Manaus, Borneo, Sumatra: what satellites saw and missed

In October, smoke over Manaus got so thick that the city closed its schools, with PM2.5 above 135 µg/m³. Weeks earlier, smoke from Borneo and Sumatra drifted to Malaysia and Singapore.

One part of the story gets less attention: what satellites can and cannot see.

We mapped every NASA FIRMS fire alert from August to October 2026 in Kalimantan and mainland Sumatra, and in the nine states of Brazil’s Legal Amazon: 1.18 million alerts in Indonesia, 1.22 million in the Legal Amazon.

Figure 1. FIRMS fire alerts per cell of about 1 km (0.009°), August to October 2026, Kalimantan and Sumatera (top) and the Legal Amazon (bottom), forest in 2020, and  Cities above 1 million people.

Indonesia peaked first. Its busiest week ended on 20 September; the Legal Amazon’s on 11 October. In late October, Indonesia was below 3,000 alerts a day, while the Amazon region still had 10,000 to 20,000. That fits the El Niño pattern described in 2017, in which Indonesia dries out before the central Amazon.

The fires were concentrated. Central Kalimantan alone had 536,000 alerts, 45% of Indonesia’s total, and one in six ~1 km land cells in Kalimantan had at least one alert. Kota Palangkaraya had about 1,700 alerts per 100 km², the highest of any district in either region. In Brazil, Pará led with 473,000.

Figure 2. FIRMS fire alerts per 100 km² per regency or city (Indonesia) and municipality (Brazil), August to October 2026.

Most alerts fell in areas mapped as forest in 2020: 54% in Indonesia, 65% in the Legal Amazon. In Indonesia, that share rose every month, from 45% in August to 61% in October.

What do these numbers measure? A FIRMS alert is a hot pixel seen by a passing satellite: 375 m for VIIRS, 1 km for MODIS. It is not a fire count or a burned area. One fire can produce many alerts; a fire under thick smoke or cloud may produce none.

So daily counts jump. On 9 October, alerts in Kalimantan and Sumatra almost halved in a day. On 22 October they hit the season’s peak, 38,000. A drop can mean fires are going out, or that sensors cannot see through the smoke.

Night matters too. 54% of Indonesian alerts were detected at night, against 34% in the Amazon, which fits smouldering peat. Peat burning below the surface can be missed altogether.

Radar satellites like Sentinel-1 fill part of the gap. Smoke and clouds do not block them, and they show where the canopy changed, though they do not see heat. Space4Good uses this radar data in its FORESTER system and sends alerts to forest teams, who check them on the ground. The system was tested in Indonesia and Brazil, the two places on the map.

The takeaway: a fire map shows where satellites saw heat, not everything that burned. Counts swing with smoke and cloud, not only with fires, and in Indonesia the fires moved further into forest as the season went on. Heat data says when and where to look. Radar and people on the ground show what was lost.

The fires will slow when the rain comes: in Indonesia this month, in the Amazon around December or January. The gap in the data will still be there next season.

Maps produced by Space4Good.

Featured image of forest fire from The New York Times.