Indonesia's haze season, explained
Published 19 October 2026
Most years, smoke from fires in Sumatra and Kalimantan reaches cities hundreds of kilometres away. When it happens, why, and how to read it on Kabut.
When the haze comes
Indonesia has two seasons that matter here: a wet season, roughly November to March in Sumatra and Kalimantan, and a dry season in the middle of the year. The haze belongs to the dry season. Fires usually start to show up around July, rise through August and September, and fade once the rains return, often in late October or November.
The timing changes a lot from year to year. In a wet year there may be no haze worth the name. In a dry year it can last for weeks and cover much of western Indonesia. The difference usually comes down to rain, and rain in this part of the world follows the Pacific.
Why some years are much worse
In an El Niño year the rains over Indonesia arrive late and are lighter than usual. The dry season runs long, the ground dries out further, and fires that would have been put out by the first storms keep burning. The worst recent haze years line up with El Niño: 2015, when smoke covered large parts of Sumatra, Kalimantan and the neighbouring countries for weeks; 2019; and 2023. A positive Indian Ocean Dipole, which also dries the region out, made 2019 worse.
So when forecasters say an El Niño is coming, people in Riau, Jambi, South Sumatra and Central and West Kalimantan take it as a warning about the air.
What is burning
Most haze fires are set by people, not lightning. Fire is the cheapest way to clear land for planting, and burning on degraded land and plantation concessions is common in the dry season, even though it is against the law.
What makes Indonesian haze so dense is peat. Large areas of lowland Sumatra and Kalimantan sit on peat: waterlogged layers of plant matter that built up over thousands of years. Kept wet, peat does not burn. Drained by canals for agriculture, it dries out and becomes fuel. A peat fire burns downward into the ground as well as across it, and it smoulders rather than flames, so it gives off far more smoke than a forest fire of the same size. It is also very hard to put out. The peat fires guide covers that in more detail.
Where the smoke goes
Smoke does not stay over the fire. During the dry season the prevailing winds over western Indonesia blow from the south and southeast, so smoke from Sumatra tends to drift north and east, toward Peninsular Malaysia and Singapore, and smoke from Kalimantan toward Sarawak and Brunei. The same winds carry it over Indonesian cities on the way: Pekanbaru, Jambi, Palembang, Pontianak, Palangka Raya and Banjarmasin are the ones that usually see the worst of it.
That is why haze is called transboundary. A fire in one province can be the main source of pollution in a city in another province, or in another country. The ASEAN Agreement on Transboundary Haze Pollution exists because of it, and Singapore and Malaysia publish their own haze readings during bad years.
It also means the air in a city can turn bad quickly when the wind shifts, even if no fire is close. Night and early morning are often the worst hours: the air near the ground is still, and smoke collects in it.
Reading the haze on Kabut
Kabut puts two kinds of evidence side by side: what is in the air, and where the fires are.
The air. Each city page shows the current US AQI and the PM2.5 concentration, the fine particles that make up most of smoke. In a haze event PM2.5 is the number to watch; the PM2.5 guide explains why. Some cities have a monitoring station and some are estimated by an atmospheric model. During dense haze a model reading can sit below what a station would measure, so the page always says which one you are looking at.
The fires. The map and the home screen show fire hotspots from NASA's FIRMS system, detected by the VIIRS instrument on the Suomi NPP satellite. A hotspot is a spot the satellite saw as unusually hot when it passed over. Three things to know about the count:
- It covers the last 24 hours, not now. The satellite sees a given place only a few times a day, and the data reaches Kabut about three hours after each pass. A fire that started this morning may not be on the map yet.
- It undercounts. Cloud hides fires from the satellite, and a low, smouldering peat fire can be too cool to register. A small count does not prove the air is clean.
- It is counted once. Kabut collects detections in overlapping areas so no land is missed, then removes duplicates, so the total on the home screen and the one on the map are the same number.
A city page also shows the distance to the nearest detection and the direction the wind is coming from. A fire 30 km away and upwind matters more than one 10 km away and downwind.
What to do during a haze event
Follow the advice for the reading's class on your city's page. It is the same everywhere in Kabut, in English and Indonesian. In short:
- From Unhealthy upward, limit time outdoors, avoid hard exercise, and wear an N95 outside. A cloth mask does not stop PM2.5; see the masks guide.
- Keep windows shut during the worst hours, and use an air purifier if you have one. The indoor air guide covers how to make one room cleaner.
- Children, older adults, and people with asthma or heart conditions should ease up earlier, from the Sensitive groups class.
- Seek care for breathing difficulty, chest pain or dizziness. Don't wait for the air to clear.
Kabut is not an official warning service. For official statements, follow BMKG, the Ministry of Environment and your local government. The methodology page explains how every number on the site is made.