Singapore got some respite from the haze yesterday as changing winds and heavy rain cleared the white pall from the skies.
But experts warn that the relief is temporary, as wind patterns could still turn unfavourable and bring polluted air from the region.
The 24-hour Pollutant Standards Index (PSI) reading at 6pm yesterday was in the moderate range of 80 to 94 - the first time it had inched out of the unhealthy band since last Friday. At 9am, the three-hour PSI registered a reading of 26 - possibly one of the lowest in recent weeks.
The National Environment Agency (NEA) said the improvement was due to widespread showers early in the morning.
Dr Erik Velasco, a research scientist from the Singapore-MIT Alliance for Research and Technology, said rain in Sumatra and Kalimantan in Indonesia also helped to slow the fires' intensity and expansion. "More rain is forecast for the following days along large sectors of both islands. If the rain is long and intense, it will help to extinguish the fires," he told The Straits Times.
He said that previous instances of rain had not helped much as they were short and not very intense.
"Only on a few occasions were they capable of reducing the pollution for a few hours, but because the winds were blowing from the south-west and south, the haze quickly returned."
Assistant Professor Winston Chow of the National University of Singapore's geography department said north-easterly winds were the main reason for yesterday's improved conditions.
"The rain helped somewhat in improving air quality, but the northeast instead of southerly winds were more important in blowing the haze away temporarily."
The change in wind direction was due to the current inter-monsoon season.
During this season, winds are very variable, Prof Chow said, and Singapore could experience hazy conditions again if winds revert to a generally southerly direction. The inter-monsoon season is expected to transition into the north-east monsoon in mid-November, and people can expect to breathe easier when this happens, Prof Chow said.
The NEA said that for today, prevailing winds are expected to be weak and blowing from the north-east or east. Thundery showers are also expected in the early morning today.
"Occasional slightly hazy conditions can still be expected and visibility could be slightly reduced if PM2.5 concentration levels are elevated," it said in yesterday's update.
The air quality for today is expected to be in the high end of the moderate range, and could enter the low end of the unhealthy range.
Dr Velasco said: "We are getting close to the north-north-east monsoon onset. Wind direction will fluctuate less and once the main winds come from the north, the haze threat over Singapore will decrease. However, if the fires don't stop, any meteorological anomaly may bring back the haze, and I would recommend following NEA's warnings closely."