Map from Google - Indonesia

Indonesia is a vast archipelago made up of thousands of islands, covering a vast area around the equator between Southeast Asia and Australia: it covers more than 5,000 kilometres from west to east, similar to the distance between the west and east coast of Canada. Some consider the Western New Guinea as part of Oceania, and in this case Indonesia would be a transcontinental country.
The climate of Indonesia is equatorial almost everywhere, ie hot, humid and rainy throughout the year. In some areas there is a dry season, more or less marked, therefore that will be the best for a visit. As usually happens in hot countries, the rains occur in the form of downpour or thunderstorm, which that sometimes can cause flooding.
There are also mountains and volcanoes, often very high, in which the temperature naturally decreases with altitude.
Being located near the equator, in Indonesia the day lasts 12 hours throughout the year, and the sun sets quite soon. However, the equatorial sun is very strong, especially in the mountains.
The temperature is stable throughout the year, with lows around 22/25 °C and highs around 30/32 °C all year round, so the main difference is found in precipitation, which is due to the location in a hot and humid area, but also to the regime of the monsoon, and the impact that it has on the different areas. In fact, there is the northeast monsoon 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 slope exposure. To this wide variety of situations, which is not always easy to follow, should be added that the rains do not always follow the same pattern from year to year, as we will see when dealing with El Niño.

1- Equatorial climate

Indonedia, areas with an equatorial climate
The climate is equatorial, with abundant rainfall throughout the year, in the following areas: the coast and the western slopes of Sumatra, the southern slopes of western Java, almost all of Borneo (except the south-east) and the greater part of Irian Jaya (aka Western New Guinea).
Here, it is difficult to find a better time to visit it.
The southern coast of Sumatra is very rainy. Here, around 4,000 millimetres of rain per year fall, and it is difficult to find an acceptable period. The worst period, however, is from September to December, when more than 400 mm of rain per month fall. The only city of some importance in this area is the very rainy Padang, which is relatively less rainy from June to August, although the decrease in precipitation does not occur every year.
The island of Borneo (in the Indonesian provinces called Kalimantan) is mostly very wet and covered with an impenetrable rainforest. In the wettest regions, the inland area and the north-east, it is difficult to find a month in the rainfall doesn't go above 250 mm. However, in general here the period (relatively) less rainy runs from June to August.
Here are the average temperatures of Pontianak, in the west coast of Borneo.
Average temperatures - Pontianak
Min (°C)232323232323232223232322
Max (°C)323333333333333333333232

Here is the average precipitation.
Average precipitation - Pontianak
Prec. (mm)2602152552902552102001802953304003003200

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

2- Sub-equatorial climate

Indonesia, areas with a sub-equatorial climate
In this area (comprising inland and northern Sumatra, south-eastern Borneo, the northernmost tip of Sulawesi, the north-central Maluku islands and the northern and western 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 per month. This less 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 of rain per year fall. Here the wettest period is from September to January, when more than 150 mm per month fall, then there is a slight recovery of the rains in April and May. So, here we have two less rainy periods, which are therefore those to be preferred: from June to August, when less than 100 mm per month fall, and from February to March, when precipitation is around 115/120 mm per month. The worst month is December, with as much as 320 mm. The amount of sunshine is never great, 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.
A little more to the south, but always in the north coast of Sumatra, in Medan, still around 2,000 mm per year fall, but the rainfall is more evenly distributed throughout the year. 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.
Average precipitation - Medan
Prec. (mm)140901051301751301351802102602452302030

In Medan the weather is hot and muggy throughout the year, and the breeze is less intense than in Banda Aceh.
Average temperatures - Medan
Min (°C)222323242324242322232323
Max (°C)323233333333333332323131

Borneo is rainy throughout the year, but in the south-east, it enjoys a relative decline in rainfall between June and October, when the rains, however, hover between 100 and 150 mm per month.
In the northern slope of Sulawesi (see Manado), the rainfall has a relative decrease from July to September, when it amounts to about 110/140 mm per month.
In the Central Maluku, islands like Buru, Ambon and Seram have reversed rainy seasons, so they are very rainy from May to September, especially in June and July, when they can receive more than 400 mm of rain per month, while the least rainy period runs from October to February, when however, precipitation still exceeds 100 mm per month. This pattern of rainfall is also recorded at the western tip of Papua, around Sorong.
Here is the average precipitation in Ambon.
Average precipitation - Ambon
Prec. (mm)1551351651752854454002452451101051502615

And here is the average temperature.
Average temperatures - Ambon
Min (°C)222223232322222222222324
Max (°C)323232323029292930313333

3- Tropical climate

Indonesia, areas with a tropical climate
In this area of Indonesia, the climate can be defined tropical, because we can find a period with moderate or even poor rains. So, here it is possible to speak of a "best time", at least for the rains, while the heat can be intense anyway.
Jakarta, the capital of Indonesia, is located on the north side of Java, which is sheltered from the south-west monsoon. Jakarta is hot and humid, with stable temperatures all year round. The heat is intense in the city, which is big and therefore has a noticeable urban heat island effect, while the breezes bring a bit of relief, especially in neighbourhoods close to the coast. The rainfall amounts to 1,800 mm per year; the rains are abundant from December to March, with a maximum of 300 mm in January and February. The driest period, therefore the best one, is from June to September.
Here is the average precipitation in Jakarta.
Average precipitation - Jakarta
Prec. (mm)300300210145115956545651101402051795

The temperatures are lower from November to March, when highs are around 29/30 °C, but at this time the humidity is higher; from April to October they are around 31.5/32.5 °C. The temperature rarely drops below 20 °C at night, while sometimes it can touch 35/36 °C during the day.
Average temperatures - Jakarta
Min (°C)242425252525252526262524
Max (°C)303032333331323233333130

Even in Bandung, there is a clearly defined dry season. Moreover, being located at 700 metres above sea level, this city has also a milder climate, with maximum temperatures around 27/29 °C, and minima around 17/19 °C. Here the best period is from July to September, and the driest month is August with an average of 50 mm of rain. Here are the average temperatures.
Average temperatures - Bandung
Min (°C)191919191817171717181819
Max (°C)272627272727272828282727

Semarang is located on the northern coast of Java, east of Jakarta. Here the wettest month is January with even 430 mm, 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 less rainy than the western one. In Surabaya, a hot and sunny city, 1,500 mm of rain per year fall, with a maximum of more than 200 mm per month from December to March; on the other hand, little rainfall occurs from July to September, and around 50/60 mm in June and October. Considering the heat that is intense especially in September and October, here the best period is from June to August.
We find a similar situation in the island of Bali, located east of Java, of which we have dealt with separately because of its tourist importance. Here, 1,700 mm of rain per year fall, the heat is less intense because of the breeze, and all in all the best period is from June to September.
East of Bali, the rainfall decreases further, to around 1,300/1,400 mm per year along the coasts (while it remains higher in inland areas, especially where there are highlands, as happens for example in Ruteng, in the island of Flores, where it rains a lot from October to April), but in the southernmost islands of the Sunda Islands (or Nusa Tenggara), like in Sumba, precipitation is even lower than 1,000 mm 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. Even some small islands east of Timor, like Kisar, are semi-arid. Here the least rainy period, which is very pronounced, goes from May to November, even though between September and November it's definitely hot.
On the island of Timor, from 1,200 to 1,500 mm 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, the southern Maluku (Tanimbar, Kai, Aru) become rainier, surpassing 2,000 mm per year, but they still have a dry season, though a little shorter, from August to October.
In the southern part of the island of Sulawesi (Celebes), in Makassar (Ujung Pandang) it rains a lot from December to March, more than 400 mm per month, and even 700 mm in January, but rainfall drops below 100 mm per month from June to September; further east, in Kendari, it drops below 100 mm per month from August to November.
In north-central Sulawesi, the situation with regard to rainfall changes. In Luwuk, unlike what happens elsewhere in Indonesia, there is no rain-soaked month: the rainiest is March, with 155 mm, while the rainfall drops below 70 mm in September and October; in Gorontalo, the least rainy months are August and September, the rainiest is May, with about 150 mm. We are in a fairly sheltered area, which also includes the Gulf of Tomini and Walea in the Togian islands, where there is an interesting coral reef.
In the southernmost part of New Guinea (see Merauke), there is a clearly defined period of little rain from June to November.

4- Mountains

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 metres high, and despite being near the equator, it's home to some snow fields around the top, however, in retreat because of global warming.
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 thunderstorm in the world, even 322 per year! The rains are more abundant along windward sides, therefore 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 a cold weather is only found at high altitudes. In Wamena, New Guinea, at 1,650 metres above sea level, the average daily temperature is around 19/20 °C all year round. At 2,500 metres, the daily average is around 15 °C, so at night it gets cold.

Cyclones and El Niño

The vast majority of Indonesia, being at the equator, is not in the path of tropical cyclones. However, some areas, in particular the southernmost areas (Java, Bali, Sunda Islands, Timor, southern New Guinea), can be affected by the Southern Hemisphere cyclones, between November and May (but more likely between January and March). 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 high waves.

In addition, the climate in Indonesia is influenced by the phenomenon known as El Niño, which brings drought and an increase in temperature both of air and sea, an event which may cause coral bleaching. The drought caused by El Niño is pronounced between June and August, and even more between September and November; due to this phenomenon, the fires that are usually set 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, while temperatures may become cooler than usual between June and August.


For swimming, the sea in Indonesia is warm enough all year round, with little variation in temperature between the months. In some sea areas, like in the straits of Sape and Lombok, there can be strong currents, while in others there can be sharks.
Sea temperature - Gorontalo
Sea (°C)292829293029292929292929

When to go

Generally, the best time to visit Indonesia is from June to September, but as we have seen, there are areas where the situation is different, especially the central Maluku, which in this period receive the maximum 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, choosing of course the least rainy period.

What to pack

All year round: tropics-friendly, loose fitting clothing, a scarf for the breeze, a light sweatshirt for air-conditioned places. Light raincoat or umbrella in equatorial climate areas all year round, as well as in tropical climate areas during the rainy season.
In Bandung, around 1,000 metres, you can add a sweatshirt and a light jacket; for high mountains, warm clothes, warm jacket, raincoat, hiking shoes.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.

See also the climate of Bali.