JAKARTA - Rain arrived in parts of Sumatra and Kalimantan, taking away the smoke and haze that choked the region for weeks, and providing the provinces with much needed but temporary relief, Indonesia's weather forecast agency BMKG said on Wednesday (Sept 25).

It rained over two days from Tuesday in six provinces affected by the fires: Riau, Jambi, South Sumatra, West Kalimantan, Central Kalimantan, and South Kalimantan.

Winds blowing in from the South China Sea had contributed to the formation of rain clouds over Sumatra and Kalimantan, the agency's head of public meteorology Fachri Radjab told The Straits Times.

"The wind carried a lot of water vapour, which allowed the formation of rain clouds," he said.

Mr Fachri noted that the number of hot spots was reduced significantly as a result of the rains.

There were 3,150 hot spots across the archipelago on Monday.

But by Tuesday, it had dropped sharply to 1,982 before slipping to 1,744 hot spots on Wednesday, the National Disaster Management Agency (BNPB) said in a statement.

In Riau province, which is close to Singapore, the 24-hour Pollutant Standards Index (PSI) showed air quality at midnight on Tuesday had hit the 146-mark, which is in the moderate range.

It improved to within the good range at 28.27 on Wednesday at 1pm, according to BMKG.

This is a vast improvement from Sunday when the PM10 PSI in Pekanbaru, the capital of the Indonesian province of Riau, crossed the 700-mark, a record high level which entered the dangerous range.

It prompted the local authorities on Monday to declare a state of emergency in the province, to fight the haze and secure more aid from the central government.

PM10 refers to particulate matter. The higher the concentration, the poorer the air quality.

The recent rains brought much-needed relief to residents of Jambi as well, with PM10 concentration readings settling at the 99.57-mark in the moderate range on Tuesday night, before improving further to reach the 48.12-mark at 1pm on Wednesday.

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Although the monsoon season is in mid-October, BMKG said rains seen earlier this week will continue for the next three days over parts of Indonesia, including Riau, Jambi, West Kalimantan and the northern regions of Central Kalimantan.

The agency said the monsoon season is marked by rainfall greater than 50mm per day in 30 consecutive days.

Mr Fachri said that clouds forming over Sumatra and Kalimantan may help efforts to induce rain through cloud-seeding operations.

"We haven't entered the monsoon season yet but cloud seeding has sped up the arrival of rains, with more clouds for seeding."

Until the recent rains, Indonesia's emergency response team has relied on four aircraft to perform cloud-seeding operations over Sumatra and Kalimantan.

Indonesia, home to the world's third-largest tropical rainforest after the Amazon and Congo Basin, is struggling to tackle the fires engulfing Sumatra and Kalimantan which are spreading the haze and worsening air quality in neighbouring South-east Asian countries, particularly Singapore and Malaysia.

From January to August, fires raged across 328,724ha of land, of which 27.3 per cent was peatland, according to the BNPB.

In Riau, 49,266ha of land was impacted by the fires, of which more than 80 per cent of them was peatland.

Indonesia has deployed 29,039 personnel to battle the fires, with 48 helicopters conducting water-bombing operations every day.