A tropical storm brewing over the South China Sea has been blamed for blowing the current haze towards Singapore.
The storm acts like a "magnet", which pulls the wind in a direction different to that of the south-southeasterly winds usually expected during this period, said Assistant Professor Winston Chow of the National University of Singapore's (NUS) geography department.
He was responding yesterday to a weather and haze situation briefing by the National Environment Agency (NEA) on Tuesday.
During the briefing, the NEA said the presence of the tropical storm in the South China Sea has brought about a shift in winds to blow from the south-south-west direction. As a result, haze was blown in from Sumatra, Indonesia, causing air quality in Singapore to deteriorate.
The Republic is in the midst of the south-west monsoon season, during which winds are expected to blow from the south and south- east.
But the tropical storm "very subtly alters the monsoonal pattern", said Prof Chow. "It shifts the wind direction in Singapore from mostly the south or south-east to south- west instead."
However, the NEA also gave good news during Tuesday's briefing. It said that the wind direction could change for the better tomorrow, when the storm reaches land and prevailing winds are forecast to blow from the south-east.
Prof Chow said tropical storms are usually fuelled by the evaporation of sea water in the open sea. But when they reach land, their source of fuel is cut, and they start to dissipate.
Yesterday, air quality in Singapore improved slightly. The 24-hour Pollutant Standards Index (PSI) hovered in the low end of the unhealthy range for most of yesterday, compared with earlier this week. On Monday, the 24-hour PSI was in the range of 146-173 at 3am.
Air quality is considered unhealthy when 24-hour PSI readings are in the range of 101-200, and very unhealthy when 24-hour PSI readings are between 201 and 300. When they cross 300, air quality is deemed hazardous.
The NEA attributed the improvement to "a shift in the prevailing winds to blow from the south-east".
Yesterday afternoon's showers also brought a temporary respite from the haze, the NEA said.
"Rainfall removes particles from the air, leading to a temporary reduction," said Assistant Professor Jason Cohen from the NUS civil and environmental engineering department. "But if the fires are still burning, the fresh particles will continue to move in after the rain clears."
Moderate to dense haze is still persisting in central and southern Sumatra.
The NEA said that hazy conditions are expected to persist, and thundery showers are forecast over some areas of Singapore in the late morning and early afternoon today.
The 24-hour PSI for today is expected to be in the low end of the unhealthy range, and could cross the mid-section of the unhealthy range "if denser haze from Sumatra is blown in by unfavourable winds".