Index

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• Introduction
• Equatorial climate - Pontianak
• Sub-equatorial climate - Medan, Ambon
• Tropical climate - Jakarta, Bali and eastern islands
• Sea temperature
• Mountains
• Cyclones
• El Niño
• Best Time
• What to pack

Introduction

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Indonesia is a vast archipelago made up of thousands of islands, stretching along the Equator from Southeast Asia to Australia: it covers more than 5,000 kilometers (3,000 miles) from west to east, a distance similar to the one found between the west and east coasts of Canada. Some consider Western New Guinea as part of Oceania, and in this case, Indonesia would be a transcontinental country.
The climate of Indonesia is almost everywhere equatorial, ie hot, humid and rainy throughout the year. In some areas, there is a dry season, more or less marked, which is therefore the best period for a trip. As usually happens in hot countries, the rains occur in the form of downpours or thunderstorms, which sometimes can cause flooding.
There are also mountains and volcanoes, often very high, where the temperature naturally decreases with altitude.
Being that Indonesia is located near the Equator, the day lasts 12 hours throughout the year, and the sun sets quite soon. However, the sun's rays are very strong, especially in the mountains.
The temperature is stable, with lows around 22/25 °C (72/77 °F) and highs around 30/32 °C (86/90 °F) all year round. The main difference is found in the rainfall, whose quantity and distribution are due to the location in a hot and humid area, but also to the monsoon regime, and the impact that it has on the different areas. The northeast monsoon takes place from December to March, and the southwest monsoon from June to September. These winds have a different impact depending on the presence of mountains and on slope exposure. However, the rains do not always follow the same pattern from year to year, as we will see when dealing with El Niño.

Equatorial climate

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The climate is equatorial, with abundant rainfall throughout the year, in the following areas: the western coast and the western side of Sumatra, the southern side of western Java, almost all of Borneo (except the south-east) and much of Western New Guinea (formerly Irian Jaya).
Here, it is difficult to find a best time to visit.
The southern coast of Sumatra is very rainy. Here, around 4,000 millimeters (155 inches) of rain fall per year, and it is difficult to find an acceptable period. The worst period, however, is from October to December, when rainfall amounts to more than 400 mm (16 in) per month. The only city of some importance in this area is Padang, which is relatively less rainy from June to August, although the decrease in precipitation does not occur every year.
Off the south-west coast of Sumatra, we find the Mentawai Islands.
The island of Borneo (in the Indonesian part called Kalimantan) is mostly very wet and covered with an impenetrable rainforest. In the wettest regions, i.e. the inland area and the north-east, it is difficult to find a month when the rainfall is below 250 mm (10 in). However, the (relatively) least rainy period generally runs from June to August.

Pontianak

Here are the average temperatures of Pontianak, on the west coast of Borneo.

And here is the average precipitation.

Since it often rains all year round, the amount of sunshine in Pontianak is not very good.

The northern part of the Moluccas (Maluku Islands) and the center-north of Western New Guinea have also an equatorial climate. Here, it usually rains relatively less from July to October.

Sub-equatorial climate

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In this area (comprising the interior and the northern side of Sumatra, south-eastern Borneo, the northernmost tip of Sulawesi, the north-central Maluku islands and the westernmost part of New Guinea), the climate can be considered sub-equatorial, since there is no real dry season, but there's a time of the year when the rainfall becomes less abundant, that is, around 100 mm (4 in) per month. This least rainy period is therefore the one to be preferred, however, it's not the same everywhere.
In the northern part of the island of Sumatra, in Banda Aceh, almost 2,000 mm (80 in) of rain fall per year. Here the wettest period is from September to January, with more than 150 mm (6 in) per month, then there is a secondary peak in the rains in April and May. So, here we have two relatively dry periods, from June to August, when precipitation drops below 100 mm (4 in) per month, and from February to March, when it is around 115/120 mm (4.5/4.7 in) per month. The worst month is December, with as much as 320 mm (12.6 in). The amount of sunshine is not great all year round, however, it is slightly better from February to July, and worse in November and December. The weather is sultry throughout the year, but the breezes temper a bit the heat, especially from June to August.

Medan

A little more to the south, but still on the north coast of Sumatra, in Medan too, around 2,000 mm (80 in) of rain fall per year, but precipitation is more evenly distributed over the seasons. Here the best months are February and March, but also June and July, while the rainiest period begins already in August, and lasts until December.
Here is the average precipitation in Medan.

In Medan, the weather is hot and muggy throughout the year, and the breeze is less intense than in Banda Aceh.

In Medan, the least sunny period is the wettest one, from October to December. In the rest of the year, the sun shines for a decent (but not exceptional) number of hours.

Borneo is rainy throughout the year, but in the south-east, there's a relative decline between June and October, when the rains, however, still hover between 100 and 150 mm (4 and 6 in) per month.
On the northern side of Sulawesi (see Manado), the rainfall has a relative decrease from July to September, when it amounts to about 110/140 mm (4.3/5.5 in) per month.
The Central Malukus, on islands such as Buru, Ambon and Seram, experience a reversed rainy season, so they are very rainy from May to September, especially in June and July, when they can receive more than 400 mm (16 in) of rain per month, while the least rainy period runs from October to February, when, however, precipitation still exceeds 100 mm (4 in) per month. This rainfall pattern is also observed at the western tip of Papua, around Sorong.

Ambon

Here is the average precipitation in Ambon, an island of the Maluku Islands.

And here is the average temperature.

In Ambon, the sky is often cloudy in the wettest period, especially from June to August.

Tropical climate

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In this area of Indonesia, the climate can be defined as tropical because there is a period with moderate or even poor rains. So, here it is possible to find a best period, at least as regards to the rains, while the heat can be intense anyway.

Jakarta

Jakarta, the capital of Indonesia, is located on the north side of Java, which is sheltered from the south-west monsoon. Here, the weather is hot and humid, with stable temperatures all year round. Owing to the so-called urban heat island effect, the heat is more intense within the big city, while sea breezes bring a bit of relief especially in the neighbourhoods close to the coast. Rainfall amounts to 1,800 mm (70 in) per year; the rains are abundant from December to March, with a maximum of 300 mm (12 in) in January and February. The driest period, which is therefore the best one, is from June to September.
Here is the average precipitation in Jakarta.

Temperatures in Jakarta are lower from November to March, when highs are around 29/30 °C (84/86 °F), but at this time, the humidity is higher; from April to October, they are around 31/33 °C (88/91 °F). The temperature rarely drops below 20 °C (68 °F) at night, while sometimes it can reach 35/36 °C (95/97 °F) during the day.

In Jakarta, the sun often shines in the long period when it does not rain a lot, from April to October.



In Bandung too, there is an evident dry season. Moreover, being located at 700 meters (2,300 feet) above sea level, this city has also a milder climate, with highs around 27/29 °C (81/84 °F) and lows around 17/19 °C (63/66 °F). Here the best period is from July to September, and the driest month is August, with an average of 50 mm (2 in) of rain. Here are the average temperatures.

Semarang is located on the northern coast of Java, east of Jakarta. Here the wettest month is January, with up to 430 mm (17 in) of rain, while the best period is from June to August; September has still little rain, but it is hotter. In fact, in this as in other parts of Indonesia, there is a slight increase in temperature in September and October.
The eastern part of Java is not as rainy as the western one. In Surabaya, a hot and sunny city, 1,500 mm (60 in) of rain fall per year, with a maximum of more than 200 mm (8 in) per month from December to March; on the other hand, little rainfall occurs from July to September, and around 50/60 mm (2/2.4 in) in June and October. Considering the heat that is intense especially in September and October, the best period here is from June to August.

Bali

We find a similar situation on the island of Bali, located east of Java, of which we have spoken separately because of its tourist importance. Here, 1,700 mm (67 in) of rain fall per year, the heat is less intense than in big cities because of the breeze, and all in all, the best period is from June to September.
East of Bali (see Lombok, Sumbawa, Komodo, Flores, Sumba), the dry season is longer, and the rainfall decreases further, to around 1,300/1,400 mm (51/55 in) per year along the coasts (while it remains higher in inland areas, especially where there are highlands, as happens for example in Ruteng, on the island of Flores, where it rains a lot from October to April). On the southernmost islands of the Lesser Sunda Archipelago (or Nusa Tenggara), such as in Sumba, precipitation is even scarcer and drops below 1,000 mm (40 in) per year. The island of Sumba is therefore almost dry, especially along the north and east coasts, where the total annual rainfall drops to 800 mm (31.5 in). Some small islands east of Timor, such as Kisar, are semi-arid as well. Here, the driest period, which is very pronounced, is from May to November, even though it's definitely hot between September and November.
On the island of Timor, from 1,200 to 1,500 mm (47 to 60 in) of rain fall along the coast; in August and September, the rains are poor, but in general, they are acceptable from May to October. Some areas of the island are semi-arid.
East of Timor, on the southern Malukus (Banda, Tanimbar, Kai, Aru), the rainfall is more abundant, and it surpasses 2,000 mm (80 in) per year, but there is still a dry season, though a little shorter, from August to October. For example, in Tual, on the Kai Islands, 2,600 mm (102 in) of rain fall per year, with heavy rains from December to May, 110 mm (4.3 in) in July, and less than 100 mm (4 in) per month from August to October.
In the southwestern part of the island of Sulawesi (Celebes), in Makassar (Ujung Pandang), it rains a lot, more than 400 mm (16 in) per month, from December to March, with a peak of as high as 700 mm (27.5 in) in January, but rainfall drops below 100 mm (4 in) per month from June to September.
In other areas of Sulawesi, precipitation drops below 2,000 mm (80 in) per year. In the center-north, in Luwuk, there is no month with very abundant rainfall: the rainiest month is March, with 155 mm (6.1 in), while the rainfall drops below 70 mm (2.8 in) in September and October. In Gorontalo, the least rainy months are August and September, while the rainiest is May, with about 170 mm (6.7 in). We are in a fairly sheltered area, which also includes the Gulf of Tomini and the Togian Islands, where there is a coral reef which that invites you to go diving.
In south-eastern Sulawesi, precipitation drops below 2,000 mm (80 in) per year as well; in Kendari, it drops below 100 mm (4 in) per month from August to November. In the Wakatobi archipelago, an acronym for the islands of Wangi-wangi, Kaledupa, Tomia, and Binongko, rainfall is quite rare from July to October.
In the southernmost part of New Guinea (see Merauke), there is a clearly defined period of little rain from June to November.

Sea temperature

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The sea in Indonesia is warm enough to swim in all year round, with little temperature variations between the months. In some sea areas, such as the straits of Sape and Lombok, there can be strong currents, while in others, it's useful to mention the presence of sharks.
The following table refers to the sea temperature in Gorontalo, but provides a good indication for the whole country.

Mountains

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In Indonesia, there are several mountain ranges. The island of Java is dotted with volcanoes; the highest peak, however, is in Western New Guinea: Puncak Jaya, belonging to the Maoke Mountains, is 5,000 meters (16,400 ft) high, and despite its position near the Equator, it is home to some snow fields around the top which, however, are in retreat because of global warming.
In the areas near the mountains, there's a higher risk of showers and thunderstorms: Bogor, near Jakarta, is the city with the highest number of days with thunderstorms in the world, as many as 322 per year! The rains are more abundant along the windward sides, that is, along the northern slopes during the northeast monsoon (December to March), and the southern slopes during the southwest monsoon (June to September).
The temperature naturally decreases with altitude, but cold weather is only found at high altitudes. In Wamena, New Guinea, 1,650 meters (5,400 feet) above sea level, the average daily temperature is around 19/20 °C (66/68 °F) all year round. At 2,500 meters (8,200 ft), the daily average is around 15 °C (59 °F), and at night it gets cold.

Cyclones

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Almost all of Indonesia, being near the Equator, is not in the path of tropical cyclones. However, the southernmost areas, and more rarely the northernmost ones, can be affected by them.
The southern islands (Java, Bali, Sunda Islands, Timor, southern New Guinea), can be affected by the Southern Hemisphere cyclones, between November and May (but more likely, between December and mid-April). Cyclones, when they don't directly hit a particular area with their load of rain and wind, can still cause damage to tourist facilities because of abnormal waves.
On the other hand, north of the Equator, the island of Sumatra, especially in the northernmost part, can be affected by cyclones when they are in the early stages of their formation in the Andaman Sea or in the Bay of Bengal, or it can be affected in a more marginal way by the typhoons that come from the Pacific and pass over the Gulf of Siam. In this case, cyclones are formed from April to December, with two peaks at the beginning and at the end of the period (so, in April-June and October-December). However, given the warm sea, they can, in theory, form all year round.

El Niño

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In addition, the climate in Indonesia is influenced by the phenomenon known as El Niño, which brings drought, as well as an increase in air and sea temperature, an event which may cause coral bleaching. The drought caused by El Niño is pronounced between June and August, and even more so between September and November; owing to this phenomenon, the fires that are usually lighted during the dry season can become uncontrollable, and cause the spread of smoke in large areas.
The opposite phenomenon, La Niña, brings more rains than usual, as well as temperatures cooler than usual between June and August.

Best Time

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Generally, the best time to visit Indonesia runs from June to September, but as previously mentioned, there are areas where the situation is different, especially the central Malukus, which in this period receive the highest amount of rainfall of the year, while they are less rainy from November to February.
For a beach holiday, the most suitable areas are those with a tropical climate, of course if you choose the least rainy period.

What to pack

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All year round: bring tropics-friendly, loose-fitting clothing, a scarf for the breeze, and a light sweatshirt for air-conditioning. You can add a light raincoat or an umbrella, in areas with an equatorial climate all year round, and in areas with a tropical climate during the rainy season.
In Bandung, around 1,000 meters (3,300 feet), you can add a sweatshirt and a light jacket; in high mountains, bring warm clothes, a warm jacket, a raincoat, and hiking shoes.
When going to the reef, you can bring snorkeling equipment, including water shoes or rubber-soled shoes.