Fires have been raging in the forests of Indonesia, but it has been hazy one moment and clear the next in Singapore.

This is because when it comes to haze, Singapore is at the mercy of the winds. Yesterday, the weatherman cautioned that hazy conditions could persist in the days ahead, with dry weather expected over central and southern Sumatra.

The National Environment Agency (NEA) said: "The prevailing winds are forecast to continue blowing from the south-east or south, and Singapore may still experience slightly hazy conditions."

Air quality in Singapore crept into unhealthy levels for the first time in three years last Saturday.

Winds brought smoke haze from the fires in central and southern Sumatra.

But this changed on Sunday, when the wind started blowing from the south-east instead, providing Singapore with some relief from the pollution down south.

The relatively clear skies continued yesterday, with the Pollutant Standards Index - a measure of air quality in Singapore - hovering at moderate levels throughout the day. At such levels, normal activity can continue, say the authorities.

At 9pm, the 24-hour PSI was 84 in southern Singapore, in the moderate range. At that time, the one-hour PM2.5 reading in the same region was 48, in the normal range.

The main air pollutants during periods of haze are tiny particulate matter known as PM2.5. The one-hour PM2.5 concentration readings provide an indication of the current air quality.

Haze in Singapore usually originates from forest fires in the region.

Indonesia, in particular, has been criticised in recent years for not doing enough to prevent and suppress the forest fires in the country.

The fires are usually started by farmers using the slash-and-burn method. To make matters worse, the fires tend to occur on peat soil.

These carbon-rich ecosystems are usually waterlogged. But cash crops such as oil palm and acacia - trees grown for the production of pulp and paper - do not do well in waterlogged soil, leading farmers to drain the land, making it more flammable.

In 2015, the annual fire season coincided with El Nino - which makes the weather in South-east Asia hotter and drier than usual - and this climate phenomenon caused the forest fires in Indonesia to spiral out of control.

It resulted in the region's worst haze crisis on record.

This year, South-east Asia is also experiencing drier-than-usual weather due to persistent cooler-than-usual sea surface temperatures in the eastern Indian Ocean, off the west coast of Sumatra. This discourages the formation of rain clouds over the region.

Associate Professor Koh Tieh Yong, a weather scientist at the Singapore University of Social Sciences, said: "That year was one of the strongest El Nino events on record, whereas the current cooling of the Indian Ocean is moderate by historical standards."

Singapore is in the midst of its south-west monsoon season.

During this period, surface winds are expected to blow from the south-east or south, and on occasion, shift to blow from the southwest, NEA said.

In an update yesterday, it said the monsoon rain band is forecast to remain over northern South-east Asia, away from Singapore. "The prevailing dry weather is expected to continue into the second fortnight of the month," NEA added.

The fires burning in Indonesia do not necessarily mean Singapore will experience haze. What is key during this period, said Prof Koh, is where the fires are burning and where the wind is blowing from.

Satellite images have shown a surge in the number of hot spots on Indonesia's Sumatra Island and in Kalimantan over the past week.

But the recent haze experienced here came mainly from south Sumatra, which lies directly south of Singapore, Prof Koh said. Southerly winds brought pollution from the fires there to Singapore last week.

Meanwhile, other fires burning in central Sumatra had caused a dense build-up of particulate matter in the surrounding air. Turbulence near the surface helped diffuse the pollutants to nearby Singapore.

Thankfully, Singapore got a breather when the winds changed direction on Sunday to blow from the south-east instead.

"It is unlikely that the fires in Kalimantan would affect Singapore because they are quite a distance away from us in the south-east direction, unlike Sumatra," said Prof Koh.

Even if south-easterly winds brought haze from Kalimantan to Singapore, the smoke plume would diffuse in all directions on its way over, leaving much lower concentrations of pollutant particles, he added.