For most of Sumatra and Kalimantan, not before the rainy season starts, and BMKG says it will start later than normal across 61.08% of Indonesia this year (BMKG, 2026). One shower will not end it either. Smouldering peat needed rain of about 4 mm an hour to go out in lab tests (Lin et al., 2020).
Part of our Haze and air quality by city guide.
The morning after the first real downpour, the road is still wet and the sky is pale blue. You open the visor. By the afternoon ride home, the smell of smoke is back.
That is the question riders keep typing: when will the haze be gone? No agency publishes an end date.
What they do publish is the rain forecast, and the physics of peat fires tells you how to read it. This post answers the seven questions people ask most, with the studies behind each answer.
Key takeaways
- BMKG expects the 2026/2027 rainy season to start later than normal in 529 of Indonesia's seasonal zones, 61.08% of its land, with most of Sumatra and Kalimantan peaking in December (BMKG, 2026).
- In lab tests, smouldering peat only went out under rain of about 4 mm an hour or more, so persistent light rain could not put it out (Lin et al., 2020).
- Peat groundwater in Central Kalimantan fell by 1.0 to 1.5 m in 1997/98, 2015 and 2019, which is why fires keep burning below a wet surface (Sulaiman et al., 2023).
- El Niño raised the modelled risk of a 1997 or 2015 scale fire year in Indonesian Borneo by a factor of 2.7, but that risk existed in every ENSO phase (Lam et al., 2026).
Has it been this bad before? Yes, in 1997 and 2015
By airport visibility records in Sumatra and Kalimantan, 2015 ranked after 1997, and alongside 1991 and 1994, among the worst smoke years on record. That year's smoke plume stayed in the upper troposphere for 2 months (Field et al., 2016).
had persistent exposure to unhealthy air during the 2015 fires, with an estimated 11,880 excess deaths from short-term exposure.
Source: Crippa et al., Scientific Reports, 20162026 is already on that list's scale in places. The forestry ministry's fire monitor logged more than 9,000 hotspots in August, the highest August since its records began in 2015. On 15 September, five regions of Singapore recorded unhealthy air for the first time since September 2019 (Kompas, 2026).
The health toll is climbing too. Indonesia's health ministry counted 174,694 acute respiratory infection cases in haze-affected areas between August and 21 September. More than 40,000 were toddlers (Ministry of Health via The Jakarta Post, 2026).
Respiratory infection cases in Indonesia's haze provinces, 2026
Cumulative cases counted since August, as reported by the health ministry.
| Date | Cases |
|---|---|
| 1 September | 50,891 |
| 9 September | 113,336 |
| 21 September | 174,694 |
If you ride in Sumatra, our Palembang haze season guide shows how much worse the dawn air is than the afternoon.
When will the haze be gone this year? Not before the late rains
For most of Sumatra and Kalimantan, the smoke is likely to hang on until the rainy season is properly under way. This year that season is late.
On 22 September, BMKG said the 2026/2027 rainy season will start later than normal in 529 seasonal zones, covering 61.08% of Indonesia's land. Only 11 zones start early (BMKG, 2026).
BMKG found stations with more than 60 days without rain in 25 provinces. The longest run, 131 days, was at Bantul (BMKG, 2026).
The timing BMKG gives for the fire provinces:
- November onset for parts of Sumatra and Kalimantan. November starts cover the largest share of land, 24.66% (BMKG, 2026).
- December peak for most of Sumatra and most of Kalimantan (BMKG, 2026).
- January 2027 peak for southern Riau, parts of West Kalimantan and most of Central Kalimantan (BMKG, 2026).
BMKG also asked health services to prepare for prolonged unhealthy air because of the late start. It flagged September and October as the months to watch for fires (BMKG, 2026).
Singapore and Malaysia get the smoke that drifts. Their air clears when the source provinces get sustained rain or the wind turns, so the same late forecast applies to them.
Won't the first heavy rain clear it? Only if it falls hard enough
The haze ends the day it rains.
Rain washes smoke out of the air for a few hours. The peat underneath only goes out under rain of about 4 mm an hour, kept up long enough (Lin et al., 2020).
There are two different "4" numbers in the research, and they are easy to mix up.
- 4 mm a day is the dryness line. Below it, for weeks, fires run away (Field et al., 2016).
- 4 mm an hour is the rain rate that put smouldering peat out in the lab (Lin et al., 2020).
A drizzly day can deliver 4 mm in total and never reach 4 mm in any single hour. It looks like rain. To the peat it barely counts.
Lin's fitted formula shows how long the rain has to last. By our arithmetic on it, rain at 4 mm an hour needs about 6.8 hours and 27 mm in total. At 50 mm an hour it needs about 22 minutes and 18.5 mm (Lin et al., 2020). Try other rates below.
Rain check: how long must it rain to put out peat smoulder?
Move the slider to a rain rate. The readout uses the lab formula from Lin et al. (2020) for the top 15 cm of peat.
Bar length: hours of steady rain needed, scaled to 7 hours.
At 4 mm an hour, lab peat smoulder needed about 6.8 hours of steady rain, about 27 mm in total.
| Rain rate (mm/h) | Time needed | Total rain (mm) |
|---|---|---|
| Below 4 | Did not go out | n/a |
| 4 | 6.8 hours | 27 |
| 10 | 2.4 hours | 23.5 |
| 50 | 22 minutes | 18.5 |
Check the hourly rain forecast, not the daily chance of rain. A 90% chance of light showers will not end a peat haze. A few hours of heavy rain might.
Why does the haze come back after it rains? The fire is underground
Rain does two separate jobs. It scrubs smoke out of the air, which is why the sky looks clean an hour later. It only stops the smoke at its source if it soaks the peat deeply enough.
In bad years that source sits a long way down. Peat groundwater fell by 1.0 to 1.5 m in 1997/98, 2015 and 2019 (Sulaiman et al., 2023). The lab tests that found the 4 mm an hour line only covered the top 15 cm (Lin et al., 2020).
So a shower can wet the surface while dry peat below keeps smouldering. When the rain stops, that smoke rises again.
Why a shower clears the sky but not the source
What you see vs what is happening
Haze plays tricks on the eye. Here is what the sky suggests, next to what the research found.
| What you see | What is happening |
|---|---|
| A blue sky after a downpour | Rain washed the smoke out of the air for a few hours. The peat below can keep smouldering (Lin et al., 2020). |
| A rainy day on the forecast | A day can bring 4 mm in total and never reach 4 mm in a single hour. Smouldering peat only went out at about 4 mm an hour in lab tests. |
| Wet ground after a storm | In the worst fire years peat groundwater sat 1.0 to 1.5 m lower, so dry peat can burn well below a wet surface (Sulaiman et al., 2023). |
| A haze season that ends in October | BMKG expects a late rainy season across 61.08% of Indonesia, with a December peak for most of Sumatra and Kalimantan (BMKG, 2026). |
You cannot make it rain harder. You can decide what air reaches your lungs on the ride home.
Ride with filtered air
Filters haze PM2.5 on every ride
Easi Breezi mounts on your scooter or motorbike and feeds H11 HEPA-filtered air into your helmet. Bike-powered, no batteries, no charging.
Why is 2026 running later than normal? Two oceans are drying the region
Two ocean patterns are drying the region at once. BMKG measured the Pacific Niño3.4 sea surface anomaly at +2.83 in late August, which it calls a very strong El Niño. The Indian Ocean Dipole read +0.376 in August and is heading positive (BMKG, 2026).
The US Climate Prediction Center agrees.
On 10 September it gave a greater than 90% chance of a very strong El Niño through the northern winter. It put a 75% chance on October to December being historic, stronger than any event since 1950 (NOAA CPC, 2026). Rainfall over Indonesia remained suppressed.
That combination matters. Across 1900 to 2019, rain and stored water on the Maritime Continent fell furthest when El Niño and a positive Indian Ocean Dipole came together (Xiao et al., 2022).
Dry months feed the fires. Once rain stays below 4 mm a day for long stretches, fire activity climbs steeply, and that sensitivity appears to have grown over Kalimantan (Field et al., 2016).
Will it happen again next year? It can, in any year
A late, smoky season is most likely in an El Niño year, but it is not limited to one.
In a large set of climate simulations, El Niño raised the risk of a 1997 or 2015 scale fire year in Indonesian Borneo by a factor of 2.7. The same simulations found that risk in every ENSO phase (Lam et al., 2026).
The recent run of combined El Niño and positive Indian Ocean Dipole years has been more frequent since 1965. Climate models, though, do not project more of them under severe warming, which points to natural swings rather than a steady climb (Xiao et al., 2022).
So the honest answer is uneven: some years clean, some years like this one, with no guarantee either way. For the long view across several seasons, read is haze getting worse every year.
When the rains come, they work
This is the part of the forecast worth holding on to. BMKG expects the rainy season to start in November in parts of Sumatra and Kalimantan, and to peak in December for most of both islands (BMKG, 2026).
When rain of about 4 mm an hour keeps falling for several hours, smouldering peat goes out (Lin et al., 2020). The smoke stops at its source, and the air over Singapore and Malaysia clears with it.
Until then, the days between showers are the ones to plan for.
What should riders do until the haze is gone?
Indonesia's health ministry gives three pieces of advice: cut outdoor activity when the air worsens, wear a mask, and drink enough water (Ministry of Health via The Jakarta Post, 2026). A rider cannot always stay indoors, so the mask and the timing do most of the work.
The weeks to watch are the in-between ones. After a shower the sky looks clean, so visors open and masks come off. The peat is still burning, and the smoke can be back by the ride home.
Check a live reading before you trust the sky. Our live haze map for riders pulls current PM2.5 across the region, and the Pontianak haze guide covers what a hazardous day looks like on a bike.
What the studies actually measured
Most haze answers online give a month. The research gives something more useful: the physical conditions that have to happen before the smoke stops. Four studies cover it.
Lin and colleagues set peat smouldering in the lab, then rained on it at controlled rates. Below roughly 4 mm an hour, the fire kept going. Their words are plain: persistent light rain cannot suppress it (Lin et al., 2020).
They also fitted a formula for how long the rain must last. The harder it rains, the shorter the time. Even under violent rain, the total needed never dropped below about 13 mm (Lin et al., 2020).
Sulaiman and colleagues looked underground. They logged peat groundwater in Central Kalimantan for 27 years. In the big fire years of 1997/98, 2015 and 2019 it dropped by 1.0 to 1.5 m (Sulaiman et al., 2023). Dry peat that deep can burn well below the surface a shower wets.
Field and colleagues looked at the other end: what starts the runaway. Fire in Indonesia climbs steeply once rain stays under 4 mm a day for long stretches (Field et al., 2016).
Lam and colleagues ran a large set of climate simulations for Borneo. El Niño made a 1997 or 2015 scale fire year 2.7 times as likely, but such a year could happen in any phase (Lam et al., 2026).
The research behind this
Peer-reviewed studies this post is built on. Each card says what was measured, what was found, and what it cannot tell you.
Can rain suppress smouldering peat fire?
- What they measured
- Lab experiments that rained on smouldering peat in the shallow layer, up to 15 cm deep, at different rain intensities.
- What they found
- The minimum rain intensity that put the fire out was roughly 4 mm an hour. The total rain needed fell towards a minimum of 13 mm under violent rain.
- What it can't tell you
- Lab scale and the top 15 cm only. Deep peat and real water tables were not modelled.
Indonesian fire and smoke in 2015 and El Niño drought
- What they measured
- Satellite fire, smoke and carbon monoxide records, airport visibility in Sumatra and Kalimantan, and dry-season rainfall from satellites and gauges.
- What they found
- Fire responds sharply once rain stays below 4 mm a day for long periods, and that sensitivity appears to have grown over Kalimantan. The 2015 smoke plume lasted 2 months.
- What it can't tell you
- It explains when fires run away, not the date any single season ends.
Peat groundwater as an El Niño alert
- What they measured
- 27 years of groundwater level readings in Central Kalimantan peat forest, set against El Niño and Indian Ocean Dipole records.
- What they found
- Groundwater dropped sharply by 1.0 to 1.5 m in 1997/98, 2015 and the 2019 positive IOD year.
- What it can't tell you
- Central Kalimantan sites only. It does not measure how long the peat takes to rewet once rain returns.
Major Borneo fires under every ENSO phase
- What they measured
- A large ensemble of global climate simulations, used to estimate the risk of fire years as severe as 1997 and 2015 in Indonesian Borneo.
- What they found
- El Niño raised that risk by a factor of 2.7, but the risk existed whatever the ENSO phase.
- What it can't tell you
- A model estimate of risk, not a forecast for this season.
Who breathed the 2015 smoke
- What they measured
- A high-resolution weather and chemistry model of the autumn 2015 fire smoke, checked against satellite and ground readings.
- What they found
- 69 million people had persistent exposure to unhealthy air, with an estimated 11,880 excess deaths from short-term exposure.
- What it can't tell you
- Modelled exposure. The deaths are estimated from risk curves, not counted.
Frequently asked questions
When will the haze go away?
When the rainy season is under way in the fire provinces. In 2026 BMKG expects that to be late, with a November start in parts of Sumatra and Kalimantan and a December peak for most of both islands (BMKG, 2026).
How long will the haze last?
No agency gives an end date. The smoke lasts until enough heavy rain falls to soak the peat, which BMKG's forecast puts later than normal this year across 61.08% of Indonesia (BMKG, 2026).
Is the haze gone after it rains?
Often only for a few hours. Rain clears the air, but light rain cannot put out smouldering peat, which needed about 4 mm an hour in lab tests (Lin et al., 2020).
When will the haze end in Singapore and Malaysia?
Their haze drifts from Sumatra and Kalimantan, so it ends when those fires are damped by sustained rain or the wind shifts. The late Indonesian forecast is the one to watch.
Will the haze get worse before it gets better?
It can. BMKG flagged September and October for fire risk, and 556 zones covering 61.91% of Indonesia's land were still in dry season on 22 September (BMKG, 2026).
What to do this week
- Read the hourly rain forecast. Look for rain of 4 mm an hour or more lasting several hours (Lin et al., 2020). Drizzle does not count.
- Do not trust a clear sky after a shower. Check a live PM2.5 reading before you open the visor.
- Move your worst ride. If you can, shift the longest trip to the hours your local readings are lowest.
- Filter the air inside your helmet. The Easi Breezi unit runs off the bike and feeds your helmet air through an H11 HEPA filter that stops 95% of PM2.5, whatever the sky looks like.
BMKG's next seasonal updates will show whether the rain arrives on the forecast. Until then, plan for smoke through the in-between days.
Sources
- BMKG (2026). El Niño sangat kuat sedang terjadi, musim hujan 2026/2027 diprediksi datang lebih lambat. Press release, 22 September 2026. bmkg.go.id
- Lin S, Cheung YK, Xiao Y, Huang X (2020). Can rain suppress smoldering peat fire? Science of the Total Environment 727:138468. doi:10.1016/j.scitotenv.2020.138468
- Sulaiman A, Osaki M, Takahashi H, et al. (2023). Peatland groundwater level in the Indonesian maritime continent as an alert for El Niño and moderate positive Indian Ocean dipole events. Scientific Reports 13:939. doi:10.1038/s41598-023-27393-x
- Lam T, Catto JL, Kay G, Dunstone N, Barciela R, Harper AB (2026). Major fires in Indonesian Borneo are possible under all ENSO phases. npj Natural Hazards 3:46. doi:10.1038/s44304-026-00209-4
- Field RD, van der Werf GR, Fanin T, et al. (2016). Indonesian fire activity and smoke pollution in 2015 show persistent nonlinear sensitivity to El Niño-induced drought. PNAS 113(33):9204. doi:10.1073/pnas.1524888113
- Crippa P, Castruccio S, Archer-Nicholls S, et al. (2016). Population exposure to hazardous air quality due to the 2015 fires in Equatorial Asia. Scientific Reports 6:37074. doi:10.1038/srep37074
- NOAA Climate Prediction Center (2026). ENSO diagnostic discussion, 10 September 2026. cpc.ncep.noaa.gov
- Xiao HM, Lo MH, Yu JY (2022). The increased frequency of combined El Niño and positive IOD events since 1965s and its impacts on maritime continent hydroclimates. Scientific Reports 12:7532. doi:10.1038/s41598-022-11663-1
- Aldika AB (2026). Indonesian fire haze worsens air quality in Singapore. Kompas, 22 September 2026, reporting NEA Singapore and SiPongi data. kompas.com
- Indonesian Ministry of Health, via The Jakarta Post (2026). Respiratory problems climb past 170,000 cases in Indonesia. 24 September 2026. thejakartapost.com
Written by Ash, mechanical engineer and founder of Easi Breezi, building an active HEPA filtration system for motorcycle helmets (patent pending). Based between Hong Kong and Bali, riding daily in the traffic this blog writes about.