Indonesia Geography & Climate

Indonesia is the largest archipelagic state in the world, comprising between 13,000 and 17,000 islands — Sumatra, Java, Sulawesi and parts of Borneo and New Guinea among them — spread over 1.9 million square kilometres of land and far more sea, between the Indian and Pacific oceans. It straddles the equator and spans three time zones.

The big islands

Java is small relative to its neighbours and holds more than half the population, making it one of the most densely settled places on earth. Sumatra, Borneo — shared with Malaysia and Brunei as Kalimantan — Sulawesi with its four peninsulas, and Western New Guinea make up most of the rest of the land area.

The Ring of Fire

Indonesia has around 130 active volcanoes, more than any other country, strung along the collision between the Indo-Australian and Eurasian plates. Krakatoa erupted in 1883 with a sound heard four thousand kilometres away; Tambora in 1815 produced the year without a summer in Europe. Earthquakes are frequent and the 2004 Indian Ocean tsunami began off Sumatra.

The Wallace Line

A biological boundary running between Bali and Lombok, and between Borneo and Sulawesi, marks where Asian fauna stops and Australasian begins — tigers and elephants to the west, marsupials and cockatoos to the east, across a strait you can see over.

Rainforest and reef

The country holds the third-largest area of tropical rainforest in the world, and sits at the centre of the Coral Triangle, which has the greatest marine biodiversity on the planet. Both are under heavy pressure from logging, palm oil and fishing.

Indonesia straddles the equator, so temperature is close to constant — around 26 to 32°C at sea level in every month — and the seasons are defined entirely by rainfall and by the monsoon that drives it.

The two monsoons

The dry season, roughly May to September, comes with the south-east monsoon blowing off Australia: less rain, lower humidity, calmer seas. The wet season usually runs October to March, bringing the north-west monsoon and heavy convective downpours, usually in the afternoon and usually over within an hour or two rather than lasting all day.

Regional exceptions

The pattern is different in eastern Indonesia, where the wet season usually runs April to August — so Maluku and northern Sulawesi are wettest when Java and Bali are dry. Sumatra’s west coast is wet almost year round, and Raja Ampat’s calmest months are October to April, the opposite of Komodo’s.

Altitude

The only escape from the humidity is upward. The hill towns of Java — Bandung, Malang, the Dieng plateau — and the Balinese interior around Bedugul are several degrees cooler, and the volcano summits are genuinely cold before dawn, which catches out people who climbed Bromo or Ijen in a T-shirt.

Haze

In the dry season, fires set to clear land produce smoke haze that drifts across the region, particularly over the Riau Islands, central Sumatra and Kalimantan. It can disrupt local and regional air travel and push air quality to levels that matter for health, and because much of it burns in peat-rich soil some fires run all year. It varies enormously from year to year and is worst in El Niño years.

Sea and storms

Indonesia lies close enough to the equator that tropical cyclones almost never form over it, which is unusual for the region. Rough seas in the wet season are the practical hazard for island crossings.