The rainy season, originally expected in early October in Riau province, will now start in the middle of the month at the soonest, due to the el-Nino phenomenon, said Bibin Julianto, a weather forecaster at the meteorological, climatological and geophysical office in Pekanbaru.

Riau is the second closest province to Singapore, after the Riau Islands province where Batam is located.

Last June, Dumai city in Riau was the epicentre of the haze, and Singapore as well as Malaysia saw record-high Pollutant Standards Index (PSI).

Jambi province, another hotspot area, may see rainy season start three to four weeks late, said state weather forecaster Nurangesti, who goes by only one name.

Whether Singapore and Malaysia will experience a prolonged period of hazy weather will depend on the wind directions.

In the next two to three days, the westerly winds (from west to east) and the south-easterly winds (from south-east to north-west) will be predominant above Riau, Bibin told The Straits Times.

"The westerly winds are going in the Singapore direction," he said, but added that this is a transition period where the wind direction could change.

Haze from forest and plantation fire in Jambi, South Sumatra and Riau is being blown to this region, as Indonesia is focusing fire fighting efforts in the southern part of Sumatra, where most of the fires have been detected in the past weeks.

"The number of hotspots is actually declining now, but the haze from the previous episodes (of forest and plantation fires) was stuck in the air because there was not enough rain, and is now being blown to Singapore," said Agus Wibowo, head of data at the national disaster mitigation agency (BNPB), which coordinates national fire fighting efforts.

BNPB is doing cloud-seeding and water bombing operations in South Sumatra, the centre of the firefighting operations for Sumatra. A military hercules and several helicopters have been stationed there to help with the operations, said Agus.

Riau has had rainfalls and is seeing PSI falling to below 50 on Tuesday, according to Bibin, while Jambi's PSI is below 100. PSI levels above 100 is considered unhealthy, and between 51 and 100 are deemed moderate.

On Tuesday morning, there are 10 hotspots detected in Sumatra by the MODIS satellite - four in South Sumatra, four in Riau, one in Jambi and one in North Sumatra.

"A small number, but more could have been undetected by the satellite due to the thick haze. A lot of the haze above Riau came from South Sumatra," Agus said.

In Palangkaraya, Central Kalimantan, the local authority continued to shut schools until Thursday due to the thickening haze, Kompas daily reported. Schools have been closed since last week when the PSI reached very unhealthy levels.

Almost all 10 flights leaving and going to Palangkaraya were cancelled on Monday as visibility level fell to as low as 200 metres. (***)

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