elevation and monsoon patterns. Average temperatures at or near sea level range from about 23̊ C to 31̊ C. 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. 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. [Source: Library of Congress]

Indonesia is one the rainiest places on earth. The west coast of Sumatra get above 400 centimeters of rain a year. Other places that receive a lot of rain include northwest Kalimantan, West Java, Papua and some parts of Sulawesi. Other islands such as Sumba and Timor receive relatively small amounts of rain. Temperatures are determined by elevation and nearness to the sea. The temperatures are generally cooler on the coast and in the mountains than there they are in the interior and in the lowlands.

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.

Wet and Dry Seasons in Indonesia

There are two major seasons in Indonesia—1) the hot dry season and 2) the rainy monsoon season. The wet season for most of Indonesia is from September to March and the dry season is from March or June (depending on the area) to September. But the times of these season varies from place to place because western Indonesia, eastern Indonesia, and Borneo are all influenced by different monsoon wind patterns. The rainy season is different than the one in Southeast Asia and India, which usually lasts from May to September.

From November to May winter monsoon winds blow in from the northeast and brings moisture from the South China Sea and the Pacific to north and northeast Indonesia while land on the sheltered southern side of the mountains have a dry season. From June to October the summer monsoon that brings rains to India also brings rain and winds from the south and southeast to the southern and western sides of Sumatra and Java. From central Java eastward the winds comes from Australia and they have little moisture and consequently this region is dry from June to October.

The wet season on Java is from September to March and the dry season is from March to September. The further south you go the later the monsoons begin and end—Sumatra (September to March), Java (October to April), Bali and Nusa Tengarra (November to May). Rains tend to fall in short afternoon downpours during the rainy season, when the countryside is lush and green and beautiful but the jungles are full of leeches and dirt roads in remote areas become impassable. In the dry season, road travel is easier but the countryside is often brown and dusty. Rainfall patterns can vary from island to island and within islands and are affected the mountains and prevailing winds, with the windward sides of mountains getting the most rain. Indonesia is not hit very often by typhoons because it lies south of the typhoon belt. Timor located south enough to be within the cyclone belt. But even so cyclones are rare there.

Rainfall and Wind Patterns in Indonesia

Extreme variations in rainfall are linked with the monsoons. The dry season is influenced by the Australian continental air masses, and the rainy season 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. [Source: Library of Congress]

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.

Climate in Sumatra, Sulawesi and the Moluccas

The rainy season and dry seasons are not very distinct in northern Sumatra. The wet season runs from September to December. The hot, dry season is from May to August. In Medan the wettest months are October, November and December. The driest months are February, March and April but there isn’t that much difference between the wet months and the dry months. In southern Sumatra the rains start in November and reach their peak in January and February. The west coast of Sumatra is very wet, some places get above 400 centimeters of rain a year. Straddling the equator, Sumatra is very hot throughout the year. Fortunately the places visited by tourist tend to be in the highlands, where the climate is noticeably cooler, or near the oceans, which is tempered by sea breezes.

The rainy season in most of Sulawesi is from November until April, with the highest rainfalls in December and January. In the southeast the wettest months are around April, May and early June. The Moluccas have the opposite rainy season patterns as most of Indonesia. The dry season lasts from October to March. The wet season lasts from late April to September in central and southern Maluku. The wet season lasts from December to March in northern Maluku.

Climate in in Bali and Eastward

The wet and dry season are progressively later as one moves eastward from Bali. In some places the difference between the two seasons is very pronounced, with floods in the wet season and droughts in the dry season. On Bali, the rainy season is between October and March; the dry season, between April and September. On Lombok, the rainy season is between November and March; the dry season between May and August. The greatest rainfall is on the western side of the island.

On Komodo, the dry season lasts from May through November and wet season lasts from December to April. The rains are generally heaviest in January and February.The wet season in most of Flores is from December to April, with the dry season from May yo November. Rainfall rarely exceeds 200 centimeters. On eastern part of Flores the rainy season lasts from November to May with the heaviest rains falling in January and February. The area gets about 125 centimeters of rain a year. The rainy season in Alor lasts from October to April.

The climate of Timor is influenced by the western monsoon, which brings rain, and the easterly monsoon which brings dry weather. The mountains create a rain shadow effect. The rainy season is from January to April. The dry season is May to December. The average temperature on Timor is 80 degrees F (25 degrees C). However there is great variation since in coastal regions temperatures get as high as 95 degrees F (33 degrees C) while in the high mountains it may remain relatively cool, about 70 degrees F (23 degrees C). Indeed during the months of June and July the temperature in the mountains can plummet to around 36 degrees F (5 degrees C) during the night time. During these months the mountains see steady rain fall and the air tends to be damp and humid. [Source: Andrea K. Molnar, Northern Illinois University, Department of Anthropology and Center for Southeast Asian Studies, May 2005 ,]

Climate in Kalimantan and Papua

Kalimantan is hot, damp and rainy throughout the year. Much of this region does not have a pronounced rainy or dry seasons. In Balikpapan rain fall amounts are high throughout the year. In other places the wettest months are October to March and the driest period is from July to September although in many cases the difference between the wet montsh and dry months are not that different.

Much of Papua is out of range of the monsoon cycles. The wet and dry patterns are irregular, with many places getting over 200 centimeters of rain a year. The driest months tend be between May and October although heavy rains can fall in that time and often do. Along the northern coast it rainiest and often very windy from November to March. Along the southern coast it rainiest and often very windy from April to October.

The highlands are temperate and reasonably comfortable with mean temperatures averaging between 26°C and 15°C. The best times to visit the Baliem Valley is after March when the weather is a little drier. The lowlands are considerable hotter than the highlands. Southern Papua is located at the edge of the monsoon belt. The hottest month is December, the coolest is June. Rainfall regularly exceed 450 centimeters a year. Some places in the highlands get more than 600 centimeters a year.

The highlands of Papua do get some cold spells. In February 2006, Earth News reported: “Remote villages in Indonesia's easternmost province of Papua have been enveloped by a freak cold waves in recent weeks, resulting in cold-related ailments that have killed nearly 100 people. Temperatures dipped to as low as 41 degrees Fahrenheit in mountain communities where readings are typically above 68 degrees. Health officials say the stress caused by the cold has created outbreaks of acute pneumonia, tuberculosis, dysentery and diarrhea. Emergency cold-weather shelters were being rushed to the area, along with medical supplies. [Source: Earth News, February 22, 2006]

El Niño

Indonesia periodically experiences droughts, sometimes accompanied by devastating forest fires, caused by El Niño. The El Niño of 1997-98 produced a severe drought that created severe crop damage and food shortages in Papua New Guinea and parts of Indonesia. Worldwide hundreds of people died from famine, cholera and lack of clean drinking water during the 1997-98 El Niño.

El Niño is a periodic climate condition that occurs an average of every five years. It is strongest in the Pacific but has global ramifications. Caused when a dominate high pressure system over the Pacific collapses, it causes wind directions and ocean currents in the Pacific to change direction, throwing off prevailing winds and bringing drought to Indonesia, Southeast Asia, southern Africa and Australia, heavy rains and floods to Peru and east Africa, typhoons to Japan, stormy weather to the United States and disruptions to monsoons in India. [Source: Curt Suplee, National Geographic, March 1999

The name El Niño (Spanish for "the Christ Child") was coined by Peruvian fishermen in the port of Callao north of Lima in early 1970s because the warm air and water associated with change usually first appeared around Christmas. In the 20th century there were 23 El Niños. During the 50 years period between 1950 and 2000, El Nino condition existed 31 percent of the time.

Scientist have said that El Niños have been getting progressively worse. More powerful and far-reaching than originally suggested, the El Niño 0f 1982-83 was the strongest of century. It was still creating havoc in 1990s. After that El Niños have been more frequent. The 1997-98 El Niño was the worst on record.

Image Sources:

Text Sources: New York Times, Washington Post, Los Angeles Times, Times of London, Lonely Planet Guides, Library of Congress, Compton’s Encyclopedia, The Guardian, National Geographic, Smithsonian magazine, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Wall Street Journal, The Atlantic Monthly, The Economist, Global Viewpoint (Christian Science Monitor), Foreign Policy, Wikipedia, BBC, CNN, and various books, websites and other publications.

Last updated June 2015

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